Geology test 3

What is meant by hot-spot volcanism?

...A concentration of heat in the mantle, capable of producing magma that, in turn, extrudes onto Earth's surface. The intraplate volcanism that produced the Hawaiin islands is one example.

Law of superposition-

...The law simply states that in an underformed sequemce of sedimentary rocks, each bed is older than the one above and younger than the one below.

What are the four changes in a volcanic area that are monitored in order to detect the movement of magma?

1) Changes in the pattern of volcanic earthquakes 2) expansion of a near-surface magma chamber which leads to inflation of the volcano 3) changes in the amount and/or composition of the gases that are released from a volcano 4)an increase in ground temperature caused by the implacement of new magma

To provide a reliable radiometric date a mineral must remain in a closed system from the time of its formation until the present. Why is this true?

A correct date is not possible unless there was neither the addition nor loss of parent or daughter isotopes.

What is a tsunami? How is one generated?

A tsunami is a seismic sea wave formed by the displacement of the ocean floor during an earthquake.

Distinguish between anticlines and synclines ,domes and basins, anticlines and domes.

Anticlines are folds with two, well-defined limbs dipping in opposite directions away from a long, linear, fold axis. Strata are raised or buckled upward along the axial part of the fold relative to their elevations farther out on the limbs; thus after erosion, older strata are exposed along the axial part of the fold. Synclines are folds with two, well-defined limbs that dip inward toward a long, linear, fold axis. Strata are lowered or buckled down in the axial region; thus after erosion, younger strata are exposed in the axial portions of synclines. Domes are more or less circular zones of upraised rocks in which the beds follow the geometry of a dome and dip away in all directions from a high point or apex; unlike an anticline, the dome structure does not have an axis. Geometrically, a basin may be thought of as an inverted dome. The strata dip inward in all directions toward the central, most downbuckled point in the structure. Anticlines and synclines have long, roughly parallel limbs and linear axes. Limbs of domes and basins make circular outcrop patterns, and the crests of domes and the lowest parts of basins are points, not axes. It is very important to distinguish between structural and topographic basins and domes

Why is radiometric dating the most reliable method of dating in the geologic past?

Because the rates of decay for many isotopes have been precisely measured and do not vary under the physical conditions that exist in Earth's outer layers.

Contrast the movements that occur along normal and reverse faults. What type of stress is indicated by each fault.

Both are dip-slip movements in which one block moves up and the other down along the fault surface. Assume that dip-slip faults with vertical dips (the fault surface is vertical) are normal faults. For dip-slip faults with inclinations or dips other than vertical, the hangingwall-footwall designation is very useful. The hangingwall block is the block that is entirely above the fault surface, and the footwall block is entirely below. In normal fault movement, the hangingwall block slides down along the fault surface with respect to the footwall block. Horizontal distances between points in the blocks are increased (stretched) and the stresses are tensional. In reverse fault movement, the hangingwall block slides upward along the fault surface with respect to the footwall block. Horizontal distances between points in the two blocks are decreased (shortened) and the stresses are compressional.

Extensive pyroclastic flow deposits are associated with which volcanic structures?

Calderas.

What type of plate boundary is associated with the ring of fire?

Convergent

Compare a volcanic crater to a caldera.

Craters are formed by gradual accumulation of volcanic debris on the surrounding rim. Calderas are formed by a a collapse or ejection of the summit area of a volcanoe.

At which type of plate boundary is the greatest quantity of magma generated?

Divergent

How is brittle deformation different from ductile deformation?

Ductile deformation, also known as plastic deformation, occurs when a material accumulates strain that is not reversible. Plastic deformation takes the form of a permanent change in the size and/or shape of a rock that has passed its elastic limit. Fracture occurs when material that is accumulated strain, or deforming, finally break. This stage is termed brittle deformation. Of course, like ductile strain, brittle deformation is permanent.

How are joints different from faults?

Faults and joints are both fractures in rock. Along faults, the fracture-bounded blocks have been displaced (offset) from their unfractured positions; the blocks are not significantly displaced along joints. Joints typically come in sets; a joint set is a group of fractures in a given area that more-or-less exhibit a common orientation (strike and dip). Multiple joint sets may be present in any given area. Joints usually exhibit a strong control over differential weathering and erosion. The fractures, being zones of weakness and accessible to water, weather and erode faster than stronger, unfractured bedrock.

In addition to the destruction created directly by seismic vibrations, list 3 other types of destruction associated with earthquakes.

Fire, landslides and ground subsidence, and seismic sea waves (tsunamis) are all capable of adding to the destructive nature of earthquakes.

