Transcript PowerPoint - Cal State LA - Instructional Web Server

Earth Structure
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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
Why Continents Higher
and Ocean Floor Lower
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Isostacy—Principle that less dense
continental crust sits higher on the
mantle while denser oceanic crust sits
lower
http://www.youtube.com/watch?v=G9rN
7qhUQZg&feature=player_detailpage
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
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Continents seem to fit together like pieces of a puzzle
Continental Drift
Fossils
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Similar distribution of fossils such as the Mesosaurus
Continental Drift
Mountains
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Mountain ranges match across oceans
Continental Drift
Glaciation
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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
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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
 Youngest sea floor is at mid-ocean ridge
 Oldest sea floor away from mid-ocean ridge
Evidence for Seafloor Spreading
Paleomagnetism
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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
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John Tuzo Wilson combined ideas of continental drift and
seafloor spreading into “Plate Tectonics”
Principles of Plate Tectonics
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Earth’s outermost layer composed of thin rigid
plates moving horizontally
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
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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