Earth Sciences: Textbook Questions

What are some examples of pyroclastic debris?

Pyroclastic debris includes pumice, lapilli, blocks, and bombs.

What is pyroclastic flow?

Pyroclastic flows are deadly, fast-moving clouds of volcanic ash and gas. They're usually a feature of rhyolitic volcanoes.

Why do magnetic anomalies form?

Reversals of Earth's magnetic polarity take place while sea-floor spreading occurs.

How would we know sediment was deposited in a current?

Ripple marks, dunes, and cross bedding are all consequences of deposition in a current.

How does geologic evidence point to the theory of plate tectonics?

Rock units on different continents with similar structural style line up in continuous chains if Pangaea is reconstructed.

How does Earth's surface area remain constant?

The amount of oceanic plate consumption worldwide, averaged over time, equals the amount of sea-floor spreading worldwide.

Name the classes of sedimentary rock.

The classes are clastic, biochemical, and chemical. Clastic rocks consist of cemented grains. Biochemical rocks consist of shells or shell fragments. Chemical rocks consist of minerals precipitated from solution.

What is the streak of a mineral?

The color of a mineral in powdered form.

Describe the composition of both continental and oceanic lithosphere.

The crustal part of the continental lithosphere consists largely of low-density felsic and intermediate rock. The crustal part of the oceanic lithosphere consists largely of relatively high-density mafic rock. The mantle part of both consists of very high-density ultramafic rock.

Where is the epicenter of an earthquake?

The epicenter marks the point on the Earth's surface directly above the earthquake.

What's the relationship between cooling rate and texture?

The larger the grain, the more slowly it must've cooled.

What are some examples of hazards caused by volcanoes?

The lava flows, pyroclastic debris, explosions, lahars, landslides, earthquakes, tsunamis, and gases all act as hazards.

What is layering called in sedimentary and metamorphic rocks, respectively?

The layering in sedimentary rocks is called bedding, whereas the layering in metamorphic rocks is called metamorphic foliation.

What is the lithosphere made up of?

The lithosphere consists of the crust plus the top (cooler) part of the upper mantle.

What does the lithosphere sit on top of?

The lithosphere floats on a relatively soft, or "plastic," layer called the asthenosphere.

How does the space for a pluton develop?

The magma comprising a pluton may rise as a buoyant blob, filling space produced by crustal stretching. Plutons may intrude by stoping. When blocks of wall rock fall into the magma, some dissolve but others may remain as xenoliths.

How did deep-sea drilling provide evidence of the sea-floor-spreading model?

The sediment layer got progressively thicker away from the axis and the age of the oldest sediment just above the basalt got progressively older away from the axis.

Identify the four steps in clastic sedimentary rock formation.

The steps are weathering, transportation, deposition, and lithification.

How does subduction cause melting?

The water released by the subducting plate causes the asthenosphere to melt and form magma, which can either rise directly or get trapped below.

Name two ways geologists can identify minerals.

They can examine a thin section with a petrographic microscope or use more high-tech instruments, such as an electron microprobe.

How do we classify crystalline igneous rocks?

They initially classified by colour index, which refers to the amount of ferromagnesian minerals present. Then, they're sorted by grain size.

Where do sedimentary rocks occur?

They only occur in the upper part of the crust.

Explain how the military acquired images of sea-floor bathymetry.

They used sonar, which measures water depth using sound waves. A series of bathymetric profiles were overlapped to construct maps of the sea floor.

What's the difference between magma and lava?

They're both just molten rock. We call it lava once it reaches the Earth's surface.

What kind of scheme do we use to classify rocks?

We use the genetic scheme. This means we focus on the origin of rocks.

What theory did Alfred Wegener propose?

Wegener argued that the continents were once merged into a supercontinent called Pangaea that later broke up to produce smaller continents that "drifted" apart.

How does the distribution of climatic belts support the theory of plate tectonics?

Wegener found large coal deposits, which were former tropical swamps, dating back to the late Paleozoic Era in the Canadian Arctic. This only makes sense if the area used to be equatorial.

How does a hot-spot track form?

When the overlying plate moves over a fixed plume, the movement slowly carries the volcano off the top of the plume, so that it becomes extinct.

How does paleomagnetism develop?

As lava cools, tiny magnetite crystals begin to align with the Earth's magnetic field. Once solidified, they collectively produce a measurable field.

Why is the ocean deeper over older ocean floor than younger ocean floor?

As lithosphere thickens and gets cooler and denser, it sinks down into the asthenosphere.

Describe the differences between basaltic, andesitic, and rhyolitic lava flows.

Basaltic lava has low viscosity and can flow long distances. Andesitic lava is too viscous to flow far and tends to break up as it flows. Rhyolitic lava is so viscous that it may pile up in a dome-shaped mass.

How do we know that oceanic crust is younger than continental crust based on sediment layers?

But even at its thickest, the sediment layer on top of the ocean floor is too thin to have been accumulating for the entirety of Earth history.

What's mineral cleavage?

