Physical Geology Exam 1 End of Chapter Reading Q/A
How does subduction trigger melting?
Flux melting of the asthenosphere above the subducting slab creates a rising melt.
What is metamorphic foliation, and how does it form?
Foliation is the presence of parallel planar surfaces or layers in metamorphic rock. Under sufficient differential stress, platy or elongate grains are broken down and regrown in a preferred orientation perpendicular to maximum compressive stress. Ch. 7
Why is hornfels nonfoliated?
Hornfels forms through contact metamorphism, without the application of differential stress. Ch. 7
Bowen's reaction series:
allows a geologist to predict what minerals will be found in a given igneous rock.
A high-pressure, fast-moving mixture of pyroclastic debris including fragments and gas typically ejected skyward from a volcanic vent is called:
an eruptive cloud or column.
Give examples of intrusive igneous structure:
batholith, laccolith, sill
Define O Horizon:
consists almost entirely of plant debris with barely any mineral matter.
Which of the following processes is associated with the formation of travertine?
precipitation
Describe the three processes that are responsible for the formation of magmas.
(1) Decreasing pressure (2) The addition of volatiles (such as water and carbon dioxide) (3) Obtaining heat from a nearby source such as a neighboring body of magma Ch. 4
Shields:
1) Are composed of some of the oldest rock on Earth (Precambrian). 2) Make up large areas of Canada, southern Africa, and South America. 3) Are composed of rocks that were metamorphosed during ancient mountain-building events.
Pegmatites:
1) Can contain crystals up to tens of centimeters across. 2) Are an igneous rock type, distinguished by their unusual grain size. 3) Occur in tabular intrusions called pegmatite dikes.
Describe the three different kinds of material that can erupt from a volcano.
1) Lava is molten rock, flowing out of the volcano in liquid state and cooling and solidifying on the ground. 2) Pyroclastic debris forms when bodies of lava are shot into the atmosphere and cool during flight to form volcanic glass; pyroclastic debris ranges in size from ash to lapilli to larger pieces termed bombs. Blocks are fragments of pre-existing volcanic rock from older eruptions that are hurled by the force of eruption. 3) Gases that erupt from volcanoes are water vapor, carbon dioxide, sulfur dioxide, and hydrogen sulfide. Ch. 5
What two features characterize most metamorphic rocks?
1) Metamorphic mineral assemblages (minerals uniquely produced under the temperature and pressure regimes of metamorphism) 2) metamorphic texture (grain arrangement, often involving foliation, which is a preferred alignment of inequant grains or alternating light and dark mineral bands) are characteristic of most metamorphic rocks.
Name the soil horizons in order:
1) O 2) A 3) E 4) B 5) C
Mylonite forms by:
1) Recrystallization in a fault zone. 2) Has pronounced foliation parallel to the direction of faulting 3) Shear stress on softened rock.
Magma moves upward toward the Earth's surface:
1) Slowly, working its way up through cracks and narrow conduits. 2) Because the weight of overlying rock creates pressure that squeezes it up. 3) Because it is less dense than the surrounding rock.
Early Earth had numerous heat sources for igneous activity, including:
1) The compression of mass into a smaller volume. 2) Conversion of the kinetic energy of meteorite impact to heat energy. 3) Conversion of the kinetic energy of iron alloy sinking toward Earth's center to heat energy.
Describe the geologic settings where thermal, dynamic, and dynamothermal metamorphism take place, respectively
1) Thermal metamorphism takes place in a zone of country rock surrounding a pluton, where the country rock's mineral assemblage becomes recrystallized. 2) Dynamic metamorphism occurs in fault zones, where shearing force recrystallizes minerals at depth. 3) Dynamothermal metamorphism occurs within the cores of mountain ranges, induced by increased heat and pressure associated with crustal thickening and the shearthat arises from collision. Ch. 7
Silicates:
1) are classified on the basis of how the tetrahedrons join and share oxygen atoms. 2) Are a major component of continental crust 3) have the silicon-oxygen tetrahedron as their structural unit.
Define Differentiation of planets:
1) began with heating up component materials within these bodies. 2) Is the process by which these bodies develop internal layering. 3)was caused by heat created by the transformation of kinetic energy from collisions into thermal energy.
Define Stellar nucleosynthesis:
1) process by which stars create intermediate-weight elements. 2) Stellar nucleosynthesis is a normal process that happens during the life cycle of stars. 3) In stellar nucleosynthesis, reactions are caused by fusion.
Describe the several ways in which mineral crystals can form.
1)Crystals can solidify from a melt, freezing from hot liquid rock in the formation of igneous rock. 2)Can form through solid-state diffusion, in which new crystals are formed from the atoms that were present in a pre-existing mineral. Movement of atoms or ions to form the new structure can be driven by heat and pressure; thus, solid state diffusion is associated with metamorphic rock formation. 3)Can form through precipitation from water or from gas. Precipitation of crystals might be due to inorganic physical processes, but it might also result from the metabolic activities of organisms. Ch. 3
Crystals:
1)grow outward from a seed. 2)have an orderly internal arrangement of atoms arranged in a lattice pattern 3)display symmetry.
Describe the major categories of materials constituting the Earth. Does the crust have the same composition as the whole Earth? On what basis do geologists distinguish among different kinds of silicate rock?
1. Organic Chemicals, Minerals, glasses, rocks, grains, sediments, metals, melts, volatiles 2. The crust does not. This is due to the internal processes within the Earth that have caused the planet to become layered. This process tends to have heavier elements migrate toward the center of the planet and lighter elements towards the crust 3.Geologists distinguish silicate rocks on the basis of silica content and grain size.
What was Wegener's continental-drift hypothesis? What was his evidence? Why didn't other geologists accept Wegener's proposal of continental drift, at first?
1. The continents had once been adjoined, forming a supercontinent (Pangaea), and that they later moved apart to form their present configuration. 2. (1) Continents looked like they fit together like a jigsaw puzzle, (2) Wide distribution of fossils, (3) Rock units match up between continents across the Atlantic Ocean basin, (4) Ancient climatic belts (including an ancient glaciation) 3. Could not explain why the continents moved.
Why is the oldest oceanic lithosphere less than 200 Ma?
All oceanic lithosphere produced prior to 200 Ma has been subducted down trenches.
Compare the deposits of an alluvial fan with those of a deep-marine deposit.