List three rock structures that are associated with deformation.

Folds, faults, and joints.

Why is a volcano fed by highly viscous magma likely to be a greater threat to life and property than a volcano supplied with very fluid magma?

Highly viscous magma may produce explosive clouds of hot ash and gases that evolve into buoyant plumes called eruption columns, because of the high viscosity of the silica-rich magma, a significant portion of the volatiles remain dissolved until the magma reaches a shallow depth, where tiny bubbles begin to form and grow. The bubbles and pressure create explosive reactions. But, when fluid lava erupts, the pressurized gases escape with relative ease.

Briefly describe the difficulties in assigning numerical dates to layers of sedimentary rocks.

In general, sedimentary rocks do not contain minerals that are both suitable for dating and that crystallized when the bed was deposited. One exception would be feldspar or mica grains in volcanic ash deposited at the time of the eruption. Minerals such as glauconite crystallize as sedimentary grains but contain large quantities of nonradiogenic daughter element, making an age determination imprecise. In recent years, advances in instrumentation and the application of new geochronological methods have led to much more precision and accuracy in dating of sedimentary rocks. For example, extensive, detailed micropaleontological data and very precise Sr-87/Sr-86 measurements in Mesozoic and Cenozoic marine limestones have been correlated, resulting in very precise age assignments for many marine carbonate strata. In addition, paleomagnetic measurements combined with the known geomagnetic time scale and paleontological data can often result in very precise age assignments for some strata.

Distinguish between the Mercalli and the Richter scale.

In seismology (the study of seismic waves or vibrations in the ground / earthquakes) the Richter scale is a magnitude scale. Magnitude scales show the amount of energy released by an earthquake. In the case of the Richter magnitude this is based on the amplitude of the seismic waves measured on a seismometer and the distance of the seismometer station from the epicentre of the earthquake. The magnitude value should be the same for a given earthquake no matter where it is recorded. The Mercalli scale is an intensity scale. This gives information on the perceived ground shaking in a specific location. The value is derived from witness reports on the perceived violence of the shaking, from the damage done to buildings and other infrastructure and also based on measurements from accelerometers and other seismic recording instruments in order to make the Mercalli intensity value a less subjective parameter. The intensity and damage to infrastructure can be influenced strongly by a number of variables including local ground conditions and the use (or otherwise) of earthquake resistant design.

What is ship rock New Mexico and how did it form?

It is a volcanic neck. As erosion progresses, the rock occupying the volcanic pipe is often more resistant and may remain standing above the surrounding terrain long after most of the cone has vanished

Explain why the moment magnitude scale has gained popularity among seismologists.

Moment magnitude has gained wide acceptance among seismologists and engineers because: (1) it is the only magnitude scale that adequately estimates the size of very large earthquakes; (2) it is a measure that can be derived mathematically from the size of the rupture surface and the amount of displacement, thus it better reflects the total energy released.

Distinguish between numerical and relative dating.

Numerical or absolute dating involves the numerical age measurement in actual time units like thousands or millions of years. Relative dating involves placing sequence of rocks, geological features and events in the correct order in which they occurred, without necessarily knowing their act ages

What is an outcrop?

Outcrops are surface exposures of the local, subsurface, lithological material or bedrock. As such, they provide the basic information and data utilized in geologic mapping. Outcrops provide samples of the bedrock and exhibit those structures and features (such as stratification, cross bedding, mineral veinlets, faults, cleavage, etc.) that help a geologist interpret the geologic history of an area. In areas with extensive coverings of soil or other surficial materials (regolith, glacial deposits, landslide debris, sand dunes, etc.), information about the bedrock and subsurface lithologies may be available only through drilling.

List the major differences between P and S waves.

P waves travel through all materials, whereas S waves are propagated only through solids. Further, in all types of rock, P waves travel faster than S waves.

How is magma generated along convergent plate boundaries?

Partial melting associated with a subducting slab seems to begin at depths of about 100 km. Fluids released from the slab promote melting of hot peridotite in the overlying lithosphere. Also, materials at the top of the slab, such as sediments, hydrated volcanic rocks, and continental-rock slivers are in contact with hot, non-slab peridotite and may undergo partial melting. As the slab tip penetrates to deeper levels, upward, counter flows of hot peridotite are set in motion, resulting in decompression melting and production of basaltic magma.

Describe four natural hazards associated with volcanoes.

Pyroclastic flows-hot mixtures of gas, ash, and pumice that sometimes exceed 800 degrees celcius. Lahars- which can occur when a volcano is quiet. Mixtures of volcanic debris and water. Explosive eruptions and the rapid collapse of volcanic summits and flanks

List two conditions that improve an organisms chances of being preserved as a fossil.

