Dynamic EarthDynamic EarthClass 17Class 17

7 March 20067 March 2006

Homework, Chapter 5Homework, Chapter 5 Why do some rock layers fold and Why do some rock layers fold and

others break into faults when they are others break into faults when they are subjected to crustal forces? subjected to crustal forces?

All other things being equal, experimentation indicates that rocks subjected to low confining pressures and low temperatures, such as exist near Earth's surface, will tend to break under deforming pressure. Rocks subjected to conditions that simulate those deep below the surface will bend or fold when a similar force is applied.

Experimental Deformation of Experimental Deformation of MarbleMarble

Brittle Deformation(low confining pressure)

Ductile Deformation(high confining pressure)

Factors that affect Factors that affect deformationdeformation

TemperatureTemperature

PressurePressure

Strain rateStrain rate

Rock typeRock type

The variation of these factors determines if a rock will fault or fold.

Homework, Chapter 5Homework, Chapter 5 If you found tilted beds in the field, how If you found tilted beds in the field, how

would you tell if they were part of an would you tell if they were part of an anticline or a syncline?anticline or a syncline?

One could not usually make a determination at the site but would need to look for other outcrops of the same rocks in the surrounding area. If the outcrops define a strip of rocks that becomes older as one progresses toward its center, it is an eroded anticline. If the rocks become younger toward the center of the feature, it is an eroded syncline.

Geometry of Anticlines & Geometry of Anticlines & SynclinesSynclines

Homework, Chapter 5Homework, Chapter 5 Evidence for vertical crustal movements is Evidence for vertical crustal movements is

often found in the geologic record. Give some often found in the geologic record. Give some examples of such evidence. examples of such evidence.

Areas that are lifted above the surrounding terrain are targets for increased erosion. Thus, the extensive erosion of such areas as the Black Hills is evidence that the area has been lifted. The tilt in adjacent sedimentary beds that were revealed by erosion indicates uplift as well.

Homework, Chapter 5Homework, Chapter 5 It has been suggested that the Himalaya It has been suggested that the Himalaya

Mountains and the Tibetan Plateau were Mountains and the Tibetan Plateau were uplifted ~2,000 m about 10 million years ago. uplifted ~2,000 m about 10 million years ago. What caused this sudden uplift?What caused this sudden uplift?

Mountains are pushed up when the continental lithosphere is compressed – in the process, both the crustal and mantle parts of the lithosphere are thickened, creating a deep root beneath the mountains. The mantle portion of the root is denser than the underlying asthenosphere and eventually drops off, allowing the mountains to rise higher.

The drooling lithosphereThe drooling lithosphere

Exam ReviewExam Review Second Exam – Thursday March 9Second Exam – Thursday March 9thth

Exam will be “fill in the blank” (15 x 2 Exam will be “fill in the blank” (15 x 2 points) and short answer questions (9 points) and short answer questions (9 x 5 points); 75 points totalx 5 points); 75 points total

Questions will come from Lectures Questions will come from Lectures (including videos), reading and (including videos), reading and homeworkhomework

How How do we knowdo we know about the about the Earth’s Interior?Earth’s Interior?

By studying MeteoritesBy studying Meteorites Direct observation (rocks originating Direct observation (rocks originating

from depth)from depth) Experiments at high pressureExperiments at high pressure By studying earthquake waves By studying earthquake waves

(Seismology)(Seismology)

Structure of the Structure of the EarthEarth

Seismic velocity Seismic velocity (how fast earthquake (how fast earthquake waves travel through rocks) waves travel through rocks) depends depends on the composition of material and on the composition of material and pressure.pressure.

We can use the behavior of seismic We can use the behavior of seismic waves to tell us about the interior of waves to tell us about the interior of the Earth.the Earth.

Most common types of Most common types of earthquake waves:earthquake waves:

P-wavesP-waves and and S-wavesS-waves – – BodyBody waves waves PPrimaryrimary waves travel the fastest in the waves travel the fastest in the

crust and usually are the first waves to crust and usually are the first waves to arrivearrive

SSecondaryecondary (or S (or Shearhear) waves are ) waves are slower and therefore take longer to slower and therefore take longer to arrivearrive

Changes in Changes in P- and S- wave P- and S- wave Velocity Reveal Velocity Reveal Earth’s Internal Earth’s Internal

LayersLayers

Velocities Velocities generally generally

increase in each increase in each layerlayer

Types of Seismic WavesTypes of Seismic Waves

P-wave P-wave Shadow Shadow

ZoneZone

S-wave S-wave Shadow Shadow

ZoneZone

P wave shadow zone

S wave shadow zone

Earth’s Earth’s CORECORE Outer CoreOuter Core - Liquid Fe, ~2200 km thick, - Liquid Fe, ~2200 km thick,