Alluvial fans are wedge-shaped deposits occurring at the foot of an eroding mountain range. The sediments are typically coarse (sand, pebbles, or cobbles) and contain substantial amounts of feldspar (physically weathered from a typically granitic montane source). Deep-marine deposits are dominated by the skeletons of planktonic microorganisms (chalk derived from foraminiferans, and bedded chert derived from diatoms and radiolarians ) and clay (which settles to form finely laminated mudstone). Ch. 6
Why isn't the Earth homogenous?
Early in Earth history, all of its matter was molten. Gravity caused the heavier metals (primarily iron) to sink toward the center of the planet, forming a core distinct from the rocky mantle of the Earth.
What is the proportion of land area to sea area on Earth?
Earth consists of 30% land area as opposed to 70% sea area
What is the source of heat in the Earth?
Earth has derived heat from collisions, gravitational compression, decay of radioactive isotopes, meteors and differentiation. Ch. 4
What is Earth's atmosphere composed of? Why would you die of suffocation if you were to eject from a fighter plane at an elevation of 12 km without an oxygen tank?
Earth's atmosphere is mostly nitrogen and oxygen, with minor amounts of argon, carbon dioxide, and other gases. The atmosphere becomes less and less dense with altitude; at 12 km, oxygen molecules are too sparse to support human life
Why do some minerals occur as euhedral crystals, whereas others occur as anhedral grains?
Euhedral crystals (those with clearly defined faces and edges) develop when crystal growth occurs in unoccupied space. More commonly in igneous rocks, numerous crystals form more or less simultaneously in a tightly packed space. In this case, the competing crystals intertwine as they crystallize, forming irregular boundaries. Such crystals lack the clearly defined faces and are termed anhedral grains. Ch. 3
Why do some volcanic eruptions consist mostly of lava flows, while others are explosive and do not produce flows?
Explosive eruptions are the result of a sudden release of accumulated gas pressure within the volcano. Lavas with high proportions of volatiles, such as water vapor, produce explosive eruptions in which the lava is blasted upward by explosively expanding gas, forming pyroclastic debris out of the lava that would otherwise have flowed down the slope of the volcano. Ch. 5
What factors control the viscosity of a melt?
Felsic (silicic) melts are more viscous than mafic melts because silica tetrahedra mutually link to form complex structures that cannot move as rapidly as the simple structures found in more mafic melts. High volatile content, including water content, lessens viscosity. Cooler magmas are more viscous than hotter magmas of the same composition. Ch. 4
Describe how a clastic sedimentary rock forms from its unweathered parent rock.
First, physical and chemical weathering break up and alter the parent rock to form detrital fragments of parent material, dissolved ions, and clay. This sediment of weathered grains is then eroded from the parent surface and transported away from the source by water, wind, or glacial ice (or directly by gravity, in the case of large clasts on a slope). Ultimately, the sediment will settle out of the transport medium. Eventually, loose grains of deposited sediment may become buried under additional sediment, compacted, and cemented to form sedimentary rock. Ch. 6
Why are some minerals considered gemstones? How do you make the facets on a gem?
Gemstones are minerals that are valued for their aesthetic beauty. Facets on gems are cut by a faceting machine and do not usually represent original crystal faces or cleavage planes. Ch. 3
Why is glass not a mineral?
Glass is atomically disordered, having no fixed crystalline arrangement. Because there is no fixed spatial arrangement for the atoms within glass, glass fails the "crystalline structure" requirement in the definition of a mineral. Ch. 3
List and define the principal physical properties used to identify a mineral.
Hardness (resistance to scratching), cleavage (tendency of crystals to break along planes of weakness), color (self-explanatory), luster (qualitative assessment of the way in which the mineral reflects light), crystal form or habit (the shape of visible crystals or crystal aggregates), streak (the color of the mineral in powdered form), and specific gravity (mass per unit volume of the mineral). Ch. 3
What are the major geologic features of a convergent boundary?
If the convergent boundary is a subduction zone = a deep trench where the subducting oceanic plate bends downward in opposition to the horizontal overriding plate. Sediments scrape off the subducting plate to form an accretionary prism at the edge of the overriding plate. melting associated with the subducting plate produces either a volcanic continental arc or a volcanic island arc. If a collisional boundary is present instead of a subduction zone, then a broad, thick, nonvolcanic mountain range will be present.
What factors determine the character (e.g., thickness, texture, types of horizons, etc.) of a soil?
Important factors that determine soil character include climate, chemical nature of the substrate, moisture, steepness of the landscape, vegetation present, and the amount of time that has been available for weathering reactions.
Why don't sediments accumulate everywhere? What types of tectonic conditions are required to create sedimentary basins?
In a majority of terrestrial environments, the rate of erosion meets or exceeds the rate of sedimentary deposition. Basic subsidence allows for the development and preservation of thick sedimentary sequences (including nonmarine deposits) but requires a locally sinking lithosphere. Sinking lithosphere is most often a consequence of tectonic rifting or collision, in which the lithosphere is either stretched and thinned or subjected to a load. Ch. 6
Why do some continental-rift eruptions yield flood basalts?
In a rift zone, basaltic magma may rise rapidly through fractures and may thus not undergo substantial chemical change that commonly occurs when magma rises through continental crust. Erupting with low viscosity, basaltic lava spreads outward to form uniform sheets (similar to aqueous floods). Ch. 5
How does a soil that forms in a tropical climate differ from one that forms in an arid climate?
In arid climates, soils develop slowly and retain soluble minerals, such as calcite. In a tropical climate, heavy rainfall and leaching remove nearly all but the most insoluble minerals, such as iron oxides and aluminum oxides. IB
What is the relationship between the way in which silicon-oxygen tetrahedra bond in micas and the characteristic cleavage of micas?
In micas, each silicon-oxygen tetrahedron shares three oxygen atoms with adjacent, coplanar tetrahedra. The shared silicon-oxygen bonds within these planes are very strong, but bonds perpendicular to sheets are much weaker. Cleavage in micas thus occurs along planes with a single orientation, parallel to the sheets. Ch. 3
Define Grains:
Individual mineral crystals within rocks or loose fragments of minerals or rocks.
What are meteorites, and how does the study of them provide insight into the character of the Earth's interior?
Meteorites are objects that have fallen from space and impacted the Earth. Meteorites may be rocky or metallic or a combination of the two. Most meteorites are thought to represent early pieces of our Solar System, and thus, they can be used as a model for the Earth's interior
How can you determine the hardness of a mineral? What is the Mohs hardness scale?
Mineral hardness is determined through scratch tests. A relatively hard mineral can scratch a softer mineral, but the converse statement is not true. The Mohs hardness scale is an ordinal scale of scratch resistance, with rankings from 1 (talc) at the soft end of the scale to 10 (diamond) at the hard end. Ch. 3
What is the prime characteristic that geologists use to separate minerals into classes?
