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Earth Structure & Plate Tectonics

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Kellina Gilbreth

on 28 November 2016

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Transcript of Earth Structure & Plate Tectonics

Earth Structure & Plate Tectonics
Continental Drift
The movement of crustal plates (lithosphere) upon a liquid mantle (asthenosphere)
Types of Layers
Movement of the Mantle
Convection Currents - Within the mantle, hot matter rises towards the crust and cooler matter sinks towards the core. These "currents" pull on the crust as if it were a conveyor belt.
Earth Structure
The deeper you go into the Earth, the pressure and temperature increases.
Since its formation, Earth has been cooling down. Earth's internal heat comes from 3 sources:
Heat from radioactivity
Heat from liquid iron becoming solid in the core
Heat from colliding particles during the formation of Earth
All of the layers and layers of rock create intense pressure within the planet.
Separating the internal layers by temperature, pressure, and density.
Separating the internal layers by the physical state (solid or liquid)
Compositional and Mechanical
Crust (solid) = solid rock
Mantle (liquid) = liquid rock
Core (solid) = mostly iron
Lithosphere - outermost cool and rigid shell (all of the crust and uppermost mantle). Moves as a solid.
Moho (Mohorovicic Discontinuity) - The line between the crust and the uppermost mantle. It is defined by a change in state in the rock (solid to liquid).
Asthenosphere - warm and weak layer (upper mantle). Partial melting of rocks near the top, complete melting of rocks near the bottom. Moves as a liquid.
Mesosphere - hot and rigid layer (lower mantle). Increased pressure at this depth strengthens the rock. Moves as a very thick liquid.
Outer core - very hot and weak layer. The motion of this layer produces the Earth's magnetic field. Moves as a liquid.
Inner core - very hot and rigid. Moves as a solid.
Wave Types
P (primary) are pressure waves. They are fast and can travel through both solids and liquids.
S (secondary) are shear waves. They are slow and can only travel through solids.
Types of Crust
Less dense
Thick (20-30 miles or 30-50 kilometers)
Common igneous rocks are granite and other light-colored felsic (silicon-rich) rocks.
Older (because it is not recycled)
Continental Crust
More dense
Thin (3-4 miles or 5-7 kilometers)
Common igneous rocks are basalt and other dark-colored mafic (magnesium-rich) rocks.
Younger (because it gets recycled)
Oceanic Crust
When rocks with a low melting temperature melt.
Felsic rocks melt at a low temperature.
Mafic rocks melt at a high temperature.
Partial Melting
Temperature controls density!
Hot matter rises.
Cold matter sinks.
First proposed by Alfred Wegener in 1915 when he wrote, "The Origin of Continents and Oceans."
1) The continents seem to "fit" together (like puzzle pieces)
2) The same fossils of freshwater species were found on both sides of the Atlantic Ocean. So Wegener concluded that the continents were once joined.
3) In World War II, U-boats (submarines) were equipped with radar. The first maps of the ocean floors were made showing the Mid-Atlantic Ridge. This was proof of Wegener's theory.
A large continent that is thought to have split into smaller ones in the geologic past.
A supercontinent that existed between 550 Mya and began to break up 200 Mya (and is still breaking apart!).
Means "All-Earth" in Greek.
Laurasia - The northern half of Pangea that makes up the northern continents today.

Gondwanaland - The southern half of Pangea that makes up the southern continents today.
A supercontinent that existed between 1 Bya and finally broke up 800 Mya (long before life existed on land).
Means "Motherland" in Russian.
Plate Tectonics
The theory that the Earth's crust is made of separate plates. The lithosphere (plates) float on the asthenosphere.
Where 2 plates slide past each other.
Driving Forces
The movement of the lithosphere (plates) is due to the movement of the asthenosphere (mantle)
Ridge Push
The force at divergent boundaries when hot mantle material pushes plates up and away. This pushes the rest of the plate out. Once at the surface, the hot mantle cools to form new crust.
The beginning of the "conveyor belt"
A break in rock mass along which movement has occurred.
(3 major types)
Plate Boundaries
Major geologic activity occurs at the edges of these plates where they push into one another, pull farther apart, or move sideways with tremendous power.

At these rims are the earthquakes, mountain building, and/or volcanic activity.
Where 2 plates come together.
3 types depending on the type of crust involved.
2 continental plates come together.

Example - Indian plate collided with the Eurasian plate to form the Himalaya Mountains.

Geologic Activity -
No volcanic activity
Shallow earthquakes over a broad area around the boundary
Seafloor age is not applicable
Formation of high elevation mountains
An oceanic plate comes together with a continental plate.

