Answer four Earth Science Essays with a minimum of 200 words in APA Format

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Foundations of Earth

Science, 6e

Lutgens, Tarbuck, & Tasa

Restless Earth:

Earthquakes, Geologic

Structures, and Mountain

Building Foundations, 6e - Chapter 6

Stan Hatfield

Southwestern Illinois College

What is an earthquake?

• An earthquake is the vibration of

Earth produced by the rapid release

of energy

• Energy released radiates in all

directions from its source, the focus

• Energy is in the form of waves

• Sensitive instruments around the

world record the event

Earthquake focus

and epicenter

What is an earthquake?

• Earthquakes and faults

• Movements that produce earthquakes are usually associated with large fractures in Earth’s crust called faults

• Most of the motion along faults can be explained by the plate tectonics theory

What is an earthquake?

• Elastic rebound

• Mechanism for earthquakes was first

explained by H. F. Reid

• Rocks on both sides of an existing fault

are deformed by tectonic forces

• Rocks bend and store elastic energy

• Frictional resistance holding the rocks

together is overcome

What is an earthquake?

• Elastic rebound

• Earthquake mechanism

• Slippage at the weakest point (the focus)

occurs

• Vibrations (earthquakes) occur as the

deformed rock “springs back” to its original

shape (elastic rebound)

What is an earthquake?

• Foreshocks and aftershocks

• Adjustments that follow a major

earthquake often generate smaller

earthquakes called aftershocks

• Small earthquakes, called foreshocks,

often precede a major earthquake by

days or, in some cases, by as much as

several years

Seismology

• The study of earthquake waves,

seismology, dates back almost 2000

years to the Chinese

• Seismographs, instruments that record

seismic waves

• Record the movement of Earth in

relation to a stationary mass on a

rotating drum or magnetic tape

Seismology

• Seismographs

• More than one type of seismograph is needed to record both vertical and horizontal ground motion

• Records obtained are called seismograms

• Types of seismic waves

• Surface waves • Travel along the outer part of Earth

Seismology

• Types of seismic waves

• Surface waves

• Complex motion

• Cause greatest destruction

• Exhibit greatest amplitude and slowest

velocity

Seismology

• Types of seismic waves

• Body waves • Travel through Earth’s interior

• Two types based on mode of travel

• Primary (P) waves

• Push-pull (compress and expand) motion, changing the volume of the intervening material

• Travel through solids, liquids, and gases

Seismology

• Types of seismic waves

• Body waves

• Secondary (S) waves

• “Shake” motion at right angles to their

direction of travel

• Travel only through solids

• Slower velocity than P waves

Locating an earthquake

• Terms

• Focus—The place within Earth where

earthquake waves originate

• Epicenter—Location on the surface

directly above the focus

• Epicenter is located using the

difference in velocities of P and S

waves

Locating an earthquake

• Locating the epicenter of an earthquake • Three station recordings are needed to locate

an epicenter

• Each station determines the time interval

between the arrival of the first P wave and the

first S wave at their location

• A travel-time graph is used to determine each

station’s distance to the epicenter

Seismogram showing P, S,

and surface waves

A travel-time graph

Locating an earthquake

• Locating the epicenter of an earthquake

• A circle with a radius equal to the

distance to the epicenter is drawn

around each station

• The point where all three circles

intersect is the earthquake epicenter

Finding an earthquake

epicenter

Locating an earthquake

• Earthquake belts

• About 95 percent of the energy released

by earthquakes originates in a few

relatively narrow zones that wind around

the globe

• Major earthquake zones include the

Circum-Pacific belt and the Oceanic-

Ridge system

Measuring the size

of earthquakes • Two measurements that describe the

size of an earthquake are

• Intensity—A measure of the degree of earthquake shaking at a given locale based on the amount of damage

• Magnitude—Estimates the amount of energy released at the source of the earthquake

Measuring the size

of earthquakes • Intensity scales

• Modified Mercalli Intensity Scale was

developed using California buildings as

its standard

• The drawback of intensity scales is that

destruction may not be a true measure

of the earthquake’s actual severity

Measuring the size

of earthquakes • Magnitude scales

• Richter magnitude—Concept introduced

by Charles Richter in 1935

• Richter scale

• Based on the amplitude of the largest

seismic wave recorded

• Accounts for the decrease in wave

amplitude with increased distance

Measuring the size

of earthquakes • Magnitude scales

• Richter scale

• Magnitudes less than 2.0 are not felt by

humans

• Each unit of Richter magnitude increase

corresponds to a tenfold increase in wave

amplitude and a 32-fold energy increase

Measuring the size

of earthquakes

• Magnitude scales

• Other magnitude scales • Several “Richter-like” magnitude scales

have been developed

• Moment magnitude was developed because none of the “Richter-like” magnitude scales adequately estimate very large earthquakes