Minerals are divided on the basis of chemical composition (more precisely, the anion[s] present in the mineral). Ch. 3
What are the various reactions that can contribute to chemical weathering?
Most chemical weathering is due to chemical reactions termed dissolution, oxidation, hydrolysis, and hydration. IB
What phenomena can cause metamorphism?
Mountain building, plutonism, volcanism, faulting, meteoric impact, mantle convection, deep burial, and water-rock interactions all lead to metamorphism. Ch. 7
The geothermal gradient of Mars is 8°C/km, and the crust of Mars is 30 km thick. Do high-grade metamorphic rocks form in this crust?
No. The temperatures at the base of the crust (240°C) could only produce very low grade metamorphism. Ch. 7
Would we likely find broad regions of gneiss and schist on the Moon? Why or why not?
No. These dynamothermally altered rocks are ultimately products of horizontal stresses caused by plate tectonics, which the Moon lacks. Contact metamorphism was possible during lunar history, but not dynamothermal metamorphism. Ch. 7
Popular media sometimes imply that the crust floats on a "sea of magma." Is this a correct image of the mantle just below the Moho? Explain your answer.
No; the mantle just beneath the crust is not only solid but rigid, and along with the crust, it forms the lithosphere. Even the asthenosphere is mostly solid, though weak and ductile. Ch. 1
How does oceanic lithosphere differ from continental lithosphere in thickness, composition, and density?
Oceanic lithosphere is thinner, more mafic in its crustal component (largely basalt, whereas continental crust is granitic), and denser.
What processes may lead to hot-spot eruptions?
Opinions among geologists vary concerning the genesis of hot spots. One school of thought proposes that they arise as plumes of very hot mantle rock from the base of the mantle (core-mantle boundary), which generate massive amounts of volcanism due to transfer of heat with surrounding rock. Another school argues that other circumstances are responsible for the copious melting seen at hot spots.
Does partial melting produce magma with the same composition as the parent rock from which it was derived?
Partial melting produces melts that are more felsic than the parent rocks that melted to form them. This result arises from the fact that minerals with high temperatures of crystallization (which characterize more mafic rocks) will be preferentially left behind as solid rock during the partial melting process. Ch. 4
Contrast a pyroclastic flow with a lahar.
Pyroclastic flows are hot mixtures of suspended pyroclastic debris (ash and lapilli) and air that tumble down the sides of volcanoes at great speed. A lahar is a fast, liquid flow arising when a pyroclastic flow mixes with water from snowfields or nearby streams. Ch. 5
. Explain the difference between relative-plate velocity and absolute-plate velocity
Relative-plate velocity describes rates of motion calculated for the movement of material on one plate with respect to material from an adjacent plate (or with respect to a plate boundary). Absolute-plate velocity is calculated using age and distance data from the material on a plate, with the distance calculated from a hot spot or other fixed point of reference on Earth's surface.
Discuss the major forces that move lithosphere plates
Ridge push is a driving force that arises because elevated lithosphere at a ridge pushes downward on less-elevated lithosphere to either side. Slab pull is a driving force that arises at subduction zones due to old, cold, dense oceanic lithosphere sinking into the less dense asthenosphere.
Describe how magmas are produced at continental rifts. Why can you find both basalt and rhyolite in such settings?
Rifting force thins out the lithosphere locally, decreasing the pressure on the underlying asthenosphere. This pressure release triggers partial melting to yield basaltic magma, which may extrude to the surface rapidly to produce basalt or melt the surrounding crust to produce a more silica-rich melt that produces rhyolite. Ch. 4
What processes can cause originally solid rock to break into pieces?
Rock can be broken by jointing from exhumation, frost wedging, animal attack, root wedging, salt wedging, and thermal expansion. IB
What kinds of high-tech equipment can be used to study rocks?
Rocks are often studied with such high-tech electronic equipment as scanning electron microscopes, electron microprobes, mass spectrometers, and X-ray diffractometers. IA
How is it possible for sandstone derived from sediment deposited in a beach environment to comprise a formation that blankets a broad region?
Sea level rises and falls overtime, shifting beach environments landward and seaward. Many areas in the region would have consisted of a beach environment at different times. Ch. 6
Describe the hypothesis of seafloor spreading
Seafloor spreading is the idea that new oceanic basalt is produced at mid-ocean ridges and spreads laterally to either side. It is the push of oceanic basalts that causes the continents to drift over Earth's surface
Do all sedimentary rocks have same composition? What conditions produce evaporites?
Sedimentary rocks have diverse compositions. Evaporite formation requires a fluid with dissolved ions (typically seawater) to be evaporated to such an extent that the ions will precipitate out, typically as halide, carbonate, or sulfate minerals. Hot, dry conditions with extensive subaerial exposure are conducive to evaporite formation; these include warm, broad, shallow seas with little riverine influx and restricted circulation. Ch. 6
How did drilling into the seafloor contribute further proof of seafloor spreading? How did the seafloor-spreading hypothesis explain variations in ocean floor heat flow?
Sediments atop oceanic basalts become thicker away from mid-ocean ridges, and the lowermost (oldest) layers become progressively older with increasing distance from the ridge. Heat flow is greatest at mid-ocean ridges, which is consistent with the seafloor spreading model in which new seafloor is produced as igneous rock solidifying from rising hot magma at these spreading centers.
Describe Shield volcanoes
Shield volcanoes are gently sloped domes typically composed of basaltic volcanic rock. The basaltic lava that forms this rock is of low viscosity, so it flows readily in response to gravity and attains a low profile on solidification. Ch. 5
On what basis do mineralogists organize silicate minerals into distinct groups?
Silicates are subdivided into distinct groups on the basis of the amount of oxygen sharing between neighboring silicon-oxygen tetrahedra. Ch. 3
The Long Valley Caldera, near the Sierra Nevada range in California, exploded about 700,000 years ago and produced a huge ignimbrite called the Bishop Tuff. Mono Lake found northwest, has an island in the middle and a string of craters extending south from its south shore. Hot springs and tufa deposits can be found along the lake. Explain the origin of Mono Lake.
The depression known as Mono Lake formed as a volcanic caldera that has infilled with water. Its islands and nearby domes imply resurgent volcanic activity. Ch. 5
What is the ecliptic, and why are the orbits of planets within the plane of the ecliptic? Why is Pluto no longer considered to be a planet?