Cleavage refers to the way some minerals break along certain lines of weakness in their structure.

How do continents move?

Continents passively move apart as the sea floor between them spreads at mid-ocean ridges, and they passively move together as the sea floor between them sinks back into the mantle at trenches.

What is crystal habit?

Crystal habit refers to the shape or character of the crystal.

What factors determine lava viscosity?

Differences in viscosity depend, in turn, on composition and temperature.

Where does the space for intrusions come from?

Dikes fill space formed when crust undergoes horizontal stretching. Sills intrude between layers and may cause the uplift of the land surface.

Explain the difference between effusive and explosive eruptions.

During an effusive eruption, lava pours out a summit vent or fissure. An explosive eruption involves immense pressure buildup.

What are earthquakes?

Earthquakes are vibrations caused by shock waves that are generated where rock breaks and suddenly slips along a fault.

Explain the two steps of lithification.

First, pressure squeezes out water and air between the grains. This is compaction. Then, cementation occurs.

How do flood and pillow basalts form?

Flood basalts form when vast quantities of low-viscosity mafic lava "floods" over the landscape and freezes into a thin sheet. Pillow basalts form due to underwater eruptions of basaltic lava.

How do the locations of past glaciations support the theory of plate tectonics?

Former glaciers appear to have moved out from the middle of the ocean, but glacial ice doesn't form in the ocean. If the continents fit together into Pangaea, the glacier moved out from the South Pole.

How does the distribution of fossils support the theory of plate tectonics?

Fossils of land-dwelling species that existed during the late Paleozoic and early Mesozoic Eras occur on multiple continents.

Compare the old interpretation of transform faults with the new one required by the sea-floor spreading hypothesis.

Fracture zones formed at the same time as the ridge axis itself, and thus the ridge consisted of separate segments to start with. These segments were linked (not offset) by fracture zones. Only the fault between the two ridge segments is active.

Is glass a mineral? Why or why not?

Glass is not crystalline. In other words, the atoms in glass are not arranged into a crystal lattice.

What's hardness?

Hardness is a measure of the relative ability of a mineral to resist scratching.

Who proposed the concept of sea-floor spreading?

Harry Hess proposed the concept.

How does oceanic crust form at a mid-ocean ridge?

Hot material rises and melts into magma, which floats above solid rock. It builds up under the crust below the ridge axis in the magma chamber. As the magma cools, it turns into a mush of crystals. This can solidify at various levels.

Name the four geologic settings where melting can occur and, therefore, igneous activity.

Igneous activity occurs along volcanic arcs bordering trenches, at hot spots, within continental rifts, and along mid-ocean ridges.

Explain how the three different types of rock are formed.

Igneous rocks are formed when molten rock solidifies. Sedimentary rocks are formed when grains are cemented together or mineral crystals are precipitated out of water solutions. Metamorphic rocks are formed when preexisting rocks change character.

What type of rock makes up all of the oceanic crust and much of the continental crust?

Igneous rocks are very common on Earth.

What happens to volcanic gas in low-viscosity magma? High-viscosity magma?

In low-viscosity magma, gas bubbles can rise faster than the magma moves. This means the volcano produces a lot of steam initially. In high-viscosity magma, the gas has trouble escaping. Explosive pressures build.

Why can't continental crust be subducted?

It is too buoyant, and cannot sink into the asthenosphere.

What does the texture of a rock refer to?

It refers to the arrangement of grains within the rock.

What did paleomagnetism show?

It showed that the continents moved relative to the Earth's magnetic poles. Each continent has a different apparent polar-wander path, which is only possible if the continents move relative to each other.

What's a mantle plume?

It's a column of very hot rock that rises through the mantle to the base of the lithosphere. Here, it partially melts and produces magma that seeps up through the lithosphere to the Earth's surface.

What's a magnetic reversal?

It's a time when the Earth's field flips from normal to reversed polarity, or vice versa.

How many plates is Earth's lithosphere divided into?

It's divided into about twenty plates.

Define magnetic inclination.

It's the angle between a magnetic field line and the surface of the Earth at a given location. It is 0° at the magnetic equator and 90° at the magnetic poles.

Define magnetic declination.

It's the angle between the direction that a compass needle points and a line of longitude at a given location.

What is the Wadati-Benioff zone?

It's the band of earthquakes in a down-going plate.

What's paleomagnetism?

It's the branch of geophysics concerned with the magnetism in rocks that was induced by the earth's magnetic field at the time of their formation.

What's an accretionary prism?

It's the mass of sedimentary material scraped off a region of oceanic crust during subduction and piled up at the edge of the overriding plate. A volcanic arc forms behind it.

What does Bowen's reaction series indicate and how was it discovered?

It's the order in which specific minerals crystallize in cooling magma. Bowen cooled a melt just enough to cause part of it to solidify. Then, he "quenched" the rest and identified the crystals that formed. This was repeated at different temperatures.

What's a chron?

It's the time interval between successive magnetic reversals.

