Chapter 8: Deformation and Metamorphism

Which of the following are true regarding joints, fractures, folds, tilting, and shear zones?

Fault blocks created by a series of normal faults may rotate and tilt. Joints and folds are common along faults. During displacement in a shear zone, rocks may fold and stretch layers.

In which ways may a fault be recognized in a landscape?

Fault scarp Fault breccia Offset beds

What metamorphic and geologic structures are pictured in the area of New England shown?

Faulting that thrust rock up and toward the west High-grade (gneiss) metamorphic dome

Which of these minerals represents the highest grade of metamorphism?

Garnet

Based on the image and your knowledge of metamorphic minerals, which of the following is true?

Garnet is diagnostic of moderate- to high-grade metamorphic conditions.

______ is a physical process that may occur during metamorphism.

Grain and clast deformation Rotation Shearing

How may a geologist best work toward understanding the structural and metamorphic history of an area?

Making measurements of features and plotting them on a base map. Collecting samples to take back into the lab. Paying special attention to diagnostic features, such as a fault. Recording and sketching observations in a notebook.

Which of the following responses would most likely occur when rocks and minerals are exposed to the pressures, temperatures, and fluids at shallow levels in Earth's crust?

Many minerals are stable. Rocks behave brittlely. Low-temperature mineral veins may form.

If a rock is only subjected to heating and burial pressures during metamorphism (no differential stresses are applied), what may occur?

Metamorphism will occur without deformation. Rock will lack foliation or lineation. New minerals grow in random directions.

Which of the following might a geologist look for in a rock hand sample to make interpretations regarding deformation and metamorphism?

Minerals that are present Mineral chemistry Observed sequence of minerals

How do geologists determine the stability conditions of minerals buried at great depths?

Observing deep rocks that were uplifted due to tectonics and exposed Conducting laboratory experiments with high-temperature and high-pressure conditions

How might this map indicate geologic structures in the area?

Offset of layers of rock indicates a fault. Different exposures of rock at the surface may indicate rock beds below are tilted.

Which may be characteristics displayed by metamorphic rocks?

Shear zones that may displace metamorphic layering "Shiny" appearance due to orientation of mica minerals Foliation, cleavage, and layers that form platy, jagged outcrops and tabular slabs

Which of the following are true of the geologic map of New England?

Some faulting is found in the area. The most widespread rocks are Paleozoic (pinks and blues). Rock units generally trend north-south.

Which of the following correctly match the type of joint with its formation?

Tectonic stresses: vertical joints Uplift and reduced pressure: unloading joints Cooling, contracting stresses: columnar joints

Which of the following are causes of metamorphism?

Temperature increases due to hot fluids carrying material into, through, and out of rocks. Pressure increases associated with tectonic stresses. Pressure and temperature increases associated with increasing depth. Temperature increases from nearby magma bodies.

When joints are expressed in landscapes, they ______.

commonly occur in joint sets of similar orientation may be expressed as columnar joints due to cooling and contraction of igneous rocks may weather in blocks or lines to determine the appearance of rock outcrops

When a rock is buried deep in Earth and experiences the same force from all sides, it is under ______. When a rock is subjected to tectonic forces and experiences stress that is not the same in all directions, it is under ______.

confining pressure; differential stress

The type of metamorphism that occurs when rocks are heated locally and have little associated deformation is metamorphism, whereas metamorphism involves heating and deformation over large areas.

contact; regional

When tilted and folded layers erode, ______.

differential erosion of dipping layers can create a hogback erosion of soft and hard layers can carve a dip slope

The ______ of a rock structure such as a fault, is the angle the surface makes with the horizontal. The direction of a horizontal line on an inclined surface is the ______.

dip; strike

Fractures along which rock has slipped relative to the other side are called ______, whereas fractures that are simple cracks where the rock has pulled apart by a small amount are ______.

faults; joints

______ per unit area is ______.

force; stress

Fold and thrust belts ______.

form where thrust faults cut through a thick sequence of layered rocks displace older rocks over younger ones contain cleavage and joints

A hornfels is ______.

formed from metamorphic, igneous, or sedimentary rock created through metamorphism by heat from a magma nonfoliated

The type of metamorphic rock created by an igneous rock depends upon ______.

grade of metamorphism whether the rock was plutonic or volcanic composition of the original rock (mafic or felsic)

The formation of the Valley and Ridge Province in the eastern U.S. includes ______.

uplift and erosion a continental collision folding and faulting

When stress fields orient themselves so that rock may be pulled apart in a horizontal direction, ______ joints form. If stress fields are oriented so that rock may be pulled apart in a vertical direction, ______ joints form.

vertical; horizontal

Which of the following are true for horsts and grabens?