Rapid burial and possession of hard parts.

Most of the very largest earthquakes occur on a zone of the globe known as _____.

Ring of Fire

Describe elastic deformation.

Rock deformation in which the rock will return to nearly its original size and shape when the stress is removed.

What is scoria? How is scoria different from pumice?

Scoria is the name applied to vesicular ejecta that is a product of basaltic magma and black to redish brown. When magmas with intermediate or felsic compositions erupt explosively, they emit ash and vesicular rock called pumice. Pumice is lighter in color and less dense than scoria and many have so many vesicles they are light enough to float.

Name a prominent volcano for each of the three types of volcanoes.

Sheild: Mauna Loa Cinder Cone: Paricutin Composite Cone: Vesuvius

Compare and contrast the three main types of volcanoes (size, composition, shape, and eruptive style).

Shield volcanoes: are produced by the accumulation of fluid, basaltic lavas and exhibit the shape of a broad, slightly domed structure, resembling a warriors shield. Most begin on the ocean floor as seamounts, a few of which grow large enough to form volcanic islands. Gentle flows. Cinder Cones: built from ejected lava fragments that take on the appearance of cinders or clinkers as they begin to harden in flight. Most of the volume of a cinder cone consists of pea to walnut sized lapilli. Mostly emit lava spews that fall back as loose pyroclastic material, they sometimes emit lava from the base not the summit. They are small and hill sized. Composite Cones: Large,nearly symmetrical structure consisting of alternating layers of explosively erupted cinders and ash interbedded with lava flows. In general,composite cones are the product of gas-rich magma having an andesitic composition. A conical shape, with a steep summit area and more gradually sloping flanks, is typical of many large composite cones. Have very viscous magma that creates very explosive eruptions. In addiction to violent eruptions they also generate very fluid mudflow called lahars.

As an earthquake measuring 7 on the Richter scale released about ____ times more energy than an earthquake with a magnitude of 6.

Thirty-two

Compare stress and strain.

Stress is the force per unit area acting on any surface within a sold. Strain is an irreversible change in the shape and size of a rock body caused by stress.

Contrast compressional and tensional stresses.

Stresses describe the external and internal forces and pressures that cause a material to undergo deformation (strain). Tensional stresses act to pull the rock apart; compressive stresses act to force rock particles inward and closer to one another. Rocks under uniaxial, compressive stress (stresses act parallel to one direction only) are shortened parallel to the stress orientation and elongated vertically (thickened). Stretching may occur in the other, right angle, lateral direction if the compressed rocks are elevated topographically and not laterally confined. Horizontal, tensional stresses oriented at right angles to parallel, vertical planes cause stretching in the direction of normals to the planes, vertical shortening (thinning), and shortening along horizontal lines parallel to the vertical planes. In both cases, predeformational angles change, demonstrating shear strain. In hydrostatic compression or tension, radial distances are shortened or elongated by the same increments; thus volume changes (dilatations) occur without changes of internal angles (no shear is involved).

With which of the three types of plate boundaries does a normal fault predominate?

Tensional faults (normal faults) dominate at divergent boundaries, and faults due to compression (reverse faults) dominate at convergent margins. Transform (sliding) plate boundaries are strike-slip faults.

The san andreas fault is an excellent example a strike slip fault called a _____.

The San Andreas fault is a well-known, strike-slip (transform) fault that forms the boundary between the North American and Pacific plates between the head of the Gulf of California and the Mendocino fracture zone north of San Francisco . Canyons in the hilly terrain to the right of the fault trace are beheaded and offset, suggesting active faulting with an important, strike-slip component. A canyon showing right-lateral displacement is clearly evident in the larger-scale photo . Linear valleys, mangled and pulverized rock of the fault zone, sag ponds, seeps and springs, offset drainages, numerous earthquakes , and juxtaposition of fundamentally different bedrock assemblages are characteristic features of active, strike-slip faults

Volcanism at divergent plate boundaries is associated with which rock type? What causes rocks to melt in these regions?

The greatest volume of magma is produced along the oceanic ridge system in association with seafloor spreading. As rock rises, it experiences a decrease in confining pressure and undergoes melting without the addition of heat.

How do the eruptions that created the Columbia plateau differ from eruptions that create large composite cones?

The greatest volume of volcanic material is extruded from fractures in the crust called fissures. Rather than building a cone, these long narrow cracks tend to emit low- viscosity basaltic lavas that blanket a wide area.

