Exam 4 - Deformation and Plate Tectonics

Question 1

earthquakes

Answer 1

result from faults

Question 2

geological fold

Answer 2

A geological fold occurs when one layer or a stack of originally flat and planar surfaces are bent or curved as a result of permanent deformation

Folding is the bending or warping of stratified rocks by tectonic forces (movements in the Earth’s crust). Folds can be observed on many scales, for tiny folds as can be seen in hand specimens, to larger scales as can be seen on the sides of road cuts or canyons, or very large scale such as entire mountain ranges to features that are so large they can only be seen from airplanes or satellites.

Question 3

anticline

Answer 3

anticline is a fold in layers of rock (strata) where the concave side faces down, with strata sloping downward on both sides from a common crest

Question 4

syncline

Answer 4

A syncline is a trough-shaped fold of stratified rock in which the strata slope upward from the axis; opposite of an anticline

Question 5

plunging folds

Answer 5

Plunging folds are folds (anticlines or synclines) that are tipped by tectonic forces and have a hinge line not horizontal in the axial plane

Question 6

dome

Answer 6

A dome is a deformational feature consisting of symmetrically-dipping anticlines; their general outline on a geologic map is circular or oval

Question 7

basin

Answer 7

A basin is s structural downwarp, a doubly plunging syncline ,or more typically, a downwarp filled with sediments derived from surrounding areas. The term basin is used to describe a large-scale structural formation of rock strata formed by tectonic down warping of previously flat lying strata.

Question 8

strike

Answer 8

Strike is the direction taken by a structural surface, such as a layer of rock or a fault plane, as it intersects the horizontal. Strike is measured in degrees east or west of true north.

Question 9

dip

Answer 9

Dip is the angle that a rock layer or any planar feature makes with the horizontal, measured perpendicular to the strike and in a vertical plane. Dip angles can range from 0

Question 10

joint

Answer 10

A joint is a fracture in rock where the displacement associated with the opening of the fracture is greater than the displacement due to movement in the plane of the fracture (up, down or sideways) of one side relative to the other.

Question 11

foot wall

Answer 11

A foot wall is the underlying block of a fault having an inclined fault plane.

Question 12

hanging wall

Answer 12

A hanging wall is the block (rocks) on the upper side of an inclined fault plane.

Question 13

normal fault

Answer 13

A normal fault is a fault in which the hanging wall appears to have moved downward relative to the foot wall. The dip angle of the slip surface is between 45 and 90 degrees.

Question 14

a reverse fault

Answer 14

A reverse fault is a fault in which the hanging wall has moved up relative to the foot wall

Question 15

thrust fault

Answer 15

A thrust fault is a fault with a dip angle of 45º or less over its extent on which the hanging wall appears to have moved upward relative to the foot wall

Question 16

strike-slip fault

Answer 16

A strike-slip fault is a generally vertical fault along which the two sides move horizontally past each other. If the block opposite an observer looking across the fault moves to the right, the slip style is termed “right lateral.

Question 17

oblique-slip fault

Answer 17

Oblique-slip faults are faults that display significant components of both horizontal (strike-slip) and vertical (dip-slip) motion. An oblique-slip fault combines strike-slip motion with significant normal, reverse, or thrust offset.

Question 18

stress

Answer 18

Stress is the force acting on a rock or another solid to deform it, measured in kilograms per square centimeter or pounds per square inch.

Question 19

strain

Answer 19

Strain is the amount of deformation an object experiences compared to its original size and shape.

Question 20

crustal compression

Answer 20

Crustal compression is more likely to form thrust faults and reverse faults associated with crustal shortening, and crustal compression is typically associated with regions where mountain ranges are being pushed up

Question 21

crustal tension

Answer 21

In contrast, crustal tension is more likely to form normal faults associated with crustal extension. Continental rifting and associated crustal thinning are associated with crustal extension

Question 22

brittle vs ductile deformation

Answer 22

Rocks near the surface are cold, but the temperature deep down can be extremely hot. Cool rocks near the surface tend to shatter (forming joints and faults) when they rupture. Deep underground, the weight of overlying material adds confining pressure to hold rocks together, and if hot enough they will deform fluidly rather that fracture if heat and pressure is great enough.

Question 23

earthquake terminology

Answer 23

An earthquake is ground shaking caused by a sudden movement on a fault or by volcanic disturbance.

An epicenter is the point on the Earth’s surface above the point at depth in the Earth’s crust where an earthquake begins.

The focus is the point below the Earth’s surface where seismic waves originate during an earthquake.