The ecliptic is the plane defined by the orbits of the major planets of our Solar System. The planets reside together in this plane because they formed from matter within the rotating protoplanetary disk surrounding the early Sun. Pluto is no longer considered a planet because it has not swept its orbit clear of other objects.
Contrast the geocentric and heliocentric Universe concepts
The geocentric concept placed Earth at the center of the Universe, with the Sun and the other planets revolving around it. The heliocentric concept placed the Sun at the center, with Earth and the other planets revolving around it
How does crust form along a mid-ocean ridge?
The high-heat flux at the ridge melts mantle material to form magma, which is relatively light and rises to the surface. Some of the magma crystallizes beneath the surface (as gabbro or in thin basaltic dikes), and some erupts to form volcanic lava, which flows and ultimately solidifies to form pillow basalt.
How is the process of freezing magma similar to that of freezing water? How is it different?
The liquid state upon passage of the liquid through a critical temperature (or range of temperatures). The most striking difference is in the temperature of crystallization. Additionally, multiple minerals crystallize from magma; more mafic minerals crystallize first, and more silicic minerals crystallize later at cooler temperatures. Ch. 4
What is the difference between the lithosphere and the asthenosphere? At what depth does the lithosphere-asthenosphere boundary occur? Is this above or below the Moho? Is the asthenosphere entirely liquid?
The lithosphere is relatively cool and rigid compared to the hot, soft asthenosphere, which flows more readily. The lithosphere consists of the crust (oceanic basalt and gabbro, or continental granite), plus the uppermost mantle (peridotite) down to a depth of about 100 to 150 km. This boundary lies below the Moho. The asthenosphere is primarily solid rock, but is molten in places.
What are the characteristics of a lithosphere plate? Can a single plate include both continental and oceanic lithosphere?
The lithosphere is the rocky portion of Earth, relatively cool and rigid. The lithosphere is composed of the crust and the uppermost portion of the mantle. A single plate may consist of both continental and oceanic lithosphere in different regions.
What is Earth's magnetic field? Draw a representation of the field on a piece of paper. What causes aurorae?
The magnetic field of Earth is a region of space affected by the magnetic force of Earth (see Fig. 1.12c for a sketch). Aurorae are caused by high-energy charged particles traveling along Earth's magnetic field lines and interacting with the gases in the atmosphere.
What is the mantle composed of? Is there any melt in it?
The mantle is mostly made of an ultramafic silicate rock termed peridotite. There is a small amount of melt in the upper mantle.
Salt is a mineral, but the plastic making up an inexpensive pen is not. Why not?
The plastic is not a mineral because it lacks crystal structure and does not occur naturally. Ch. 3
Imagine that you are given two milky white crystals, each about 2 cm across. You are told that one of the crystals is composed of plagioclase and the other of quartz. How can you determine which is which?
The quickest and easiest way is a hardness test; quartz will scratch plagioclase, but the converse is not true. Quartz also lacks cleavage, whereas feldspars, like plagioclase, possess two cleavage directions. Plagioclase typically exhibits striations that can be seen when the specimen is rotated in light. Ch. 3
Why don't builders use gneiss to make roof shingles?
Unlike slate, gneiss has no tendency to cleave on planar surfaces. Ch. 7
The Moon has virtually no magnetosphere. Why?
Unlike the Earth, the Moon has no liquid metallic layer inside
What are the characteristics of volcanoes erupting at volcanic arcs? How do they contrast with volcanoes erupting at mid-ocean ridges?
Volcanic arcs produce cone-shaped summits, including large stratovolcanoes in continental volcanic arcs. Mid-ocean ridge volcanism extrudes from fractures in the crust termed fissures, which extend parallel to the mid-ocean ridge axis. Ch. 5
Define Transgression:
When sea level rises and the coast moves inward.
Explain the expanding Universe theory
When we look at distant galaxies, we find that they are all moving away from our own, with the farthest galaxies moving away the fastest. This movement suggests that the entire observable Universe is expanding outward.
Could you find a layer of metamorphic rock sandwiched between layers of sedimentary rock in a sedimentary basin? Why or why not?
Yes. You could, as a result of thermal metamorphism in aureoles above and below a sill that had intruded through the strata. Ch. 7
Could you use crushed calcite to grind and form facets on a diamond? Why or why not?
You could not; diamond is much harder than calcite, and the result of such efforts would be to powder the calcite. Ch. 3
Where would you go if you wanted to find exposed metamorphic rocks, and how would such rocks have returned to the surface of Earth after being at depth in the crust?
You would go look for the site of an ancient, greatly eroded mountain range. Metamorphism is strongly active in the base of mountain ranges. As overlying layers of sediment and rock are weathered and eroded away, isostatic pressure causes the basement to be buoyed upward until these rocks are finally exposed at the surface. Ch. 7
Dynamic metamorphism occurs:
as a consequence of shearing
Which of the following chemical reactions breaks down feldspars into clay minerals?
hydrolysis
Individual rock types within the three main groups can be distinguished from one another by physical properties, such as:
layering, size of grains or crystals, texture
These volcanic products cover 70% of Earth's surface.
mid-ocean ridge volcanism
"Wet" igneous rock melts at a lower temperature than the "dry" version of that same rock because:
the addition of volatiles in "wet" rocks lowers their melting temperature.
How do temperature and pressure change with increasing depth in the Earth?
Both temperature and pressure steadily increase with increasing depth.
Felsic (or silicic) magma
1) Is likely to form light tan, pink, or maroon rocks. 2) Crystallizes at the lowest temperatures. 3) Is more viscous than mafic magma. 4) Has about 70% silica and little magnesium and iron.
An igneous rock has a medium-gray, fine-grained groundmass with large crystals of plagioclase. What is true about it?
1) It began to crystallize at depth from intermediate-composition magma, then rose to the surface and became a lava flow. 2) The plagioclase crystals are called phenocrysts. 3) It could logically be porphyritic andesite.
What are the characteristics of a rock?
1) It is a coherent mass. 2) It is a naturally occurring material. 3) It is a collection of minerals or a body of glass
Define organic chemical:
A carbon-containing compound that makes up the majority of living matter.
Describe the two different kinds of chert. How are they similar? How are they different?
All chert is composed of microcrystalline (cryptocrystalline) quartz. 1) Biochemical chert is derived from the siliceous skeletons of microorganisms which deposit in vast layers on the sea floor after the death of the organisms. 2) Chert also occurs as a replacement mineral; groundwater may dissolve portions of limestone (when groundwater is undersaturated with respect to calcite) and fill in the resultant void space with microcrystalline quartz (with respect to which the groundwater is saturated). Bedding (layering) is absent in replacement chert. Ch. 6
The North Atlantic Ocean is 3,600 km wide. Seafloor spreading along the MidAtlantic Ridge occurs at 2 cm per year. When did rifting start to open the Atlantic?
