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Our Hazardous Environment
GEOG 1110
Dr. Thieme
Lecture 3:
Earth's Structure, Plate
Tectonics, and the Rock Cycle
Structure of Earth
Average density of
the solid Earth = 5.5 g/cm3
Rocks of continental crust
average 2.5 to 3.0 g/cm3
Dense (Fe, Ni) core required
by planetary motions
What Is Inside
Upper layer is crust; two types:
Lowest layer: iron-nickel core
(molten outer core; solid inner
Relief and
• Ocean Basins are
spreading away from
mid-ocean ridges
• Continental collisions
build mountains and
increase land surface
• ocean-ocean
• ocean-continent
• continent-continent
Earth's crust is divided into 7 major and 20 smaller plates
Plate boundaries are defined by areas of seismic activity
Tectonic Cycle: Earth's crust is constantly being recycled as
lithosphere is created at spreading ridges, rides on aesthenosphere,
and is subducted into the mantle
Tectonic Cycle
• New Ocean Crust is
produced at spreading
• Different spreading
rates along the ridge
are accomodated by
Transform Faults
• Compressional Stress
at Convergent
Boundaries results in
Folding and Faulting of
Crustal Rocks
• Earthquakes are
responses to tectonic
Divergence at Spreading Ridges
- most important
area for creation
of new crust
- sea floor
spreading apart at
gradual and
constant rate
- ocean floor
subducted at
trenches, remains
geologically young
Ocean-Continent Convergence
- deep sea
trenches on the
ocean floor
- denser plate of
oceanic crust is
continental crust
- ocean floor
Convergent Boundary - Oceanic-Oceanic
-both plates
are the same
- Aleutian,
Mariana, and
Tonga Islands
in Pacific
- Antilles in
Convergent Boundary - Continent-Continent
- both plates
are the same
- edges of
colliding plates
crumple into
- Himalayan
Mountains in
Nepal and
divergent plate boundary
convergent plate boundary
transform faults/
triple junctions
Strike-slip (Transform)
Reverse (Thrust)
Transform Boundary
• most transforms offset
spreading ridges of
oceanic crust
• the San Andreas fault
zone in California is a
transform plate boundary
• the Pacific plate is
sliding horizontally to the
northwest past a segment
of the North American
• transform plate motions
result in the strongest
shaking by earthquakes at
the Earth's surface
Continental Transform Fault
San Andreas Transform Fault running across the Carrizo Plain in California
Figure 1.18a
San Andreas Fault impounds drainage used by palm trees
Figure 1.18b
Marsh in Pool impounded by San Andreas Fault
Rock Cycle
• all types of rocks
can be changed into
other types by:
• time
• heat
• pressure
• beginning the cycle
with igneous rocks
(high temperature)
is arbitrary
Igneous Rocks - crystallized from magma which
either intruded deep beneath the surface
("plutonic") or extruded to the surface ("volcanic")
Sediment - particles and dissolved substances which
settle out of a liquid (oceans, river floodplains,
swamps), also windblown (dunes)
Sedimentary Rocks - sediment compacted by weight
of overlying layers, cemented by percolating water,
or chemically precipitated.
Metamorphic Rocks - Igneous rocks, sediment, or
sedimentary rocks altered by being subjected to
temperature or pressure conditions above those at
the Earth's surface.