Transcript Earth Structure

Earth’s Structure
Origin of the Earth
• Meteors and Asteroids
bombarded the Earth
Earth is 4.6 billion
years old
Earth’s Interior
• Core
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dense
Iron and Nickel
Inner Core - solid
Outer Core - liquid
• Mantle
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Less dense than core
Iron and Magnesium silicates
Mostly solid
Upper mantle is partially molten
• Crust
– Outermost layer
– Very thin and rigid
– Continental – granite
– Density = 2.8 g/cm3
– Oceanic – basalt
– Density = 3.0 g/cm3
Earth’s Interior
• Crust – Temperature range – 58˚C to -88˚C. Thickness of crust 6 – 70 km
• Mantle – Temperature - 870˚C-4400˚C
Thickness -2900 km
• Outer Core – Temperature - 4400˚C 6100˚C . Thickness – 2300 km
• Inner Core – Temperature - 7000˚C –
8000˚C. Thickness – 2400 km
Earth’s Interior
• Lithosphere - The rigid portion of the Earth’s
•
•
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crust (both oceanic & continental) that is broken
into plates.
“Litho” comes from the Greek word meaning
rock
Asthenosphere – the slushy (partially liquid &
solid) in the upper mantle on which the plates
“float”
“Asthenes” comes from the Greek word meaning
weak
Earth’s Structure
Earth’s Layers
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 waves
– Push and pull movement
– Travel fastest (~ 6 km/sec)
– Travel thru solids and liquids
• S waves
– Secondary waves
– Move side-to-side
– Slower (~ 4 km/sec)
– Travel thru solids only
Seismic Waves Through Earth
Continents Today
Continental Drift Theory
• In 1912 a German meteorologist,
astronomer and explorer named Alfred
Wegener presented a theory that the
continents were once joined together in
one land mass and have over millions of
years slowly drifted apart into their
present positions as we see them today.
He named the supercontinent “Pangaea”
after the Greek word meaning “all land”.
Continental Drift
Geographic Fit
• Continents seem to fit together like pieces of a puzzle
Continental Drift Theory
• Through Wegener’s research he found
evidence to support his theory:
• Fossils of Glossopteris, a seed fern, was
found in India, Australia, South America,
Africa and Antarctica
• Fossils of Mesosaurus, a small reptile were
found in freshwater deposits in Africa &
South America.
Continental Drift
Fossils
• Similar distribution of fossils such as the Mesosaurus
Glossopteris
• Fern
Continental Drift
• Layers of the mountains found in S.E.
Brazil, South America and S.W. Africa line
up layer for layer in exact format.
• Continents look like puzzle pieces that
could fit together.
Continental Drift
Mountains
• Mountain ranges match across oceans
Continental Drift
Glaciation
• Glacial ages and climate evidence
Continental Drift
Glaciation
• Glacial ages and climate evidence
Continental Drift Model
Problems
• Alfred Wegener
– Presented research to
professionals
– Did not provide a
plausible mechanism to
explain how continents
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”
Principles of Plate Tectonics
• Earth’s outermost layer composed of thin
rigid plates moving horizontally
• Plates interact with each other along their
edges (plate boundaries)
• Plate boundaries have high degree of tectonic
activity
– mountain building
– earthquakes
– volcanoes
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
• 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
Convection Currents
• In 1960, 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”
Plate Boundaries
Three types
• Divergent
• Convergent
• Transform
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:
• Ocean-continent
• Ocean-ocean
• 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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• Crust is neither created nor destroyed
• Plates slide past one another
Transform Plate Boundaries
Examples
San Andreas Fault
Calexico, California
Carrizo Plains, Central California