A graben is a block that is dropped down relative to blocks on either side. A horst is a block that is uplifted relative to blocks on either side. Horst and graben formation may be associated with multiple normal faults.

Which of the following do the images show?

All show differential stress. A shows compression. C shows shear stress.

Rocks deformed under low to moderate temperatures (less than 300oC) may develop a planar fabric along which they break called rock .

Blank 1: cleavage

Anything that causes a mass to accelerate is a(n) . This same property divided by the area on which it is applied is called .

Blank 1: force Blank 2: stress or pressure

The block of rock above an inclined fault is the wall, and the block of rock below the fault is the .

Blank 1: hanging Blank 2: footwall

These minerals have been smeared in a preferred orientation during ductile shearing to form a type of in the rock.

Blank 1: lineation

The type of rocks that are formed by changing previously existing rocks through increases in temperature, increases in pressure, deformation, or chemical reaction is rocks.

Blank 1: metamorphic or metamorphosed

The name of this metamorphic rock that forms when temperatures get so high that the rock partially melts is .

Blank 1: migmatite

Geologists can take a rock sample and use special equipment to determine the stability of the within it by subjecting it to high temperatures and pressures.

Blank 1: minerals

This block model represents a zone.

Blank 1: shear

This photograph shows how scientists determine the of a rock by gradually increasing the stress until the rock deforms.

Blank 1: strength

What type of metamorphism and deformation occur in a continental collision zone?

Burial and heating of rocks occur below thrust sheets. Thrusting causes shear zones to form. Rocks within thrust sheets are strongly folded.

Which of the following are true regarding the diagram of the different ways rocks respond to stress at shallow depths?

C shows shear stress. B shows tension. All show faults or fractures. All show brittle deformation in shallow levels of the crust.

Based on the image, how would cleavage and bedding be oriented in the middle hill; what type of fold is present?

Cleavage is vertical; bedding tilts from upper left to lower right; anticline is present.

Scientists must determine the strength of rocks to understand how they respond to stress. In which of the following ways do scientists go about determining the strength of a rock?

Deformation experiments performed in a laboratory setting Observations of naturally deformed rocks found in deep mines or drill holes

What types of deformation and metamorphism may occur in extensional settings?

Normal faulting and grabens Low-temperature, regional metamorphism in oceanic crust Contact metamorphism due to igneous intrusions Joints and fractures Metamorphism due to hot, circulating seawater

A gently dipping reverse fault is a(n) fault.

Blank 1: thrust

What may happen to a sedimentary rock as it metamorphoses to become metamorphic rock?

Crystals of minerals may become larger with higher grade metamorphism. Minerals present may change (e.g., mica may be created where clay existed). Rock texture may change (e.g., quartz grains may grow together).

A fold in which rock layers warp up in the shape of an A is a(n) , whereas a fold in which rocks fold down in the shape of a U is a(n) , and a fold that has layers dipping in only one direction is called a(n) .

anticline; syncline; monocline

Which of the following may be true for faults and folds?

When a fault changes dip, rocks thrust over the fault may fold, forming an anticline and syncline. An advancing fault may fold sedimentary layers over it, creating a monocline.