Question 24

earthquake fault

Answer 24

earthquake fault is an active fault that has a history of producing earthquakes or is considered to have a potential of producing damaging earthquakes on the basis of observable evidence.

Question 25

earthquake

Answer 25

An earthquake is the sudden and sometimes violent shaking of the ground as a result of movements within the crust associated with fault rupture or volcanic activity. Earthquakes are described in terms of magnitude and intensity (discussed below).

Question 26

fault creep

Answer 26

Fault creep is the gradual movement (displacement) displayed by a fault over time.

Question 27

a rupture zone

Answer 27

A rupture zone is the area through which fault movement occurred during an earthquake

Question 28

earthquake magnitude

Answer 28

A rupture zone is the area through which fault movement occurred during an earthquake

Question 29

earthquake intensity

Answer 29

Earthquake intensity (I) is a measure of ground shaking describing the local severity of an earthquake in terms of its effects on the Earth’s surface and on humans and their structures. The Modified Mercalli Intensity (MMI) scale, which uses Roman numerals, is one way scientists describe earthquake intensity

Question 30

crust

Answer 30

A crust is the outermost solid shell of a rocky planet or moon, which is physically and chemically distinct from the underlying mantle. The crust is mostly composed of mostly relatively low-density silicates minerals compared to the underlying mantle. The crust is a comparatively thin outer skin-like layer of the planet that ranges from about 2 miles (3 km) thick along the oceanic ridges to 40 miles (70 km) under some mountain belts on some continents. Gravity measurements show that the crust is separated into thin oceanic crust and thicker continental crust. In addition, oceanic crust is also slightly denser than continental crust. Oceanic crust has an average density of about 3.0 grams/cm3, whereas continental crust averages slightly less dense at 2.7 grams/cm3.

Question 31

mantle

Answer 31

A mantle is an inner layer of a terrestrial planet or other rocky body large enough to have differentiated in composition by density (about 3.3 grams/cm3).
On Earth, the mantle is a relatively viscous layer between the crust and the outer core.
• Earth’s mantle is composed of higher-density silicates rich in iron and magnesium that extends to a depth of about 1800 miles (2900 km).
• Large portions of rock in the mantle can flow slowly (like wax or plastic under pressure), and some of it is near the point of melting.

Question 32

core

Answer 32

A core is the innermost part of a planet. The Earth’s core is believed to be a magnetic iron-nickel rich sphere that consists of a 758 mile (1220 km) thick solid and very dense inner core that is overlain by 1400 miles (2250 km) of dense molten material in the outer core. The outer core is liquid, and heat convection here creates currents in the liquid metal that is believed to generate Earth’s magnetic field.

Question 33

lithosphere

Answer 33

A core is the innermost part of a planet. The Earth’s core is believed to be a magnetic iron-nickel rich sphere that consists of a 758 mile (1220 km) thick solid and very dense inner core that is overlain by 1400 miles (2250 km) of dense molten material in the outer core. The outer core is liquid, and heat convection here creates currents in the liquid metal that is believed to generate Earth’s magnetic field.

Question 34

asthenosphere

Answer 34

The term asthenosphere refers to a semi-fluid layer beneath the lithosphere (within the upper mantle), between about 60 to 400 miles (100-650 km) below the outer rigid lithosphere (oceanic and continental crust) forming part of the upper mantle (see Figure 5-3). The asthenosphere, although solid, is very hot and the pressure is great enough for material to be able to slowly flow vertically and horizontally. This enables sections of lithosphere to undergo movements associated with plate tectonics. Geologist use the term plastic to describe how hot solid materials, including rocks, can deform and flow slowly. Since rocks in the asthenosphere are hot, they will deform rather than fracture under pressure. This movement is driven by the heat derived from the deeper parts of the mantle and core that allow materials to flow by gravitational heat convection). Gravitational heat convection results when hot materials expand and rise, and cold materials contract and sink (Figure 5-4)

Question 35

mesosphere

Answer 35

This region is a rigid layer between the depths of about 400 to 1800 miles (650 km and 2900 km), but the rocks at these depths are very hot and capable of gradual flow. Heat from the core drives lower mantle convection.

Question 36

outer core

Answer 36

Geophysical studies show that Earth’s outer core is a liquid layer. Gravity and seismic studies suggest that the outer core is mostly of an mixture of metallic iron and nickel (similar in composition to metallic meteorites). Convective flow within this metallic outer core generates Earth’s magnetic field.

