Transcript Dynamic Earth

Mini Quiz 04.26.07
1. What happens in Earth’s interior to produce
2.
Earth’s magnetic field?
Rocks on continents are billions of years old,
while the oldest rock found on the ocean floor
is less than 200 million years old. Explain how
this is possible.
NOTE: DYNAMIC EARTH TEST
MAY 10, 2007
Geomagnetism
Dynamic Earth
Earth’s Interior
• Core
–
–
–
–
dense
Iron and Nickel
Inner Core - solid
Outer Core - liquid
• Mantle
–
–
–
–
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
mantle convection
Heat transfer from the core to the
mantle produces slow convection
of the mantle material
(in the order of centimetres per year)
Mantle convection produces lateral
forces at the boundary between the
mantle and the crust
Lateral forces at the crustmantle boundary push the
crust around – in different
directions – and this leads to
the motion of crustal plates.
The plates are moving at rates
of cm/year
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
Lithosphere
• Consists of continental,
oceanic and upper part of
mantle
• Continents composed of granitetype 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.
– 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
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”
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
Heat Flow
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”
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
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