The center of a ______ has rocks that are younger than those in the surrounding area. The center of a _____ has rocks that are older than those in the surrounding area.

basin; dome

The mechanism(s) causing regional metamorphism is/are ______.

burial and heating

Strike-slip zones ______.

can separate two plates (either being continental or oceanic) may create large shear zones may lie totally within a tectonic plate

The vertical lines indicated in this model of a fold and threat belt likely represent ______.

cleavage joints

Horizontal shear stresses, like those pictured, may produce ______.

metamorphism and ductile deformation at depth strike-slip faults in shallow parts of the crust shear zones

The confining pressure on a rock is ______ when there is high fluid pressure within the rock.

reduced

A thrust fault is a gently dipping ______ fault.

reverse

The type of stresses represented in this block model are ______ stresses.

shear

This metamorphic melange is formed due to ______.

shearing slicing folding

High fluid pressure in a rock body will ______.

act to decrease the confining pressure

True or false: A rock becomes permanently deformed when even a small amount of stress is applied to it.

False

True or false: The terms rock cleavage and foliation are always used interchangeably.

False

How would a rock respond to stresses applied at depth?

High-temperature veins form; mobilization of chemical constituents. Minerals recrystallize into larger or smaller crystals. Ductile behavior occurs; flowing of the rock.

______ is a nonfoliated metamorphic rock created by being heated from an igneous intrusion.

Hornfels

If compressive stress is applied in the directions indicated by the arrows (consider that arrows directly oppose one another), what will likely happen to the rock layers?

If rocks deform in a ductile manner, they will fold in a direction perpendicular to the stresses.

What type of rock is formed through the solid-state alteration of a previously existing rock, by heating, pressure, and/or chemically active fluids?

Metamorphic

Based on the metamorphic grade map of New England, what is true?

Metamorphic conditions were most intense in the center area and decreased toward the east and west.

In the eastern portion of central New England, metamorphosed ______ rocks that have been deformed into domes have been uplifted to the surface

Precambrian

Cleavage has a close and consistent geometric relationship to folds formed during the same event. Which of the following are true regarding folds and cleavage?

Relationships between cleavage and bedding can be used to locate folds. Cleavage typically is parallel to the axial surface of the fold.

Which of the following are forms of differential stress?

Tension Shear Compression

True or false: A rock may undergo metamorphism without deformation.

True

The type of deformation or metamorphism that is least prevalent in a continental collision zone is ______.

contact metamorphism

Which are true of faults?

Vertical compression may result in a more vertically oriented fault. Orientation of a fault depends on directions of stresses. A lateral fault may result from horizontal compression.

Which of the following is primarily responsible for the blocky and/or linear features shown?

Weathering along joint sets

Folds are most often created through what type of stress?

Compression

Which of the following are true for domes and basins?

In a basin, the rock layers dip toward the center in all directions. In a dome, the rock layers dip away from the center in all directions. Erosion of a dome exposes older rocks in the center. A basin is the structural opposite of a dome.

In which of these settings in a subduction zone does high-pressure/low-temperature metamorphism occur?

In the accretionary prism Along the megathrust boundary

Which of the following best describe lineation?

It may be due to elongated mineral crystals that grow in linear orientation. Stretched-out mineral crystals may show lineation. It can form under ductile shearing.

______ is a chemical process that can occur during metamorphism and deformation.

Remobilization Recrystallization Pressure solution

Based on the image, which of the following are true?

The slip in B is parallel to the strike. A is dip-slip fault. C is an oblique-slip fault.

How does the strength of the continental crust vary with increasing depth?

The strength of the continental crust first increases and then decreases with increasing depth.

Which of the following may be true of metamorphic rocks?

They display shear zones. They contain flattened or folded pebbles. Most metamorphic rocks have folds. They have preserved structures from before the rock was metamorphosed.

Which geologic units shown in this geologic map did not undergo extensive metamorphism and/or deformation?

Those represented by yellow shown on Long Island and Cape Cod

True or false: Metamorphism may occur without deformation.

True

Which of the following is true of rock cleavage?

Typically cuts across bedding planes Forms perpendicular to the direction of applied stress Forms when rock deforms under low to moderate temperatures

The feature shown by this aerial image is a(n) ______ and it formed as the result of a ______ fault.

offset stream; right-lateral strike-slip

Foliation and lineation is more likely to occur in rocks that have undergone ______ metamorphism.

regional

Migmatite forms in ______.

settings in which the rock begins to melt high-temperature environments

The special geologic terms used to represent the direction of a horizontal line on an inclined surface (such as in a fault) and the angle of that surface to the horizontal are ______.

strike and dip