Assuming a constant rate of seafloor spreading during the interval, the Atlantic started to open up approximately 180 million years ago. Ch. 2
Why does metamorphism happen at the site of meteor impacts and along mid-ocean ridges?
At a meteor impact site, pressure on minerals rises sharply at the time of impact, producing conditions favorable for new minerals that would not otherwise be present. Mid-ocean ridge settings provide ample opportunity for relatively cool water to interact with hot, recently formed rock. Ch. 7
Define C Horizon:
Consists of mineral derived from substrate
Why do soils develop distinct horizons?
Different levels within soil vary in moisture content and amount of organic matter. They are further differentiated by rainwater either dissolving or precipitating minerals locally. IB
What is the difference between a sill and a dike, and how to both differ from a pluton?
Dikes are intrusions that cut across layered rocks; sills are intrusions that form parallel to them. Both dikes and sills are tabular in shape; plutons are blob-shaped. Ch. 4
How does dolostone differ from limestone, and how does dolostone form?
Dolostone is composed of the mineral dolomite, (Ca, Mg)CO3, whereas limestone is generally composed of calcite, CaCO3. Dolostone contains more magnesium (a trace impurity in natural calcite) and has a different crystalline structure. Dolomite can form through chemical alteration of limestone by reaction with groundwater bearing magnesium. Ch. 6
Why is Earth round?
Gravity forces objects the size of Earth to be nearly spherical (the most compact shape, minimizing the distance of points from the center).
Why does melting take place beneath the axis of a mid-ocean ridge?
Hot asthenosphere rises to take the place of material that has spread to either side of the axis. Closer to the surface, pressure is less than at the depth from which the asthenospheric material rose, so the asthenosphere partially melts. Ch. 4
Explain why soil erosion has been exacerbated by human activity.
Humans have deforested land and tilled soil to produce agricultural output, and these activities (plus grazing by livestock) have led to increased soil erosion. IB
Define B Horizon:
Ions and clay accumulate in this horizon or subsoil.
Why do magmas rise from depth to the surface of Earth?
Magmas rise because they are less dense than the rocks that surround them, and at depth, there is great pressure that squeezes magma upward, where the pressure is less.
Recent exploration of Mars by robotic vehicles suggests that layers of sedimentary rock cover portions of the planet's surface. On the basis of examining images of these layers, some researchers claim that the layers contain cross bedding and relicts of gypsum crystals. At face value, what do these features suggest about depositional environments on Mars in the past?
Mars once had surface water, including streams and lakes or seas subject to evaporation. Ch. 6
How do geologists predict volcanic eruptions?
Measured increases in heat flow, changes in the shape of volcanoes, and increasing incidence of earthquakes and gaseous emissions may signal that an eruption is imminent. Ch. 5
Would you expect felsic pyroclastic flows to erupt from a mid-ocean ridge? Explain the reasoning that led you to your answer.
No. At the mid-ocean ridges, basaltic melts emerge quite rapidly from depth. They do not have sufficient time to undergo substantial chemical change through fractional crystallization, and there are no felsic rocks nearby that might be melted and added to the magma bodies. Ch. 4
Once formed, does a rock necessarily last for all of Earth's history?
No. Earth is an active planet with plate tectonics, surface water, and an atmosphere. Uplift, weathering, heat, stress, and melting destroy rocks and allow Earth materials to be reused in the formation of new rocks. IC
Are all stars that we see today considered to be first-generation stars? What is the evidence for your answer?
No. Many observed stars are too enriched in heavier elements to be first-generation stars
Have all rocks on Earth passed through the rock cycle the same number of times? Explain your answer.
No. Rocks buried to critical depths within the continents may be protected from both surface weathering and heating from below for long spans of geologic time. IC
Volcanic arcs do not occur where?
On divergent plate boundaries.
Explain the difference between physical and chemical weathering.
Physical weathering involves processes that mechanically break rock into fragments. Chemical weathering involves chemical reactions between rock and either chemical solutions in water or chemicals in the air. IB
Define A Horizon:
Plant debris have decayed further and have mixed with clay, silt, and sand Water percolating causes further chemcial reactions to occur, yielding ions in solution and new clay minerals
How do we identify a plate boundary?
Plate boundaries are marked by linear or arc-like segments of relatively high earthquake frequency (earthquake belts).
What does the mixture of grain sizes in a porphyritic igneous rock indicate about its cooling history?
Porphyritic igneous rocks occur when an initial phase of gradual cooling within the Earth gives way to much more rapid cooling subsequent to eruption. Ch. 4
What is the core composed of? How do the inner and outer cores differ? Which one produces the magnetic field?
The core is mostly iron; the inner part is solid, whereas the outer part is liquid. Circulation of iron atoms in the liquid outer core generates Earth's magnetic field.
Define exhumation:
The overall process by which deeply buried rocks end up back at the surface.
Metamorphic aureoles typically contain nonfoliated rock like hornfels because:
They form adjacent to an intruding pluton, which provides heat for metamorphism.
Define Regression:
When sea level falls and the coast moves seaward.
The Moho is the boundary between Earth's
crust and mantle.
Natural cracks in rock that form due to decompression as crust rises to the Earth's surface are called:
joints.
The genetic scheme for classifying rocks is based on:
origin of formation
A sedimentary basin that forms when continental lithosphere is stretched, causing down-dropped crustal blocks bordered by elongate mountain ranges, is called a:
rift basin.
Describe the steps in the formation of our Solar System according to the nebular theory.
1. The mass in our Sun and surrounding solar system condensed from a nebula (cloud of gas and dust) 2.At center of nebula, most of the mass condensed to form the Sun. 3.Sun got big enough (protostar) and hot enough to fuse H 4.Within a flat protoplanetary disk surrounding the Sun, planets arose from gravity-driven accretion and the collisions of smaller bodies termed planetesimals and protoplanets. 5. As sun got hotter, Light gases and other volatiles were ejected from the inner portion of the disk as so the terrestrial planets ended up as smaller spheres of (rock and metal). 6. Gas-giant planets incorporated abundant volatiles such as hydrogen and helium to become much more massive but less dense.
Describe the three types of plate boundaries.
: Divergent-plate boundaries exist where lithosphere on either side moves away from the boundary. At convergent-plate boundaries, lithosphere on either side comes together, bringing either subduction (if oceanic lithosphere is involved) or collision (of two continental plates). At transform-plate boundaries, plates slide past each other.