No S-waves transmitted -> S-& P-wave No S-waves transmitted -> S-& P-wave Shadow ZonesShadow Zones

Inner CoreInner Core - solid Fe (some Ni, Co, S, C), - solid Fe (some Ni, Co, S, C), ~2500 km thick~2500 km thick

How do we know?How do we know? Meteorites, Meteorites, SeismologySeismology, , MMagnetic fieldagnetic field

IsostasyIsostasy::Another key to Earth’s Another key to Earth’s

InteriorInterior Buoyancy of low-density rock masses Buoyancy of low-density rock masses

“floating on” high-density rocks; accounts “floating on” high-density rocks; accounts for “roots” of mountain beltsfor “roots” of mountain belts

First noted during a survey of IndiaFirst noted during a survey of India

The less dense crust The less dense crust “floats” on the less buoyant, “floats” on the less buoyant,

denser mantledenser mantle

MohorovicicDiscontinuity

(Moho)

Mantle TomographyMantle Tomography Uses numerous seismic dataUses numerous seismic data Uses small changes in speed of seismic Uses small changes in speed of seismic

waveswaves Faster wave motion may correspond to Faster wave motion may correspond to

denser or colder regionsdenser or colder regions Slower wave motion may correspond to Slower wave motion may correspond to

buoyant or warmer regions buoyant or warmer regions

Basics of TomographyBasics of Tomography

HotspotsHotspots Areas with volcanic activity NOT Areas with volcanic activity NOT

explained by plate tectonicsexplained by plate tectonics

Mantle beneath may be hot, wet, or Mantle beneath may be hot, wet, or chemically differentchemically different

Commonly active for long timeCommonly active for long time

Flood basalts

Hotspot tracks

Oceanic plateaus

StressStress The The forceforce that acts on a rock unit to that acts on a rock unit to

change its shape and/or its volumechange its shape and/or its volume Causes strain or deformationCauses strain or deformation StressStress

CompressionCompression TensionTension ShearShear

StraiStrainnAny change in original shape or size of an

object in response to stress acting on the object

Ductile (Plastic) Ductile (Plastic) DeformationDeformation• Permanent change in

shape or size that is not recovered when the stress is removed

• Occurs by the slippage of atoms or small groups of atoms past each other in the deforming material, without loss of cohesion

Brittle Deformation Brittle Deformation (Rupture(Rupture))• Loss of cohesion of a

body under the influence of deforming stress

• Usually occurs along sub-planar surfaces that separate zones of coherent material

Factors that affect Factors that affect deformationdeformation

TemperatureTemperature

PressurePressure

Strain rateStrain rate

Rock typeRock type

The variation of these factors determines if a rock will fault or fold.

FoldsFolds Most common ductile response to Most common ductile response to

stress on rocks in the earth's cruststress on rocks in the earth's crust

Symmetrical, Asymmetrical Symmetrical, Asymmetrical and Overturned Foldsand Overturned Folds

Faults Faults Occur when large stresses build up in the Occur when large stresses build up in the

crustcrust Most common brittle response to stress on Most common brittle response to stress on

rocks in the earth's crustrocks in the earth's crust Classified according to the kind of Classified according to the kind of

movement that has occurred along themmovement that has occurred along them Know the types of faults especially if they Know the types of faults especially if they

are important in mountain buildingare important in mountain building

Stacked Sheets of Continental Stacked Sheets of Continental Crust Due to Convergence of Crust Due to Convergence of

Continental PlatesContinental Plates

Overlapping Thrust Faults,Overlapping Thrust Faults,e.ge.g. the Himalayas. the Himalayas

Tilted Normal Fault Blocks,Tilted Normal Fault Blocks,e.ge.g. Basin and Range Province. Basin and Range Province

Structures of continentsStructures of continents1) Continents are made and deformed by 1) Continents are made and deformed by

plate motion. plate motion.

2) Continents are older than oceanic 2) Continents are older than oceanic crust.crust.

3) Lithosphere floats on a viscous layer 3) Lithosphere floats on a viscous layer below (isostasy).below (isostasy).

Age of the Continental CrustAge of the Continental Crust

Blue areas mark continental crustbeneath the ocean

Stages in Stages in the the

formation of formation of thethe

Southern Southern AppalachianAppalachian

ssFig. 17.30

India has collided with AsiaIndia has collided with Asia

Next Tuesday and ThursdayNext Tuesday and ThursdayCoastal ProcessesCoastal Processes

(March 14(March 14thth))Coastal ManagementCoastal Management

(March 16(March 16thth))

Chip FletcherChip Fletcher(No reading or homework)(No reading or homework)