Example - The Nazca (oceanic) plate collides with the South American (continental) plate to form the Andes Mountains.

Geologic Activity:
Lines of volcanic activity
Shallow earthquakes on the subducting side and deep earthquakes on the over-riding side of the boundary
No consistent seafloor age
Formation of high elevation mountains on the continent and a low elevation trench in the ocean
2 oceanic plates come together.

Example - The Philippine plate collides with the Pacific plate to form the Mariana trench and the Mariana Islands (volcanic islands)

Geologic Activity:
Lines of volcanic activity (island arcs)
Shallow earthquakes on the subducting side and deep earthquakes on the over-riding side of the boundary
No consistent seafloor age
Formation of volcanic mountains (islands) on the overriding plate and a low elevation trench on the subducting plate
Rifting Zones
2 continental plates are pushed apart.

Example - The Red Sea in East Africa is spreading wider, pushing Saudi Arabia away from the rest of the African plate.

Geologic Activity -
Sporadic volcanic activity
Shallow earthquakes along the boundary
Formation of a gap between plates (rift) and eventually formation of new oceanic crust
Formation of short mountains on either side of the rift
Where 2 plates separate, or come apart.
Mid-Ocean Ridges (MORs)
2 oceanic plates are pushed apart.

Example - Iceland is a part of the Mid-Atlantic Ridge and is where the North American plate and the Eurasian plate push apart in opposite directions.

Geologic Activity -
Sporadic volcanic activity
Shallow weak earthquakes along the boundary
Formation of new oceanic crust
Formation of short underwater mountains
Fracture Zones
2 plates slide horizontally to each other to accommodate the stresses in a divergent boundary.

Example - The Mid-Atlantic Ridge is shaped like a zig-zag. The plates push apart at the North-South boundaries, and slide against each other at the East-West boundaries.

Geologic Activity:
Sporadic volcanic activity
Shallow earthquakes on the boundary
Usually involving new oceanic crust
No elevation change
Transform Faults
2 plates slide horizontally to each other.

Example - The Pacific plate is sliding by the North American plate to create the San Andreas Fault in Southern California.

Geologic Activity -
Broad zone of volcanic activity, or no volcanic activity at all
Shallow strong earthquakes over a broad area around the boundary
No consistent seafloor age
No elevation change
Trench Pull
The force at oceanic convergent boundaries where cold crust sinks down into the mantle, pulling on the rest of the plate. Once in the mantle, the cool crust melts.
The end of the "conveyor belt"
(or Slab Pull)
Anatomy of a Fault
Contact - the place where the movement occurs

Hanging wall - If you were "inside" the fault, the segment of rock that a lantern would hang on

Footwall - If you were "inside" the fault, the segment of rock that you would stand on
Where the hanging wall drops below the footwall.
Normal Fault
Where the hanging wall rises up above the footwall

(If contact is less than 45 degrees, it is called a Thrust Fault)
Reverse Fault
Where there is no footwall or hanging wall

The 2 halves of the fault slide horizontally to each other
Strike-Slip Fault
or Transform Fault
Igneous Rocks
Rocks that have solidified from a molten mass (lava or magma).

They can either be felsic (silicon rich) or mafic (magnesium rich), depending on if they formed on the continents or in the ocean.
Igneous Rocks
Rock that solidified on the surface from lava. (Also called a volcanic rock)
Aphanitic Texture - Smooth texture with small, invisible crystals. Crystals did not have time to grow large because the rock cooled quickly.
Felsic & Aphanitic
Mafic & Aphanitic
Igneous Rocks
Rock that solidified underground from magma.
Phaneritic Texture - Rough texture with large, visible crystals. Large crystals had time to form because the rock cooled slowly.
Felsic & Phanteritic
Mafic & Phanteritic
Other Extrusive
Igneous Rock Textures
Vesicular - Small, invisible crystals and holes from escaping gas. Forms from the eruption column of a volcano (in the air).
Pyroclastic - Mostly invisible crystals with holes and chuncks of stuff. Forms from the pyroclastic flow of a volcano (on the ground).
Earth's Internal
Geologists study earthquake waves as they move through the Earth.
4) Core samples of basalt show geologists that the Earth's magnetic field has "flipped" throughout geologic time. Magnetic minerals in magma line up with the Earth's magnetic field as the rock hardens. Once mapped, geologists can see identical patterns on either side of mid-ocean ridges, further supporting Wegener's theory.
Magnetic Reversals
Mapping the
Mid-Atlantic Ridge
Identical Freshwater Fossils
Puzzle Pieces
Usually associated with divergent boundaries.
Usually associated with convergent boundaries.
Usually associated with transform boundaries.
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