The Dynamic CrustThe Dynamic Crust

Chapter 4Chapter 4

CrustCrust• The crustcrust is the solid outer rock

zone of Earth.– The crustcrust is undergoing constant

change.– Weathering and erosion– Volcanoes– Earthquakes

– TectonicsTectonics is the study of the movement of the Earth’s crust.

Tectonic PlatesTectonic Plates

SolidSolid and and LiquidLiquid Zones Zones

• The lithospherelithosphere is the crust and uppermost part of the mantle.

• The asthenosphereasthenosphere is the upper part of the mantle that behaves like a thick, plastic fluid.

• The interface between the crust and the mantle is called the Mohorovicic Mohorovicic discontinuitydiscontinuity or “Moho” Interface“Moho” Interface.

MOHOMOHO

EarthquakesEarthquakesAn earthquakeearthquake is the vibration or

shaking of the Earth’s crust caused by rapid movement of rocks in the lithosphere.

The sudden release of energy in the The sudden release of energy in the rocks as they move and break causes rocks as they move and break causes waves.waves.

Pakistan Earthquake 1971Pakistan Earthquake 1971

Pakistan Earthquake 1971Pakistan Earthquake 1971

Earthquake WavesEarthquake Waves

Earthquakes generate three types of waves.

Compressional wavesCompressional waves called primary wavesprimary waves or P-wavesP-waves.

Shear waves called Secondary Secondary waveswaves or S-wavesS-waves.

Long wavesLong waves or L-wavesL-waves.

Compressional WavesCompressional WavesP-wavesP-waves

Compressional wavesCompressional waves are like sound waves.

P-wavesP-waves cause particles in the rock to vibrate in a back and forth motion in the same direction the wave is traveling.

Shear Waves Shear Waves

Shear wavesShear waves are similar to the waves in rope when it is shaken.

S-wavesS-waves vibrate rock particles at right angles to the direction the wave is traveling.

Long WavesLong WavesL-wavesL-waves

Long wavesLong waves travel along the Earth’s surface at relatively slow speeds.

Seismic WavesSeismic Waves

• When an earthquake occurs, it generates energy waves, called seismic wavesseismic waves, that travel outward from the point in the crust where the earthquake originates.

• This point of origin is called the focusfocus of the earthquake.

SeismogramsSeismograms

Locating the Epicenter of an Earthquake

• The epicenterepicenter of an earthquake is the point on Earth’s surface directly above the earthquake focus.

Earthquake P-wave and

S-wave Travel Time Graph

Earthquake VelocityEarthquake Velocity

• The different earthquake waves have different properties as they travel through the layers of the Earth.

• Waves are bent, or refracted, as they move through materials of different densities.

• Shadow zonesShadow zones are zones on the Earth’s surface where no seismic waves are received.

Shadow ZoneShadow Zone

Earthquake StrengthEarthquake Strength

• Earthquake intensity and energy are measured on two different scales.

• MagnitudeMagnitude is the amount of energy released during an earthquake and is measured using the Richter scaleRichter scale.– Scale is 0 to a high of any number– Each step in magnitude represents and increase

of 10 times the next lower number.• IntensityIntensity is a measure of the earthquakes

effect on people and buildings and is measured using the modified Mercalli modified Mercalli scalescale.

Evidence for Crustal Evidence for Crustal MovementMovement

• Continental DriftContinental Drift theory in 1910 by Alfred Wegener suggested that the continents were once fit together as one and have since drifted apart to their present locations.

More EvidenceMore Evidence

• Rock, mineral and fossil Rock, mineral and fossil correlation also provides correlation also provides evidence that the continents evidence that the continents were once joined together.were once joined together.

Continental “Fit”Continental “Fit”

Plate TectonicsPlate Tectonics

• This theory proposes that the Earth’s crust is divided into a number of large “plates”.

• The plates are movingplates are moving across the surface in a manner that some are separating, colliding, or sliding past one another.

Earth’s Tectonic PlatesEarth’s Tectonic Plates

What Causes Movement?What Causes Movement?• Convection cellsConvection cells in the mantle cause the

plates above them to move.

Plate BoundariesPlate Boundaries

There are three basic types of plate interactions.

• DivergentDivergent • ConvergentConvergent• TransformTransform

Divergent BoundariesDivergent Boundaries

Diverging boundariesDiverging boundaries occur where two plates move apartplates move apart from each other.

• Examples are mid-ocean ridges.• Mid-ocean ridges develop valleys

called rift valleysrift valleys.

Divergent BoundaryDivergent Boundary

Convergent BoundariesConvergent Boundaries

Convergent boundariesConvergent boundaries occur where plates move toward each other.

There are many types of convergent boundaries.

• Collision boundaryCollision boundary = two continents converge and mountains form.

• Subduction boundarySubduction boundary = ocean plate and continental plate converge and denser ocean plate is pushed under continental plate and back into mantle; volcanoes and mountains form.

Convergent BoundaryConvergent Boundary

Transform BoundariesTransform Boundaries

Transform boundariesTransform boundaries occur when one plate slides horizontally past another along a single fault or group of parallel faults.

• Ex: San Andreas fault in California.

Transform Transform BoundaryBoundary

Geosyncline TheoryGeosyncline Theory

River sediment causes land to sink and remain wetlands until enough land is formed that uplifting “builds” coastline.

EvidenceEvidence

• Crustal composition and magnetic poles/reversal.

• Ocean floor spreading.• Age of igneous rocks.• Minor crustal changes – deformed

rock strata; syncline/anticline.• Displaced rock strata and fossils.• Vertical crustal movements.

EvidenceEvidence

Crustal ThicknessCrustal Thickness

The Earth’s crust is divided into two parts:– Continental crustContinental crust has ~ 20 – 40 km

average thickness; composed mostly of low density rocks.

– Oceanic crustOceanic crust has ~ 10 km average thickness; composed mostly of high density rocks.

Moving Magnetic PolesMoving Magnetic Poles

The rock “record” shows that the magnetic poles have shifted as the tectonic plates have moved.–This evidence supports the theory of plate tectonics.

Sea-Floor SpreadingSea-Floor Spreading

Oceans provide evidence of major crustal movement.

Ages of basaltAges of basalt, , igneous rock formed from cooling lava, that comprise the ocean floor show that the youngest rocks are near the mid-ocean ridges. The farther you travel from the ridges, the older the basalt gets.

Rocks also provide evidence that the Earth’s poles have reversedreversed their magnetic polaritypolarity every several thousand years.

Minor Crustal ChangesMinor Crustal ChangesDeformed rock strataDeformed rock strata, in sedimentary

rocks normally formed in layers, provide evidence of change through tiltingtilting, foldingfolding, and faultingfaulting.

Normal StrataNormal Strata

Tilting, Folding, Faulting

FoldingFolding• AnticlinesAnticlines are upward folded layers and

synclines are downward folds.

FaultingFaulting

Displaced FossilsDisplaced Fossils

Vertical Crustal MovementsVertical Crustal MovementsA bench markbench mark is a reference point to measure

change against.

Vertical Movement of Hiroshima