Chapter 11 Crustal Deformation and Mountain Building

Sizes of folds

1. broad flextures in which strata hundreds of meters thick have been slightly warped. 2. Others are very tight microscopic structures found in metamorphic rocks

Normal faults

1. dip-slip faults are classified as __ when the hanging wall block moves down relative to the footwall block 2. are associated with tensional stresses that pull rock units apart, lengthening the crust

how many types of deformation are there? 1. 2. 3.

1. elastic deformation 2. brittle deformation 3. ductile deformation

Whenever the stresses acting on a rock body exceed its strength, the rock will deform by: 1. 2. 3. 4.

1. folding 2. flowing 3. fracturing 4. faulting

Rock structures include 1. 2. 3.

1. folds 2. faults 3. joints

Weak rocks that are most likely to behave in a ductile manner (bend or flow) when subjected to differential stress include 1. 2. 3. 4. 5.

1. rock salt 2. shale 3. limestone 4. schist

The major factors that influence the strength of a rock and how it will deform include 1. 2. 3. 4.

1. temperature 2. confining pressure 3. rock type 4. time

role of temperature

1. where temperatures are high deep in Earth's crust, rocks tend to soften and become more malleable 2. where temperatures are low (at or near the surface), rocks to behave like brittle solids and fracture

how many types of differential stress? 1. 2. 3.

3 1. compressional stress 2. tensional stress 3. shear stress

Strain

Change in Shape Caused by Stress

Influence of Rock type

In addition to being influenced by the physical environment, deformation of rock is greatly influenced by its mineral composition and texture

Some outcrops have a sequence of interbedded weak and strong rock layers that have been moderately deformed by folding. example: shale and well cemented sandstone beds.

In these settings, the strong sandstone layers are often fractured and the weak shale beds form undulating folds. this structure, (one brittle and one ductile) within the same rock can be illustrated by cooling a Milky way candy bar.

A good example of topography that results when erosional forces attack folded sedimentary strata is found in the __

Valley and Ridge Province of the Appalachians

Time as a Factor

When tectonic forces are applied slowly over long time spans, rocks tend to display ductile behavior and deform by bending or flowing The same rocks may shatter if force is applied suddenly.

It is important to realize that ridges are not necessarily associated with _____, nor are valleys a characteristic of ____.

anticlines, synclines

The two most common folds are _____ and ___.

anticlines, synclines

Half-grabens

are tilted fault blocks, also contribute to the alternating topographic highs and lows in the Basin and Range Province

Folds are also described by their ____ plane, which is a surface that connects all the hinge lines of the folded strata.

axial

In simple folds, the ____ ____ is vertical and divides the fold into two roughly symmetrical limbs.

axial plane

Downward structures having a shape similar to the shape of domes are termed _______. 1. 2.

basins 1. basin of Michigan 2. basin of Illinois have gently sloping beds similar to saucers These basins are though to have resulted from accumulations of sediment whose weight caused the crust to subside (The Principle of Isostacy_

Every body of rock, no matter how strong, has a point at which it will deform by ____ or_________.

bending or breaking

small faults

can be recognized in road cuts where sedimentary beds have been offset a few meters faults of this scale occur as single discrete breaks

Overturned fold

can lie on its side so the axial plane is horizontal. These recumbent folds are common in highly deformed mountainous regions such as the Alps.

Most folds result from_____ stresses.

compressional stresses that result in a shortening and thickening of the crust.

Along ____ plate boundaries, flat-lying sedimentary strata, tabular intrusions, and volcanic rocks are often bent into a series of wavelike undulations called ____.

convergent, folds

When rocks are subjected to stresses that exceed their strength, they _____, usually by _____ or ___.

deform, bending or breaking

Like the circle shown in figure 11.3, strained bodies lose their original configuation during _____.

deformation

____________________ is a general term that refers to the changes in the shape or position of a rock body in response to differential stress.

deformation

The slopes of the large normal faults associated with the Basin and Range Province decrease with depth and eventually join to form a nearly horizontal fault called a _____ fault. These faults represent a major boundary between the rocks below, which exhibit _____deformation, and the rocks above, which exhibit mainly -______deformation.