What is the principal basis that geologists use to classify rocks into three classes? What are these classes?
:Geologists divide rocks into three classes on the basis of the way in which the rock forms. The three classes of rock are igneous, metamorphic, and sedimentary. IA
Explain the difference between a clastic and crystalline rock.
A clastic rock is composed of grains held together by cement; crystalline rocks holds together due to mineral crystals mutually interlocking. IA
What is a large igneous province (LIP)?
A large igneous province (LIP) is a region in the crust that contains an unusually large volume of igneous rock. Ch. 4
How is a hot-spot track produced, and how can hot-spot tracks be used to track the past motions of a plate?
A large volume of very hot rock from within the mantle rises at the hot spot and produces abundant magma, yielding copious volcanism to form the hot spot. Hot spots are relatively stable points, whereas the plates that overlie them, and that bear the associated volcanoes, are moving. Over periods of millions of years, as a plate slides over the hot spot, extinct volcanoes are ferried in the direction of plate motion, while new volcanoes are formed at the hot spot
Define Volatiles:
A material that easily transforms into gas at relativley low temperatures found at the Earth's surface.
What is a metamorphic grade, and how can it be determined? How does grade differ from facies?
A metamorphic grade refers to a series of temperature and (to a lesser extent) pressure regimes under which metamorphism takes place. For example, high-grade metamorphism occurs under greater temperatures (and pressures) than does low-grade metamorphism. Metamorphic grade is usually assessed on the basis of the mineral assemblage making up the metamorphic rock, as well as its foliation and other textural clues (such as grain size). Facies is a more precise term used for a restricted range of temperatures and pressures defined by the presence of key minerals. Ch. 7
What is a mineral, as geologists understand the term? How is this definition different from the everyday usage of the word?
A mineral is a naturally occurring solid, formed by geologic processes, that has a crystalline structure and a definable chemical composition. Ch. 3
What is the geologist's definition of the term rock? Can a brick be considered to be a rock? Explain your answer.
A rock is a coherent, naturally occurring solid that consists of an aggregate of minerals or a body of glass. Bricks are not rocks under this definition because they do not occur naturally. IA
What is a silicon-oxygen tetrahedron? What is the anionic group that occurs in carbonate minerals?
A silicon-oxygen tetrahedron is a pyramid-shaped structure consisting of a silicon atom surrounded by four oxygen atoms; it is an anionic group with a 4- charge. The anionic group in carbonate minerals is CO2-(superscript)3 (subscript). Ch. 3
Define Metals:
A solid composed of metal atoms.
Define Glasses:
A solid in which atoms are not arranged in an orderly pattern.
Define Minerals:
A solid, natural substance in which atoms are arranged in an orderly pattern.
What is a triple junction?
A triple junction is a point at which three plate boundaries meet.
Explain how biochemical sedimentary rocks form.
A variety of organisms in marine environments produce mineralized skeletons for their protection. When these organisms die, their skeletons often become fractured into grains by the activities of waves, currents, predators, and scavengers. Pieces of skeletal debris may accumulate on the sea floor, later to become buried and lithified through compaction and cementation. Ch. 6
If you look at the Moon, the surface appears relatively darker and smoother. These areas are individually called mare(plural: maria). Plains of igneous rock formed after huge meteors struck the Moon and formed very deep craters. These impacts occurred early in the history of the Moon, when its interior was warmer. Propose a cause for the igneous activity, and suggest the type of igneous rock that fills the mare.
Although thoroughly solid today, the Moon once had enough internal heat (from initial formation through impact, continued bombardment early in its history, and radioactive isotopes) to form melts. These melts erupted at the surface in places and cooled quickly, producing fine-grained extrusive rock. The broad area encompassed by each mare suggests that they formed from lava with low viscosity and are thus basalts (confirmed by looking at lunar rock samples). Ch. 4
Define Sediments:
An accumulation of unattached mineral grains
Define Rocks:
An aggregate of mineral crystals, grains, or a mass of natural glass.
Explain the difference between an equant and an inequant grain.
An equant grain is approximately the same linear size in all three spatial dimensions. An inequant grain has a least one dimension that is of unequal length to another. IA
How does an oceanic hot-spot volcano differ from a continental one?
An oceanic hot spot volcano consists of a vast shield composed of basaltic lava flows. A continental hot spot volcano will produce more felsic melts and more pyroclastic debris, occasionally erupting with great force to form a caldera. Ch. 5
Give examples of different kinds of rock outcrops. Can you find outcrops everywhere? Explain your answer.
An outcrop might occur as rock extending upward in the middle of a field, a ledge against the side of a cliff, a stream cut, or a roadcut or other human-made excavation. Outcrops are not found everywhere; in many places, rock is covered by sediment, soil, water, or urban development. IA
Describe the way magmas are produced in subduction zones.
Below 150 km, heat causes volatiles within the subducting oceanic, lithospheric slab to be released into the ultramafic asthenosphere of the overriding plate. Partial melting of the asthenosphere produces basaltic magma, which migrates upward through the lithosphere of the overriding plate to erupt at the surface. Depending on the amount of fractional crystallization that occurs during transit, the originally basaltic (mafic) magma may extrude as an intermediate or a rhyolitic (silicic) melt. Ch. 4
How does plate tectonics explain the peculiar combination of low-temperature but high-pressure minerals found in a blueschist?
Blueschists form at the base of thick accretionary prisms, sediments scraped off of the downgoing slab at subduction zones. Because the subducting slab is relatively cool, it adds little heat to the prism, allowing for the relatively high pressures but low temperatures in which the blueschist mineral assemblage is stable. Ch. 7
Compare the chemical formula of magnetite with that of biotite. Considering that iron is a relatively heavy element, which mineral has the greater specific gravity?
Both contain iron and oxygen, but biotite also contains a variety of other, lighter elements, including silicon, magnesium, and hydrogen. Iron is a far more abundant component of magnetite than it is of biotite. Thus, magnetite has a greater specific gravity than biotite. Ch. 3
Which minerals react with acid to produce CO2?
Carbonate minerals (such as calcite) react with acids to release CO2. Ch. 3
Why are there so many different compositions of magmas?
Changes in chemistry = fractional crystallization, partial melting, and contamination through the melting and assimilation of surrounding host rock. Melts also vary with respect to volatile content. Ch. 4
Describe cinder cones
Cinder cones are radially symmetric tephra, with steeper slopes on the sides. Cinder cones form from fountains of lava, which squirt up and freeze in midair close to the volcanic vent. Pyroclastic tephra solidifies before reaching the ground and consequently cannot flow as basaltic lava will. Ch. 5
How do you distinguish cleavage surfaces from crystal faces on a mineral? How does each type of surface form?
