Transcript Deformation of the Crust

Deformation of the Crust
Chapter 11
Deformation
• Bending
• Tilting
• Breaking
Isostasy
• When the force of gravity (weight) of the
lithosphere equals the buoyancy of the
asthenosphere
• What happens when lithosphere becomes
thinner? Thicker?
• Isostatic adjustments
Isostatic Adjustments
• Mountainous regions
• Regions of deposition (causes ocean floor
to sink – subsidence)
• Glaciers (growth or retreat)
• What happens to ocean floor when glaciers
grow?
Types of Stress
• Compression
• Squeezes rock together to occupy smaller areas
• Can bulge up above crust or deeper into crust
• Boundary type?
• Tension
• Pulls rock apart causing it to become thinner
• Boundary type?
• Shear
• Parts of the rock are pushed separate directions
• Can break, bend, or twist rocks
• Boundary type?
Types of Strain
• Permanent Strain
• Brittle (breaks)
• Ductile (bends)
Factors Affecting Strain
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Composition of rock
Temperature
Pressure
Amount of stress
Type of stress
Rate of stress
Folds
• Bend in rock resulting from stress
• Compression or shear stress
Features of a Fold
• Limbs – sloping side of fold
• Hinge – bend of the rock
• Axial plane – plane at which fold could be
split into symmetrical halves
Types of Folds
• Anticline
• Compression
• Arch-shaped
• Oldest rocks in center of fold
• Syncline
• Compression
• Bowl-shaped
• Youngest rocks in center of fold
• Monocline
• Both limbs horizontal
• One part of crust moves up, the other moves down
When Rocks Break:
• Fracture – no movement along break
• Fault – motion of the rock along the fault
plane
Types of Faults
• Normal
• Hanging wall moves downward relative to footwall
• Divergent boundary
• Step-like formations, Great Rift Valley
• Reverse
• Hanging wall moves upward relative to footwall
• Thrust fault – fault plane nearly horizontal
• Rockies and Alps
• Strike-slip
• Rocks on both sides of fault plane move horizontally
• Transform boundaries
Sizes of Faults
• Small fault – only in small area and through
few layers of rock
• Large fault system – large area, several
layers of rock
• San Andreas Fault
Earthquakes
Chapter 12
Elastic Rebound Theory
• Fault is usually locked by friction
• Stress is added and builds up until crust
deforms
• Rock fractures and snaps back into its
original shape
• Earthquake
Anatomy of an Earthquake
• Focus – where movement first occurs
• Shallow (most damage) – 70 km deep
• Intermediate – 70-300 km deep
• Deep – 300-650 km deep
• Epicenter – point on Earth’s surface
directly above focus
Seismic Waves
• Body waves
• P waves – (primary, compression)
• fastest and first to be detected
• Particles move back and forth parallel to wave
action
• Can travel through solids, liquids, and gases
• S waves – (shear, secondary)
• Particles move in side to side direction
perpendicular to direction of wave
• Can only travel through solids
Seismic Waves
• Surface Waves
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Form when P & S waves reach surface
Slowest moving
Cause greatest damage
Love – particles move side to side and
perpendicular to wave direction
• Rayleigh – elliptical and rolling motion
Seismic Waves and Earth’s Interior
• Waves speed up 30 km down because
mantle is more dense
• Speed changes of waves tells where layers
are
Shadow Zones
Earthquake Environments
• Convergent Oceanic Environments
• Overriding plates scrapes over subducting plate
• Divergent Oceanic Environments
• Spreading motion causes earthquakes along
ridge
• Continental Environments
• Stress added to rock by plates colliding,
diverging, or sliding past one another
Earthquakes Away from a Fault
• New Madrid, Missouri (1811-1812)
• Caused damage as far as South Carolina
• Caused Mississippi River to flow backwards
• Ancient fault zone (600 myo)
Measuring Earthquakes
• Seismograph
• Measures vertical motions, horizontal motions
E-W, horizontal motions N-S
• Seismogram traces earthquake motion
• Which are recorded 1st? 2nd? Last?
Locating an Earthquake
• Need measurements from 3 stations
• Difference in time P and S waves arrive
(lag-time)
• Draw circles (radius = distance to epicenter
from station)
• Point where circles intersect is location of
the epicenter
Earthquake Measurement
• Magnitude
• Amount of ground movement
• Richter Scale
• Moment Magnitude Scale (more accurate for
large earthquakes)
• Largest ever recorded: 9.5
• Felt by humans: 2.5
• Intensity
• Measures effects of earthquake
• I – XII (XII is total destruction)
• II is felt by only a few people at rest; delicately
suspended items may swing.
Tsunamis
• Earthquake with epicenter on ocean floor
• Sudden drop or rise in seafloor causes
water to suddenly drop or rise forming
waves
• Underwater landslides may also cause them
Predicting Earthquakes
• Past earthquakes
• Seismic Gaps – area along a fault where
there has been a big earthquake in the past
but none recently
• Foreshocks – tiny earthquake seconds or
weeks before earthquake
• Changes in Rocks – sensors detect any
change in the strain of the rock