detachment, ductile deformation, and brittle deformation

Rather, ridges and valleys result because of ____ ___ and ___.

differential weathering and erosion

Faults in which movement is primarily parallel to the inclination (also called dip) of the fault surface are called ___ ___ ___.

dip-slip faults

Broad upwarps in basement rock may deform the overlying cover of sedimentary strata and generate large folds. When this upwarping produces a circular or slightly elongated structure, the feature is called a ____.

dome

The Black Hills of western South Dakota is a large structural ____ generated by upwarping

dome

Folds can also be tilted by tectonic forces that cause their hinge lines to slope ____. Folds of this type are said to _____ because the hinge lines of the fold penetrate Earth's surface.

downward, plunge example: Sheep Mountain, Wyoming is an example of a plunging anticline

Crustal movements of a few meters occur along faults during major ____.

earthquakes

Fault scarps are produced by rapid vertical slips that generate ___.

earthquakes

Sudden movements along faults cause most _____.

earthquakes

Changes that result from ____ ____ are recoverable; that is, like a rubber band, the rock will snap back to nearly its original size and shape. during elastic deformation, the chemical bonds of the minerals within a rock are stretched but do not break.

elastic deformation

In the Western United States, large normal faults are associated with structures called _____________.

fault - block mountains examples: Basin and Range Province, a region that encompasses Nevada and portions of the surrounding states

Vertical displacements along dip-slip faults tend to produce long, low cliffs called _______

fault scarps

By contrast, sedimentary rocks that are weakly cemented or metamorphic rocks that contain zones of weakness, such as _____, are more susceptible to ____ deformation.

foliation ductile

Small-scale deformation of rocks by shear stresses occurs along closely spaced parallel surfaces of weakness, such as ___ surfaces and _____ fractures, where slippage changes the shape of rocks.

foliation microscopic

Igneous and some metamorphic rocks (quartzite, for example) are composed of minerals that have strong internal chemical bonds. These strong brittle rocks tend to fail by ____ when subjected to stresses that exceed their strength.

fracturing

A few structural basins may have resulted from _____ _____ impacts.

giant meteorite impacts

The weakest naturally occurring solid to exhibit ductile flow is __ ___.

glacial ice

Geologists identify the rock surface immediately above the fault as the ________ and the rock surface below the _______.

hanging wall block and Footwall

large faults like the San Andreas Fault in California

have displacements of hundreds of kilometers of consist of many interconnecting fault surfaces these structures described as fault zones, can be several kilometers wide are often easier to identify from aerial photos than at ground level

Each layer is bent around an imaginary axis called a _____ line, or simply a hinge

hinge

Folds are geologic structures consisting of stacks of originally ____ surfaces, such as sedimentary strata, that have been bent as a result of permanent deformation.

horizontal

These names were first used by prospectors and miners who excavated metallic ore deposits such as gold that had precipitated from ____ solutions along inactive fault zones

hydrothermal

glacial ice

image

mountain belts

image

tectonic activity

image

Monoclines

image Folds can be uniquely coupled with faults. example of this close association are broad, regional features called monoclines.

Brittle Deformation

image When the elastic limit (strength) of a rock is surpassed, the rock either bends or breaks. Rocks that break into smaller pieces exhibits brittle deformation.

joints in rocks

image cracks

Shear stress

image differential stress can cause rock to shear, which involves the movement of one part of a rock body past another.

tensional stress

image differential stress that pulls apart or elongates rock bodies. divergent plate boundaries ex: Basin and Range Province in the western U.S., tensional forces have fractured and stretched the crust to as much as twice is width

reverse fault

image dip-slip faults in which the hanging wall block moves up relative to the footwall block results from compressional stresses that produce horizontal shortening of the crust

Synclines

image downfolds or troughs

The miners would walk on the rocks below the mineralized fault zone or ________ and hang their lanterns on the rocks above or _____

image footwall block, hanging wall block

faults

image form where brittle deformation leads to fracturing and displacement of Earth's crust (fractures along which one rock body slides past another.)