Cleavage planes occur in parallel sets. Cleavage surfaces arise due to planes of weak bonding within the crystal and can be seen because of crystal breakage (or near breakage). Crystal faces are solitary, occurring only at the surface of the crystal. Crystal faces are formed by the growth of the crystal. Ch. 3
Why are the giant planets, which contain abundant gas and ice, farther from the Sun?
Close to the Sun, solar wind was great enough to drive off the great volumes of light gases and volatiles that are necessary for giant planets to form.
Describe the characteristics of a continental rift, and give examples of where this process is occurring today.
Continental rifts appear as elongate valleys bounded on either side by faults. Volcanism occurs along the rift as asthenosphere rises to accommodate the thinning lithosphere and melts. Rifts can be found in East Africa and in the Great Basin of the western United States
Describe the process of continental collision, and give examples of where this process has occurred.
Continental rock is not dense enough to subduct beneath an overriding, opposed continental plate and will thus collide with it, suturing itself with the adjacent plate, folding the rocks in the zone of collision, and thickening the crust locally to form a nonvolcanic mountain range. The Appalachians and Himalayas are examples of mountain ranges that formed from this process of collision
What are cross beds, and how do they form? How can you use cross beds to read the current direction?
Cross beds form from sediment deposited on the downcurrent side of dunes and ripples. These angled beds dip downward in the downcurrent direction (parallel to the lee side of the dune or ripple). Ch. 6
Mount Fuji is a 3.6 km-high stratovolcano in Japan formed as a consequence of subduction. You can see the volcano contains volcanic rocks with a range of compositions, including some andesitic rocks. Why do andesites erupt here? Very little andesite occurs on the Marianas Islands, which are also subduction-related volcanoes. Why?
Japan is not a classic volcanic island arc (rising above sea level solely due to eruptions triggered by subduction of one oceanic plate beneath another), but it represents a fragment of the Asian mainland that rifted and drifted eastward. Japan possesses characteristics of both continental and island arcs. As with continental arcs, magma from subduction-zone melting migrates upward through a thick continental sequence of intermediate to felsic rock and has opportunity to chemically evolve through fractional crystallization and assimilation. Magma beneath the Marianas Islands more rapidly migrates through a thin layer of mafic (basaltic) crust and is not as chemically altered by the time it erupts. Ch. 5
How does grain size reflect the cooling time of a magma?
Large, thermodynamically stable crystals form when a melt cools and crystallizes gradually. Rapidly cooling and crystallizing melts do not leave sufficient time for large crystals to become organized. Ch. 4
Identify some of the major volcanic hazards, and explain how they develop.
Lava flows, poisonous volcanic gases, and pyroclastic ash and lapilli are the immediate products of eruption. Ash fall occurs due to the settling of fine pyroclastics out of the atmosphere. Pyroclastic flow occurs when pyroclastics are turbulently mixed with air and swirl down the slopes of a volcano due to gravity. Lahars occur when a pyroclastic flow obtains moisture from snow near the summit or stream water. Movement of magma generates earthquakes, which may trigger landslides; in coastal settings, a landslide or earthquake might generate a tsunami. Ch. 5
What are two examples of layering that occur in rock?
Layering includes sedimentary bedding and metamorphic foliation. IA
Explain the process of fractional crystallization.
Magmas cool gradually within the Earth, and a small portion of the magma will start to solidify, forming crystals. The first crystals to form will be minerals that have a high temperature of crystallization. These tend to be iron- and magnesium-rich minerals that characterize relatively mafic rocks. The fractional crystallization process drives the chemistry of magmas to become consistently more felsic. Ch. 4
What factors control the cooling time of a magma within the crust?
Magmas that cool deep within Earth cool more slowly than those that cool near the surface. Large, globular bodies of magma cool more slowly than those that are smaller and sheet-like. Cool groundwater can absorb heat from magma and transport the heat away from the source, greatly increasing the cooling rate. Hot, circulating groundwater can have the opposite effect. Ch. 4
What are the principal layers of Earth? What happens to earthquake waves when they reach the boundary between layers?
Major layers of Earth are the very thin crust, the rocky mantle, and metallic core. Earthquake waves reaching boundary layers between these layers may be reflected or refracted (bent)
Explain how steps can be taken to protect people from the effects of eruptions.
Maps assessing volcanic hazards can be compiled and publicized. Plans for emergency evacuation can be developed. The direction of flowing lava can sometimes be diverted. Citizens can be educated concerning the hazards of living near volcanic summits and can receive incentives for relocating if practical. Ch. 5
How do grain size and shape, sorting, sphericity, and angularity change as sediments move downstream?
Mechanical forces such as tumbling and abrasion wear on sediments as they are transported downstream. Angular protuberances are especially likely to be broken off. As a result, grain size decreases, with grains becoming more spherical and more rounded (less angular). The speed at which the water in a stream is traveling decreases along its course, and the capacity of the stream to carry sediment is directly related to its rate of flow. So larger grains are deposited upstream from finer grains (grains become sorted as they travel downstream). Ch. 6
How are metamorphic rocks different from igneous and sedimentary rocks?
Metamorphic rocks are the result of heat and stress causing an alteration of texture, mineralogy, or both within a pre-existing rock, without the rock having undergone melting. Many metamorphic (but no igneous or sedimentary) rocks possess foliation. Ch. 7
How does slate differ from phyllite? How does phyllite differ from schist? How does schist differ from gneiss?
Slate and its characteristic slaty cleavage arise from the preferred orientation of clay minerals resulting from the relatively low-temperature and low-pressure metamorphism of a body of shale. Phyllite arises when significantly higher temperatures and pressures cause clay grains within slate to be recrystallized to form mica grains, which retain a preferred orientation. Unlike slate, which is rather dull, mica gives phyllite a silky luster. Schist differs from phyllite in that, as a result of greater heat and pressure, its mica grains are large, visible discrete plates, unlike the smooth sheen of tiny mica grains within phyllite. Gneiss is compositionally banded, with alternating bands or swirls of light- and dark-colored minerals, including additional minerals besides mica (quartz, feldspar, amphibole). Ch. 7
Define E Horizon:
Soil level has undergone substantial leaching, but has not yet mixed with organic material.
Explain the process of soil formation.