Provide examples of physical and chemical weathering.

Jointing, frost wedging, salt wedging, root wedging, thermal expansion, and animal attack are examples of physical weathering. Dissolution, hydrolysis, hydration, and oxidation are examples of chemical reactions that cause weathering.

What's luster?

Luster refers to the way a mineral surface scatters light.

What evidence proved the sea-floor-spreading model?

Marine magnetic anomalies and the age of the seafloor proved the sea-floor-spreading model.

How do you determine the apparent polar-wander path?

Measurements of paleomagnetism in a succession of rock layers on a continent are used to plot the paleopole positions over time.

What's the difference between an active and passive continental margin?

Active margins are plate boundaries, whereas passive margins are not. Earthquakes are common at active margins.

What causes subduction?

Aged oceanic lithosphere is denser than the underlying asthenosphere and thus can sink through the asthenosphere if given an opportunity.

What is a bed? What are strata? What about formation?

A bed is a single layer of sediment. Several beds together constitute strata. A formation is a distinct sequence of strata.

What's a divergent boundary?

A boundary at which two plates move apart from each other.

What's a convergent boundary?

A boundary at which two plates move toward each other so that one plate sinks beneath the other.

What's a transform boundary?

A boundary at which two plates slide sideways past each other.

Describe the two forms of tabular intrusions.

A dike cuts across layers vertically. A sill pushes between layers of rock and is nearly horizontal.

What's a hot spot?

A hot spot is a place where volcanism may be due to melting at the top of a mantle plume.

What's a biogenic mineral?

A mineral formed in the presence of biological cells.

What is a mineral?

A mineral is a naturally occurring solid, formed by geologic processes, that has a crystalline structure and a definable chemical composition.

Define plutons and batholiths.

A pluton is a blob-shaped intrusion. Huge batholiths consist of many plutons.

What is a rock?

A rock is a coherent, naturally occurring solid, consisting of an aggregate of minerals or, less commonly, of glass.

What's a triple junction?

A triple junction marks the point where three plate boundaries join.

What's a turbidity current and what does it result in?

A turbidity current is an underwater avalanche. Larger grains settle first, followed by progressively smaller ones. This forms a graded bed.

How would you describe the characteristics of a clastic rock?

Note the clast size. Fine-grained rocks are deposited in low energy water. Note the composition, angularity, sphericity, sorting, and packing. Is it framework or matrix supported?

When measuring the magnetic field at any point on Earth's surface, the reading includes two parts. Can you identify them?

One part is produced by the main dipole of the Earth generated by the circulation of molten iron in the outer core, and another produced by the magnetism of near-surface rock.

How do volcanic arcs and seamounts differ?

Only one island at the end of a seamount remains capable of erupting volcanically today.

What's the difference between pahoehoe and a'a' lava flow?

Pahoehoe flows have smooth, ropy surfaces, whereas a'a' flows have rough, rubbly surfaces.

Describe the five common textures of igneous rocks.

Phaneritic means visible crystals. Aphanitic means microscopic crystals. Porphyritic means a variety of crystal sizes. Larger crystals are called phenocrysts, while the mass of finer crystals is called groundmass. Glassy means no crystals. Frothy means vesicles.

What's the difference between physical and chemical weathering?

Physical weathering breaks intact rock into chunks. Chemical weathering refers to chemical reactions that alter or destroy minerals when rock comes in contact with water solutions and/or air.

What defines a plate boundary?

Plate boundaries are defined by seismic belts.

What's the difference between a positive and negative anomaly?

Positive anomalies form when sea-floor rock has the same polarity as the present magnetic field. Negative anomalies form when sea-floor rock has polarity that's opposite to the present field.

What are the products of a volcanic eruption?

Products of an eruption come in three forms - lava flows, pyroclastic debris, and gas.

What's the difference between a clastic and crystalline rock?

Rocks whose grains are stuck together by cement are called clastic, whereas rocks whose crystals interlock with one another are called crystalline.

What is sediment?

Sediments are loose fragments of rocks or minerals broken off bedrock, mineral crystals that precipitate directly out of water, and shells or shell fragments.

Explain the difference between shield and composite volcanoes.

Shield volcanoes are broad, gentle domes. Composite volcanoes (stratovolcanoes) can become quite large and consist of alternating layers of pyroclastic debris and lava.

How are extrusive igneous rocks formed?

Solidification of lava, either in lava flows on the surface or as fragments cooled in the air, produces extrusive rocks.

How are intrusive igneous rocks formed?

Solidification of magma produces intrusive rocks.

What's the difference between an equant and inequant grain?

Some grains are equant, meaning that they have the same dimensions in all directions; some are inequant, meaning that the dimensions are not the same in all directions.

Describe other special properties a mineral can have.

Some minerals react with acid. Others can be magnetic or have a particular taste. Pay attention to transparency.

What's specific gravity?

Specific gravity represents the density of a mineral, as represented by the ratio between the weight of a volume of the mineral and the weight of an equal volume of water at 4°C.