Question 37

inner core

Answer 37

Geophysical studies show that Earth’s inner core behaves like a solid, but is very dense, around 16 grams/cm3 (probably composed of a iron-nickel alloy similar to the composition of iron-nickel meteorites;

Question 38

magnetic reversals

Answer 38

The Earth’s magnetic field occasionally reverses, causing the locations of the north and south magnetic poles to switch. Current thought is that the magnetic reversals are caused by shifting currents in the liquid metallic outer core. Geophysical studies have shown that magnetic reversals have happened many times through geologic time. Magnetic reversals are preserved in the paleomagnetic record—a chronological record of magnetic reversals preserved as weak magnetic fields locked into rocks bearing magnetic minerals.

Question 39

seismic wave data and the composition of the earth

Answer 39

Earthquake Shadow Zones: Extensive study of shock waves of earthquakes and the global monitoring of underground nuclear bomb testing reveal information about the internal structure of the Earth. Earthquakes produce two types of shock waves: compression waves (called P waves) and shear waves (called S waves). Both S and P waves go through solids. However, S waves do not go through non-solids, so only P waves are received on the opposite side of the Earth. In addition, P waves are bent (refracted) when they cross the boundary between the solid mantle and liquid outer core. Zones of seismic wave shadows occur in the regions shown in Figure 5-9 between about 105° to 140° on the opposite side of the globe from a seismic shock. These shadow zones shows us that the Earth’s outer core is liquid (molten material). In contrast, the inner core is believed to consist of solid metal, possibly similar in composition of iron meteorites. In addition, the velocity that seismic waves travel depend on the density of the material they are passing through. The denser the material, the faster the seismic waves travel. (Earthquakes are discussed more in Chapter 6.)

Question 40

gravitational heat convection

Answer 40

Gravitational heat convection in the mantle is the source of forces that move, bend, and break rocks in the Earth’s lithosphere (Figure 5-11). Heat in the Earth is produced by radioactive decay of unstable isotopes as well as heat left over from when the Earth formed billions of years ago in the solar system’s nebula. Gravitational heat convection within the Earth is the conclusive power source driving plate tectonic motions.

Question 41

seafloor discoveries

Answer 41

Seismology has revealed important aspects of how lithospheric plates interact with each other, how plates form and are destroyed. In the 1930’s a Japanese scientist, Kiyoo Wadati, thought that deep earthquakes and volcanoes in Japan (and the Pacific Rim) could be explained by continental drift motions. Over time, as earthquake detection equipment (seismographs) were set up around the world and data collections were compiled, it became apparent that there were patterns that showed that nearly all earthquakes occurred in zones where chains of volcanoes and mountain ranges were most actively forming around the Ring of Fire, across southern Europe into east Asia, and along narrow belts beneath the oceans associated with mid-ocean ridges (Figure 5-30). Hugo Benioff (a USGS earthquake scientist) expanded on Kiyoo Wadati’s ideas and plotted the location of deep earthquakes to delineated large geologic structures associated with the Pacific’s Ring of Fire. It was recognized that earthquakes and volcanoes did not occur at random but at specific and concentrated spots on and within the Earth’s crust

Question 42

paleomagnetism

Answer 42

Paleomagnetism is the study of the fixed orientation of a rock’s magnetic minerals as originally aligned at the time of the rock’s formation. Paleomagnetism is usually the result of thermoremanent magnetization (magnetization that occurs in igneous rocks as they cool below a certain temperature (called the Curie Point).

Question 43

seafloor spreading

Answer 43

Seafloor spreading is the processes associated with the formation of new areas of oceanic crust. Seafloor spreading occurs through the upwelling of magma along mid-ocean ridges and its subsequent outward spreading movement on either side. As new rock forms along mid-ocean ridges it becomes attached to the lithospheric plates on either side of the spreading centers.

Question 44

plate tectonics theory

Answer 44

Plate Tectonics Theory helps to explains to some degree almost all things geological in the observable world, past and present. Plate tectonics expounds that Earth’s outer shell (lithosphere) is composed of several large, thin, relatively rigid plates that move relative to one another (Figures 5-28). The theory of plate tectonics helps explain the location of the world’s volcanoes and earthquakes. The theory combines elements of continental drift and seafloor spreading. The theory suggests that the lithosphere is divided into pieces, called lithospheric plates, and that denser ocean crust sinks below less-dense continental crust along subduction zones. Movements along fault systems that define lithospheric plate boundaries produce most observed earthquakes. The different kinds of plate boundaries are discussed below.