What is a hanging wall block and a foot wall block?

The hanging wall block is the rock surface immediately above a fault. A foot wall block is the surface below the fault.

List three factors that determine the nature of a volcanic eruption. What role does each one play?

The magma's composition, it's temperature, and the amount of dissolved gases it contains

What precautions are taken to ensure reliable radiometric dates?

The precautions that are taken to insure reliable radiometric dates are the use of cross checks which is subjecting a sample to two different radiometric methods. If the two dates agree, it is highly likely that the date is reliable, but if they are different additional cross checks are used.

When you observe an outcrop of steeply inclined sedimentary layers what principle allows you to assume that the bed were tilted after they were deposited?

The principle of original horizontality states that, in general, stratification in sedimentary beds was horizontal when the beds were deposited.

Explain what is meant by elastic rebound.

The sudden release of stored strain in rocks that results in movement along a fault

Describe the process of liquefaction.

The transformation of a stable soil into a fluid that is often unable to support building or other structures.

List 4 factors that effect the amount of destruction caused by seismic vibrations.

Their magnitude (the amount of energy released by the earthquake, which effects the energy of the seismic waves and the damage they can cause, whereby the larger the magnitude, the greater the energy and hence amplitude of seismic waves and the more damage they may potentially cause) The proximity of the epicentre to populated or urban areas (the closer the earthquake epicentre, the more energy will be retained by the earthquake waves and the greater the damage) The local ground conditions (seismic waves have a much lower amplitude in hard rock than in soft ground making the damage caused by seismic waves much greater in soft grounds. Also liquefaction may occur in soft ground where there is pore water which acts to significantly reduce bearing capacity and makes building collapse much more likely) The duration of the earthquake. The longer the duration, the greater the damage that may be caused. The construction techniques employed in the area and the enforcement of applicable construction codes or standards will affect how destructive an earthquake is (earthquake resistant designs will obviously reduce the damage caused by seismic waves, on the other hand, in countries or areas where poor building practices are followed - e.g. insufficient steel reinforcement in concrete, the use of cheap or poorly sourced aggregates which reduce the strength of concrete or the construction on poor founding materials without the requisite foundation engineering will all act to increase the damage caused by earthquakes).

What prominent features on the ocean floor are associated with deep focus earthquakes?

They are associated with destructive plate margins (subduction zones

What is the ring of fire?

This term refers to the strings and arcs of large, composite volcanoes that surround much of the Pacific Ocean. These volcanoes lie above subduction zones, where plates that comprise the Pacific Ocean floor are sinking beneath other oceanic plates or beneath plates carrying continents.

Explain the lack of a detailed time scale for the past span known as the precambrian period.

Three main factors are involved. Fossils are lacking or very difficult (single-celled organisms) to use for age determinations; many Precambrian rocks formed deep in the ancient crust and can be dated only by radiometric methods; and Precambrian rocks are deeply eroded and/or buried by Phanerozoic rocks. Thus compared to younger strata, Precambrian rocks are less accessible and their geologic record is much less detailed.

Are volcanoes in the ring of fire generally described as relatively quiet or violent? Name a volcano that would support your answer.

Very large, composite volcanoes (stratovolcanoes), like those around the Pacific margin, typically erupt explosively. The 1991 eruption of Pinatubo in the Philippines was the secondmost powerful eruption of the twentieth century, being surpassed only by the 1902 eruption of Santa Maria in Guatemala. The 1980 eruption of Mount St. Helens is another good example.

Describe William Smith's important contribution to geology?

William Smith's important contribution to the science of geology is he discovered that each rock formation in the canals contained fossils unlike those in the beds either above or below. Also, he realized that sedimentary strata in widely separated areas could be identified by their distinctive fossil content. He was an English engineer and canal builder during the late 1700s and early 1800s.

Describe pahoehoe and aa lava.

aa has surfaces of rough, jagged blocks with dangerously sharp edges and spiny projections. By contrast, pahoehoe flows exhibits smooth surfaces that often resemble the twisted braids of ropes.

How do volcanic bombs differ from blocks of pyroclastic debris?

blocks are made of hardened lava, and bombs are made of incandescent lava.

What is meant by the term correlation?

establishing the equivalence of rocks of similar age in different areas.

What is the source of magma for intraplate volcanism?

mantle plumes- a mass of hotter than normal mantle material that ascends toward the surface, where it may lead to igneous activity. These plumes of solid yet mobile material may originate as deep as the core- mantle boundary.

List the four factors that effect rock strength.

temperature, confining pressure, rock type, time