• Derived from the amount of displacement that occurs along a fault

Earthquake destruction

• Amount of structural damage

attributable to earthquake vibrations

depends on

• Intensity and duration of the vibrations

• Nature of the material upon which the

structure rests

• Design of the structure

Earthquake destruction

• Destruction from seismic vibrations

• Ground shaking

• Regions within 20 – 50 kilometers of the

epicenter will experience about the same

intensity of ground shaking

• However, destruction varies

considerably mainly due to the nature of

the ground on which the structures are

built

Damage caused by the 1964

Anchorage, Alaska quake

Figure 6.13

Earthquake destruction

• Liquefaction of the ground • Unconsolidated materials saturated with

water turn into a mobile fluid

• Tsunamis, or seismic sea waves

• Destructive waves that are often

inappropriately called “tidal waves”

Earthquake destruction

• Tsunamis, or seismic sea waves

• Result from vertical displacement along a fault located on the ocean floor or a large undersea landslide triggered by an earthquake

• In the open ocean height is usually less than 1 meter

• In shallower coastal waters the water piles up to heights over 30 meters

Formation of a tsunami

Earthquake destruction

• Landslides and ground subsidence

• Fire

Earth’s layered structure

• Layers are defined by composition

• Three principal compositional layers • Crust—The comparatively thin outer skin

that ranges from 3 kilometers (2 miles) at the oceanic ridges to 70 kilometers (40 miles in some mountain belts)

• Mantle—A solid rocky (silica-rich) shell that extends to a depth of about 2900 kilometers (1800 miles)

Earth’s layered structure

• Layers are defined by composition

• Three principal compositional layers

• Core—An iron-rich sphere having a radius

of 3486 kilometers (2161 miles)

Earth’s layered structure

• Layers defined by physical properties

• With increasing depth, Earth’s interior is characterized by gradual increases in temperature, pressure, and density

• Main layers of Earth’s interior are based on physical properties and hence mechanical strength

Earth’s layered structure

• Layers defined by physical properties

• Lithosphere (sphere of rock) • Consists of the crust and uppermost mantle

• Relatively cool, rigid shell

• Averages about 100 kilometers in thickness, but may be 250 kilometers or more thick beneath the older portions of the continents

Earth’s layered structure

• Layers defined by physical properties

• Asthenosphere (weak sphere) • Beneath the lithosphere, in the upper

mantle to a depth of about 600 kilometers

• Small amount of melting in the upper portion mechanically detaches the lithosphere from the layer below allowing the lithosphere to move independently of the asthenosphere

Earth’s layered structure

• Layers defined by physical properties

• Mesosphere or lower mantle

• Rigid layer between the depths of

660 kilometers and 2900 kilometers

• Rocks are very hot and capable of very

gradual flow

Earth’s layered structure

• Layers defined by physical properties

• Outer core

• Composed mostly of an iron-nickel alloy

• Liquid layer

• 2270 kilometers (1410 miles) thick

• Convective flow within generates Earth’s

magnetic field

Earth’s layered structure

• Layers defined by physical properties

• Inner core

• Sphere with a radius of 3486 kilometers

(2161 miles)

• Stronger than the outer core

• Behaves like a solid

Earth’s layered structure

Deformation

• Deformation is a general term that

refers to all changes in the original

form and/or size of a rock body

• Most crustal deformation occurs

along plate margins

• Deformation involves

• Stress—Force applied to a given area

Deformation

• How rocks deform

• General characteristics of rock deformation • Elastic deformation—The rock returns to

nearly its original size and shape when the stress is removed

• Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation)

Folds

• During crustal deformation rocks are

often bent into a series of wave-like

undulations called folds

• Characteristics of folds

• Most folds result from compressional

stresses which shorten and thicken the

crust

Folds

• Common types of folds

• Anticline—Upfolded or arched rock layers

• Syncline—Downfolds or troughs of rock layers

• Depending on their orientation, anticlines and synclines can be described as • Symmetrical, asymmetrical, or

recumbent (an overturned fold)

Anticlines and synclines

Folds

• Other types of folds

• Dome

• Upwarped displacement of rocks

• Circular or slightly elongated structure

• Oldest rocks in center, younger rocks on

the flanks

Folds

• Other types of folds

• Basin

• Circular or slightly elongated structure

• Downwarped displacement of rocks

• Youngest rocks are found near the center,

oldest rocks on the flanks

Faults

• Faults are fractures in rocks along which appreciable displacement has taken place