Soils form when surficial sediments are altered by the activities of organisms and chemical interaction with rainwater. IB
Describe Stratovolcanoes
Stratovolcanoes are large, conic volcanoes. The walls of stratovolcanoes are composed of alternating layers of tephra and lava rock. Stratovolcanoes are the result of multiple eruptions, which in turn have produced both effusive lava and explosive pyroclastics, whereas the smaller cinder cones represent single volcanic events. The high viscosity of andesitic lava yields the steep slope of stratovolcanoes. Ch. 5
What is the Big Bang, and when did it occur?
The Big Bang is an explosive phase of expansion of matter and space that occurred at the beginning of our Universe, 13.8 billion years ago.
Why are the marine magnetic anomalies bordering the East Pacific Rise in the Pacific Ocean wider than those bordering the Mid-Atlantic Ridge?
The East Pacific Rise is spreading faster, so it produces a greater width of basalt in the time intervals between polarity reversals.
What is the Moho? Describe the difference between continental crust and oceanic crust?
The Moho is the crust-mantle boundary, recognized by an abrupt change in seismic-wave velocities. Continental crust is thicker, more silicic, and more variable in chemistry than oceanic crust.
Describe how the Moon was formed
The Moon formed when a protoplanet approximately the size of Mars collided with Earth early in the history of the Solar System. The force of the impact ejected material similar in composition to Earth's mantle. This mantle-like mass cooled and solidified, resulting in our Moon.
Why doesn't weathering take place on the Moon?
The Moon has no atmosphere and no surface water, so there is no weathering on the Moon. IB
Is there a rock cycle on the Moon? Why or why not?
The Moon has no atmosphere, no surface water, and no great internal heat, so there is no lunar weathering, no plate tectonics, and no active igneous processes. Thus, there is currently no rock cycle on the Moon. IC
Describe the pattern of marine magnetic anomalies across a mid-ocean ridge. How is this pattern explained?
The alternating sequence of positive and negative anomalies continues in either direction outward from the ridge, forming a pattern that possesses mirror-image symmetry about the ridge axis. Negative anomalies are derived from bodies of rock that crystallized and cooled during times when Earth's magnetic field had a polarity opposite to that of today. The marine magnetic anomalies are produced by the combination of seafloor spreading through time with magnetic polarity reversals through time.
How do apparent polar-wander paths show that the continents have moved?
The apparent polar-wander paths for two different continents do not agree concerning the position of the ancient magnetic north pole. Although it is possible for the magnetic pole to wander, it is impossible for it to be in two places at the same time. Therefore, the discrepancy must be explained as a consequence of drifting continents.
The Gulf Coast of the United States is a passive-margin basin that contains a very thick accumulation of sediment. Drilling reveals that the base of the sedimentary succession in this basin consists of redbeds. These are overlaid by a thick layer of evaporite. The evaporite, in turn, is overlaid by deposits composed of sandstone and shale. In some intervals, sandstone occurs in channels and contains ripple marks, and the shale contains mudcracks. In other intervals, the sandstone and shale contain fossils of marine organisms. Be a sedimentary detective, and explain the succession of sediment in the basin.
The redbeds suggest deposition in a subaerial, well-oxygenated environment. The continent was invaded by shallow seas that readily evaporated in the hot, arid climate to yield the thick evaporite sequence. A series of marine transgressions (leading to deposition of the units with marine fossils) and regressions (yielding the cross-bedded channel sandstones and shales bearing mudcracks) followed. The lack of arkose and conglomerate in the sequence suggests that at no time during deposition did any collisional orogenic events take place. Ch. 6
Define the rock cycle, and give three examples of pathways through it.
The rock cycle is the name given to the transformation of Earth materials from one rock to another over time. Any rock from any of the three classes (igneous, sedimentary, metamorphic) may be altered to form a new rock from the same class or a new rock from a different class. 1) an igneous rock being uplifted, weathered to form sediment, and its sediments eroded, deposited, buried, and cemented to form sedimentary rock that is later metamorphosed. 2) a metamorphic rock that is melted, with the melt solidifying later to form an igneous rock that is subsequently uplifted and weathered, with the sediment formed eroded, transported, buried, and cemented to form a sedimentary rock. 3)a sedimentary rock that is melted, with the melt solidifying to form an igneous rock that is subsequently metamorphosed. IC
Recent observations suggest that the Moon has a very small, solid core that is less than 3% of its mass. In comparison, Earth's core is about 33% of its mass. Explain why this difference might exist.
The theory of Moon formation. A small protoplanet collided with Earth early in its history. The core of the impactor sank into the Earth, contributing to Earth's large core, and the Moon formed from ejected material that was mostly mantle-like in composition. As the Moon cooled, only a small amount of iron was available to sink and form a lunar core.
What process in the mantle may be responsible for causing hot-spot volcanoes to form?
These volcanoes may form when an isolated, cylindrical plume of hot material from the lower mantle rises upward. When it reaches the lithosphere, pressure is low enough to initiate partial melting, forming basaltic (mafic) melt from the ultramafic rock. The melt rises farther to reside in a magma chamber in the crust until it erupts, forming a volcano. Ch. 4
What are thin sections, how are they examined, and what do they allow you to see?
Thin sections are slices of rock that are 0.03 mm thick. They are examined with a petrographic microscope, and they allow you to see minute details of texture and mineral composition of the rock. IA
Do you think that you would be likely to find a long belt, hundreds of kilometers across and thousands of kilometers long, in which the outcrop consists of high-grade hornfels? Why or why not?
This scenario is unlikely; hornfels typically forms through thermal (contact) metamorphism, as an altered rind (aureole) of former shale that surrounded a cooling intrusion. These aureoles are never hundreds of kilometers wide. In the broader-scale regional (dynamothermal) setting, shale would be altered to form a variety of foliated rocks, such as slate, phyllite, schist, and gneiss. Ch. 7
Why are transform-plate boundaries required on an Earth with spreading and subducting plate boundaries?
Transform-plate boundaries are necessary to connect either a pair of ridge segments producing seafloor spreading in opposite directions, or a pair of trenches with opposite directions of subduction, or a ridge to a trench.
Describe how a turbidity current forms and moves. How does it produce graded bedding?
Turbidity currents form when sediment becomes unstable on a subaqueous slope and tumbles downward, pulling a current of water with it. After a while, the velocity of the turbidity current slows. The heaviest (largest) particles settle out first, whereas smaller, lighter particles stay in suspension for longer periods. Ultimately, these smaller particles settle atop the coarser grains to produce graded bedding, a grain-sized gradient from coarse (near the base of the bed) to fine (at the top of the bed). Ch. 6