Transcript Earth Structure - Cal State LA

Plate Tectonics
 The theory --the surface of the Earth is
broken into large plates. The size and
position of these plates change over
time. The edges of these plates, where
they move against each other, are
sites of intense geologic activity, such
as earthquakes, volcanoes, and
mountain building.
 Plate tectonics is a combination of two
earlier ideas, continental drift and seafloor spreading. Continental drift is the
movement of continents over the
Earth's surface and in their change in
position relative to each other. Seafloor spreading is the creation of new
oceanic crust at mid-ocean ridges and
movement of the crust away from the
mid-ocean ridges.
http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/introduction.html
Earth Structure
Mexico quake-- magnitude at 7.0, and epicenter was in
the western Pacific state of Michoacan. Its depth was about
40 miles
Indonesia-- earthquake off coast of Indonesia Wednesday,
April 11 with magnitude of 8.6, sparking tsunami warnings
Lithosphere
Consists of continental,
oceanic and upper part of
mantle
 Continents composed of
granite-type rock, quartz
and feldspar minerals,
density+2.8g/cm3
 Oceanic crust formed of
basalt; basalt rich in
iron/magnesium minerals,
density+3.0 g/cm3
 Lithosphere is rigid layer of
crust and mantle overlying
partially-molten
asthenosphere
Continental Drift Evidence
 Researchers noted geographic fit of continents
– e.g. Africa and S. America
– Atlantic formed by separation of Africa from S. America
 Seuss, 1885, proposed super continent by studying fossils,
rocks, mountains
 Wegener and Taylor, early 1900’s, proposed continental drift
and Pangaea
 Evidence supporting the idea that the continents had drifted.
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Geographic fit of continents
Fossils
Mountains
Glaciation
Continental Drift
Geographic Fit
Continents seem to fit together like pieces of a puzzle
Continental Drift
Fossils
Similar distribution of fossils such as the Mesosaurus
Continental Drift
Mountains
Mountain ranges match across oceans
Continental Drift
Glaciation
Glacial ages and climate evidence
Objections
to the
Continental Drift Model
Wegener did not
provide a plausible
mechanism to explain
how the continents
could have drifted.
Seafloor Spreading
Continental drift reexamined in 1960’s with new
information
 New theory developed – Seafloor spreading
Supporting evidence for seafloor spreading
– World seismicity
– Volcanism
– Age of seafloor
– Paleomagnetism
– Heat flow
 Theory combining continental drift and seafloor spreading
termed “Plate Tectonics”
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Seafloor Spreading
New sea floor created at the mid-ocean ridge and
destroyed in deep ocean trenches
Evidence for Seafloor Spreading
World Seismicity
Earthquake distribution matches plate boundaries
Evidence for Seafloor Spreading
Volcanism
Volcanoes match some plate boundaries; some are hot spots
Evidence for Seafloor Spreading
Age of Seafloor
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Youngest sea floor is at mid-ocean ridge
Oldest sea floor away from mid-ocean ridge
Evidence for Seafloor Spreading
Paleomagnetism

Earth has a magnetic field - Probably caused by rotation of
solid inner core in liquid outer core (both mostly Fe)
When rocks cool at the Earth’s surface, they record Earth’s
magnetic field (normal or reverse polarity)
Evidence for Seafloor Spreading
Paleomagnetism
Paleomagnetic studies indicate alternating
stripes of normal and reverse polarity at the
mid-ocean ridge.
Seafloor Spreading
Heat Flow
In 1960, convection currents were proposed as driving
force to move continents
Theory of Plate Tectonics
John Tuzo Wilson combined ideas of continental drift and
seafloor spreading into “Plate Tectonics”
Principles of Plate Tectonics
Earth’s outermost layer composed of thin rigid
plates moving horizontally
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Plates interact with each other along their edges
(plate boundaries)
Plate boundaries have a high degree of tectonic activity
– mountain building
– earthquakes
– volcanoes
Plate Boundaries
Three types
Plate Boundaries
Divergent
Plates move away from each other
New crust is being formed
Divergent Plate Boundaries
Examples
East African Rift
Mid-Atlantic Ocean Ridge
Plate Boundaries
Convergent
Three Types:
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Ocean-continent
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Ocean-ocean
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Continent-continent
Plates are moving toward each other
Crust is being destroyed
Convergent Plate Boundaries
Mount Fuji, Japan
Examples
Mount Lassen, California
Andes, South America
Plate Boundaries
Transform
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Plates slide past one another
Crust is neither created nor destroyed
Transform Plate Boundaries
Examples
Calexico, California
San Andreas Fault
Carrizo Plains, Central California
Stop Here
Origin of the Earth
Meteors and
Asteroids bombarded
the Earth
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Gravitational compression
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Density Stratified planet
Earth’s Interior
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Core
– dense
– Nickel and Iron
– Inner core – solid
– Outer core – liquid
Mantle
– Less dense than core
– Iron and Magnesium silicates
– Partially molten
Crust
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Outermost layer

Continental – granite-type,
density=2.8 g/cm
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Oceanic – basalt, density=3.0 g/cm

Rigid
Evidence of Internal Structure
 Density
– calculate density of Earth
– Speculate on probable compositions
 Meteorites
– Use composition and age to
determine composition and age of
Earth
 Seismic waves
– Travel times and direction give
indication of internal structure of
Earth
Granite
Basalt
Types of Seismic Waves
 P waves
– Primary, pressure, push and pull
– Travel fastest of the seismic waves
(average 6 km/sec)
– Travels through solids and liquids
 S waves
– Secondary, shaking, side-to-side
– Slower (average 4 km/sec)
– Travel through solids only
Seismic Waves Through Earth