• Sudden movements along faults are the cause of most earthquakes

• Classified by their relative movement which can be

• Horizontal, vertical, or oblique

Faults

• Types of faults

• Dip-slip faults • Movement is mainly parallel to the dip of the

fault surface

• May produce long, low cliffs called fault scarps

• Parts of a dip-slip fault include the hanging wall (rock surface above the fault) and the footwall (rock surface below the fault)

Faults

• Types of dip-slip faults

• Normal fault

• Hanging wall block moves down

relative to the footwall block

• Accommodates lengthening or

extension of the crust

• Larger scale normal faults are

associated with structures called

fault-block mountains

Normal fault

Faults

• Types of dip-slip faults

• Reverse and thrust faults

• Hanging wall block moves up relative to

the footwall block

• Reverse faults have dips greater than 45o

and thrust faults have dips less than 45o

• Strong compressional forces

Reverse fault

Faults

• Strike-slip fault

• Dominant displacement is horizontal and parallel to the strike of the fault

• Types of strike-slip faults • Right-lateral—As you face the fault, the

opposite side of the fault moves to the right

• Left-lateral—As you face the fault, the opposite side of the fault moves to the left

Strike-Slip fault

Faults

• Strike-slip fault

• Transform fault

• Large strike-slip fault that cuts through

the lithosphere

• Accommodates motion between two

large crustal plates

The San

Andreas

Fault

System

Mountain building

• Orogenesis—The processes that

collectively produce a mountain belt

• Include folding, thrust faulting,

metamorphism, and igneous activity

• Compressional forces producing

folding and thrust faulting

• Metamorphism

• Igneous activity

Mountain building at

convergent boundaries

• Island arcs

• Where two ocean plates converge and one is subducted beneath the other

• Volcanic island arcs result from the steady subduction of oceanic lithosphere • Continued development can result in the

formation of mountainous topography consisting of igneous and metamorphic rocks

Volcanic island arc

Mountain building at

convergent boundaries

• Andean-type mountain building

• Mountain building along continental

margins

• Involves the convergence of an oceanic

plate and a plate whose leading edge

contains continental crust

• Exemplified by the Andes Mountains

Mountain building at

convergent boundaries

• Andean-type mountain building

• Building a volcanic arc • Subduction and partial melting of mantle

rock generates primary magmas

• Differentiation of magma produces andesitic volcanism dominated by pyroclastics and lavas

• A large percentage of the magma never reaches the surface and is emplaced as plutons

Andean-type plate margin

Subduction and

mountain building

• Andean-type mountain building

• Development of an accretionary wedge

• An accretionary wedge is a chaotic

accumulation of deformed and thrust-

faulted sediments and scraps of oceanic

crust

• Prolonged subduction may thicken an

accretionary wedge enough so it protrudes

above sea level

Continental collisions

• Two lithospheric plates, both carrying continental crust

• Continental collisions result in the development of compressional mountains that are characterized by shortened and thickened crust

• Most compressional mountains exhibit a region of intense folding and thrust faulting called a fold-and-thrust-belt

Continental collisions

• Himalayan Mountains • Youthful mountains—Collision began

about 45 million years ago

• India collided with Eurasian plate

• Similar but older collision occurred

when the European continent collided

with the Asian continent to produce the

Ural mountains

Continental collisions

• Appalachian Mountains

• Formed long ago and substantially

lowered by erosion

• Resulted from a collision among North

America, Europe, and northern Africa

Terranes and

mountain building • Another mechanism of orogenesis

• The nature of terranes

• Small crustal fragments collide and

merge with continental margins

• Accreted crustal blocks are called

terranes (any crustal fragments whose

geologic history is distinct from that of

the adjoining terranes)

Terranes and

mountain building

• The nature of terranes

• Prior to accretion some of the fragments

may have been microcontinents

• Others may have been island arcs,

submerged crustal fragments, extinct

volcanic islands, or submerged oceanic

plateaus

Terranes and

mountain building

• Accretion and orogenesis

• As oceanic plates move they carry

embedded oceanic plateaus, island

arcs, and microcontinents to Andean-

type subduction zones

• Thick oceanic plates carrying oceanic

plateaus or “lighter” igneous rocks of

island arcs may be too buoyant to

subduct

Collision and

accretion

of an

island arc

Terranes and

mountain building

• Accretion and orogenesis

• Collision of the fragments with the

continental margin deforms both blocks

adding to the zone of deformation and to

the thickness of the continental margin

• Many of the terranes found in the North

American Cordillera were once

scattered throughout the eastern Pacific

End of Chapter 6