Question 45

how plate tectonics work

Answer 45

Over time, the newly formed ocean crust cooled and moved slowly away from the mid-ocean ridges (see Figure 5-27). These areas where new crust is forming and moving apart are called spreading centers. New ocean crust forms and moves away from spreading centers over time (as inferred from Figure 5-26). Since new ocean crust is forming, old crust has to be disappearing somewhere, and it turned out that the old crust was sinking back into the mantle along extensive fault zones associated with the deep ocean trenches. Earthquakes caused by friction along the subduction zone reveal that crust is slowly sinking back into the mantle. These great fault systems are called subduction zones (illustrated in two plate-tectonics models, Figures 5-29 and 5-30). Subduction zones are locations where cool and dense ocean crust sinks back into the mantle (asthenosphere). As the crust sinks it gradually heats up. Water and gases trapped in the sinking crust cause partial melting (forming magma) which rises (due to its lower density through zones of weakness in the lithosphere). Some of this rising magma accumulates in magma chambers, whereas some of it may actually rise all the way to the surface to form volcanoes.

Question 46

divergent boundaries

Answer 46

Divergent boundary (where plates are pulled apart by tensional forces)—When plates diverge, spreading centers form creating new oceanic crust. Examples include mid-ocean ridges in world’s ocean basins.

Question 47

convergent boundaries

Answer 47

Convergent boundary (where plates are pushed together by compressional forces)—When lithospheric plates collide… mountains belts form - examples include the Himalayas, Alps, and ancient Appalachian Mountains when the ancient continent of Pangaea formed. When continents collide with ocean crust… subduction zones with deep ocean trenches and volcanic arcs form - examples include the Andes Mountains, Aleutian Islands, Japan, Philippines, Indonesia, the ancient Sierra Nevada and modern Cascades Range in northern California, Oregon, and Washington.

Question 48

transform boundaries

Answer 48

where plates slide past or are rotational)—When plates slide past each other creating fault systems along plate margins. Examples include the San Andreas Fault in California and major faults in Pakistan, Turkey, and along the Jordan River/Dead Sea.

Question 49

spreading centers

Answer 49

A spreading center is a linear area where new crust forms where two crustal plates are moving apart, such as along a mid-oceanic ridge. Spreading centers are typically seismically active regions in ocean basins associated with mid-ocean ridges (MOR) and may be regions of active or frequent volcanism (Figure 5-31). Spreading centers are associated with divergent plate boundaries. The youngest rocks on the ocean floor are mostly located along mid-ocean ridges

Question 50

hotspot

Answer 50

A hotspot is a place in the upper mantle of the Earth at which extremely hot magma from the lower mantle upwells to melt through the crust usually in the interior of a tectonic plate to form a volcanic feature.

Hotspots can occur beneath any crustal type (OC or CC).

The Hawaiian Hotspot has existed for about 60 million years; the youngest part of the Emperor Seamount Chain.

Hotspots can exist in about the same place for 10’s of millions of years

There are hundred of hotspots located around the world. Some are larger and more active than others.
Most hotspots are located under the interior sections of lithospheric plates, but some occur near divergent plate boundaries.
Paleomagnetism in rocks on the ocean floor associated with hotspots provides a method for determining speed and direction of plate motions.
We are not sure of the exact mechanism that forms hotspots, there are some ideas

Question 51

continental accretion

Answer 51

Accretion is a process by which material is added to a tectonic plate or a landmass over time. This material may be sediment, volcanic arcs, seamounts or other igneous features, or blocks or pieces of continental crust split from other continental plates (Figures 5-55 and 5-56). Over geologic time (measured in many millions to billions of years), volcanic arcs form and may end up crushed onto (or between) colliding continents along plate boundaries, allowing continental land masses to grow. Pieces of continental land masses may be ripped away and carried to other locations. For instance, Baja California and parts of southern California west of the San Andreas Fault are being ripped away from the North American continent and are slowly being carried northward. These rocks may eventually pass what-is-now San Francisco, and perhaps 70 to 100 million years from now will be crushed and accreted into the landmass currently known as Alaska!

Question 52

terrane

Answer 52

In geology, the word terrane is used to describe a fragment of crustal material that formed on, or has split away from, one part of a tectonic plate and then accreted onto crust in another location or onto a different plate. Terranes are large crustal blocks (slivers of crust the size of mountains) that are bounded on the sides by great faults, usually strike-slip faults. Throughout regions like the Coast Ranges of California there are numerous terranes, and the rocks of one terrane are usually very different in composition to the terrane next to it. In the Coast Ranges, terranes are stacked up like books on a shelf, bounded by earthquake-prone faults.