Down dropped block called ____

image grabens

Movements along these faults produced alternating uplifted fault blocks called __

image horsts = hill

Ductile deformation

image is a type of solid-state flow that produces a change in the shape of an object without fracturing. ex: modeling clay, beeswax, taffy, and some metals

folds

image wave-like undulations

Elastic deformation

image when stress is applied gradually, rocks initially respond by deforming elastically

Confining pressure

image when stress is applied uniformly in all directions.

Anticlines

image usually arise by upfolding or arching, of sedimentary layers and are sometimes spectacularly displayed along highways that have been cut through deformed strata.

The vast majority of faults are remnants of past deformation and are ____.

inactive

In the stable ___________ of the continents, rocks reveal a history of deformation which shows that these areas were once sites of former mountain belts.

interior

compressional stress

is differential stress that squeezes a rock mass as if placed in a vise. are most often associated with convergent boundaries. when plates collide, Earth's crust is generally shortened horizontally and thickened vertically. produces mountains

Stress

is the force that acts to deform rock bodies

Strain

is the resulting deformation or distortion or change in the shape in the rock body

Structural domes can also form by the intrusion of magma ____. In addition, the upward migration of buried salt deposits can produce salt domes like those surrounding the Gulf of ____.

laccoliths, Mexico

By contrast, at transform fault boundaries, such as the San Andreas Fault, shear stress causes _____ segments of Earth's crust to slip ______ past one another.

large horizontally

Particularly, prominent features of the Colorodo Plateau, ____ are large, steplike folds in otherwise horizontal sedimentary strata.

monoclines

Here erosion has stripped away the highest portions of the overlying sedimentary beds, exposing ____ igneous and metamorphic rocks in the center.

older

This is opposite order of a dome, such as the Black Hills, where the ____ rocks form the core.

oldest

Examples of monoclines

on the Colorado Plateau include the East Kaibab Monocline, the Raplee Anticline, The Waterpocket Fold San Rafael Swell

asymmetrical

one or both limbs are tilted beyond the vertical

When flat-lying sedimentary layers are uplifted and tilted, their ______ change, but their shapes are often retained. Differential stress can also change the shape of a rock body, referred to as _____

orientations, strain

Plate motions and the interactions along _____ margins generate the tectonic forces that cause rocks to deform.

plate

Most crustal deformation occurs along _____ boundaries.

plate.

Role of Confining Pressure

pressure, like temperature, increases with depth as the thickness of the overlying rock increases confining pressure squeezes the materials in Earth's crust, which makes it stronger and thus harder to break. Rocks that are deeply buried are held together by the immense pressure and tend to bend rather than fracture.

For example, in the Valley and Ridge Province, resistant sandstone beds remain as imposing ____ separated by valleys cut into more easily ____ shale or limestone beds.

ridges, eroded

The basic geologic features that form as a result of the forces generated by the interactions of tectonic plates are called ____ ____ or ____ _____.

rock structures or geologic structures

Salt domes are economically important rock structures because when ____ migrates upward, the surrounding oil-bearing sedimentary strata deform to form ____ reservoirs.

salt, oil

in fact, rock salt is so weak that large masses of it often rise through overlying beds of ___ ___, much as hot magma rises towards Earth's surface.

sedimentary rocks

however, the axial plane often leans to one side so that one limb is ____ and _____ than the other.

steeper and shorter

Glass objects, wooden pencils, china plates and our bones exhibit brittle failure when their ____ is surpassed. Brittle deformation occurs when stress breaks the chemical bonds that hold a material together.

strength

Geologists use the term ____ to describe the forces that deform rocks.

stress

The limb of an anticline is also a limb of the adjacent _____.

syncline

Shifting lithospheric plates gradually change the face of our planet by moving continents across the globe. The results of the ____ ____ are perhaps most strikingly apparent in Earth's mountain belts.

tectonic activity

symmetrical

the limbs are mirror images of each other

differential stress

when stress is applied unequally in different directions

Because large basin contain sedimentary beds sloping at low angles, they are usually identified by the age of the rocks composing them. The ____ rocks found near the center, and the oldest rocks are at the flanks.

youngest