Download The Dynamic Crust

The Dynamic Crust
Chapter 4
Crust
• The crust is the solid outer
rock zone of Earth.
– The crust is undergoing
constant change.
– Weathering and erosion
– Volcanoes
– Earthquakes
– Tectonics is the study of the
movement of the Earth’s crust.
Tectonic Plates
Solid and Liquid Zones
• The lithosphere is the crust and
uppermost part of the mantle.
• The asthenosphere 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
discontinuity or “Moho” Interface.
MOHO
Earthquakes
An earthquake 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
rocks as they move and break
causes waves.
Pakistan Earthquake 1971
Pakistan Earthquake 1971
Earthquake Waves
Earthquakes generate three types of
waves.
Compressional waves called
primary waves or P-waves.
Shear waves called Secondary
waves or S-waves.
Long waves or L-waves.
Compressional Waves
P-waves
Compressional waves are like
sound waves.
P-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 are similar to the waves in
rope when it is shaken.
S-waves vibrate rock particles at right
angles to the direction the wave is
traveling.
Long Waves
L-waves
Long waves travel along the Earth’s
surface at relatively slow speeds.
Seismic Waves
• When an earthquake occurs, it
generates energy waves, called
seismic waves, that travel
outward from the point in the
crust where the earthquake
originates.
• This point of origin is called the
focus of the earthquake.
Seismograms
Locating the Epicenter of an Earthquake
• The epicenter 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 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 zones are zones on the Earth’s
surface where no seismic waves are
received.
Shadow Zone
Earthquake Strength
• Earthquake intensity and energy are
measured on two different scales.
• Magnitude is the amount of energy
released during an earthquake and is
measured using the Richter scale.
– Scale is 0 to a high of any number
– Each step in magnitude represents and
increase of 10 times the next lower number.
• Intensity is a measure of the
earthquakes effect on people and
buildings and is measured using the
modified Mercalli scale.
Evidence for Crustal
Movement
• Continental 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 Evidence
• Rock, mineral and fossil
correlation also provides evidence
that the continents were once
joined together.
Continental “Fit”
Plate Tectonics
• This theory proposes that the
Earth’s crust is divided into a
number of large “plates”.
• The plates are moving across the
surface in a manner that some are
separating, colliding, or sliding
past one another.
Earth’s Tectonic Plates
What Causes Movement?
• Convection cells in the mantle cause
the plates above them to move.
Plate Boundaries
There are three basic types of plate
interactions.
• Divergent
• Convergent
• Transform
Divergent Boundaries
Diverging boundaries occur where
two plates move apart from each
other.
• Examples are mid-ocean ridges.
• Mid-ocean ridges develop valleys
called rift valleys.
Divergent Boundary
Convergent Boundaries
Convergent boundaries occur where
plates move toward each other.
There are many types of convergent
boundaries.
• Collision boundary = two continents
converge and mountains form.
• Subduction 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 Boundary
Transform Boundaries
Transform 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
Boundary
Geosyncline Theory
River sediment causes land to sink
and remain wetlands until enough
land is formed that uplifting
“builds” coastline.
Evidence
• 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.
Evidence
Crustal Thickness
The Earth’s crust is divided into
two parts:
– Continental crust has ~ 20 – 40 km
average thickness; composed mostly
of low density rocks.
– Oceanic crust has ~ 10 km average
thickness; composed mostly of high
density rocks.
Moving 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 Spreading
Oceans provide evidence of major
crustal movement.
Ages 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 reversed their
magnetic polarity every several
thousand years.
Minor Crustal Changes
Deformed rock strata, in sedimentary
rocks normally formed in layers,
provide evidence of change through
tilting, folding, and faulting.
Normal Strata
Tilting, Folding, Faulting
Folding
• Anticlines are upward folded layers
and synclines are downward folds.
Faulting
Displaced Fossils
Vertical Crustal Movements
A bench mark is a reference point to measure
change against.
Vertical Movement of Hiroshima