Plate Tectonics Chapter 9.2 – 9.3

Plate Tectonics Proposed in 1965 by Tuzo Wilson = combination of Wegener & Hess’s ideas.Proposed in 1965 by Tuzo Wilson = combination of Wegener & Hess’s ideas. Convection Currents move the lithospheric plates causing geologic activity – (mountain building, volcanic eruptions and earthquakes)Convection Currents move the lithospheric plates causing geologic activity – (mountain building, volcanic eruptions and earthquakes)

Tectonic Plates: Pieces of lithosphere that move around on top of the asthenosphere Tectonic Plates

About the tectonic plates… –There are 7 large plates and many smaller ones Not all plates are the same shape and size –Plates can contain either oceanic or continental crust, or both About the Tectonic Plates

Plates you need to know –North American Plate –Arabian Plate –African Plate –South American Plate –Pacific Plate –Indian-Australian Plate –Antarctic Plate –Eurasian Plate –Caribbean Plate –Nazca Plate Plates you need to know

Juan de Fuca Fiji Cocos Scotia

Plate Tectonics Reference Table You need to know how to read this table!!

Convection Currents Upper Mantle - COOLER Lower Mantle - HOTTER The magma in the lower mantle is warmer. It expands, becomes less dense and rises. When it reaches the upper mantle it cools, contracts, becoming more dense and sinks. This constant rise and fall of magma causes the convection currents that drive plate tectonics. The crust plates ride along on top of these convection currents.

The plates have boundariesThe plates have boundaries As they move they affect one another.As they move they affect one another. Convergent Boundary Divergent Boundary Transform Boundary

Plate Boundaries!! Divergent Convergent Transform

Tectonic Boundaries Tectonic plates are constantly moving in different directions –Where two plates meet is called a boundary Each boundary has certain geological characteristics and processes associated with it

Divergent Boundary When Plates Divide or SeparateWhen Plates Divide or Separate

Sea Floor Spreading –Sea floor spreading: A divergent boundary in the ocean Creates new oceanic crust –This can cause ocean basins to grow wider This is the reason the Atlantic Ocean is getting larger

Step 1 – Rift Valley Magma pushes up cracks in the crustMagma pushes up cracks in the crust Convection causes the separated plates to move and form a rift valley.Convection causes the separated plates to move and form a rift valley.

Step 2 – Add Water The valley widens and begins to be filled with water. (Red Sea)The valley widens and begins to be filled with water. (Red Sea)

Step 3 – Ocean Formed Eventually an ocean formsEventually an ocean forms –At the rate of 6 cm per year Ex. Mid Atlantic Ridge

Divergent Boundaries Map

Transform Boundary Two Plates slide past each otherTwo Plates slide past each other Also called strike-slip boundaryAlso called strike-slip boundary Ex: San Andreas Fault in CaliforniaEx: San Andreas Fault in California

San Andreas Fault The Pacific Plate moves NorthThe Pacific Plate moves North The North American Plate moves SouthThe North American Plate moves South

San Andreas Fault Causes stress that leads to Earthquakes in CaliforniaCauses stress that leads to Earthquakes in California

Convergent Boundaries 2 Plates colliding into each other.2 Plates colliding into each other. Example: Himalayan Mountains formed when India collided with AsiaExample: Himalayan Mountains formed when India collided with Asia

Oceanic – Continental Convergence 1. Collision!!!1. Collision!!! 2. Continental Crust is less dense so Oceanic crust sinks (subducts) into the asthenosphere.2. Continental Crust is less dense so Oceanic crust sinks (subducts) into the asthenosphere.

Trenches Trench: A long, steep- sided valley on the ocean floor caused by a convergent plate boundary –Caused by subduction zones –Formed where the ocean plate meets the continental or oceanic plate –Destroys the ocean floor –Challenger Deep in the Mariana Trench is the deepest point in Earth's oceans (6.8 miles below sea level)

Oceanic – Continental Convergence 3. Subducting Ocean Plate creates a V- shaped Trench.3. Subducting Ocean Plate creates a V- shaped Trench. 4. Oceanic Plate reaches the asthenosphere and melts.4. Oceanic Plate reaches the asthenosphere and melts.

Oceanic – Continental Convergence 5. The melted magma begins to rise through the mantle and crust.5. The melted magma begins to rise through the mantle and crust. 6. It will rise to make a volcanic eruption.6. It will rise to make a volcanic eruption. 7. Lava cools forming a continental volcanic arc.7. Lava cools forming a continental volcanic arc.

Oceanic – Continental Examples Andes Volcanic Mountains (South America)Andes Volcanic Mountains (South America) Cascades Mountain Range (Mt. St. Helens)Cascades Mountain Range (Mt. St. Helens)

Oceanic – Oceanic Boundary 1. Collision!!!1. Collision!!! 2. The more dense ocean plate (older) subducts into the asthenosphere.2. The more dense ocean plate (older) subducts into the asthenosphere.

Oceanic – Oceanic Boundary 3. Oceanic crust melts into magma.3. Oceanic crust melts into magma. 4. Magma becomes buoyant and rises.4. Magma becomes buoyant and rises. 5. This eventually forms volcanic islands in the water.5. This eventually forms volcanic islands in the water.

Example of Oceanic-Oceanic Boundaries ALUETIAN ISLANDS

Example of Oceanic-Oceanic Boundaries Northern Mariana Islands

Continental – Continental Boundary 1. Collision!!!1. Collision!!! 2. Neither plate subducts because they are both low in density.2. Neither plate subducts because they are both low in density.

Continental – Continental Boundary 3. Crust is compressed and deformed, forced upward forming a mountain range.3. Crust is compressed and deformed, forced upward forming a mountain range.

Mountains Mountains: Formed when two continental plates push into each other –You can tell relative ages of mountains based on how eroded they are—sharp, pointy mountains are younger, rounded mountains are older The Appalachian Mountains were formed around 480 mya The Rocky Mountains were formed about 80 mya

Example: Continental- Continental Convergence Mt. Everest! (Himalayan Mts.)Mt. Everest! (Himalayan Mts.)

Convection Currents basic driving force –Hot, less dense magma rises toward the crust while cooler, more dense magma sinks –Plates move because of the uneven distribution of the Earth’s heat –Mantle plumes are masses of hotter-than- normal mantle material that ascend toward the surface, where they may lead to igneous activity. –The unequal distribution of heat within Earth causes the thermal convection in the mantle that ultimately drives plate motion. Causes of Plate motion

The pulling of old crust down in the mantle because by the downward flow of more dense magma in the convection current Is a mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along. It is thought to be the primary downward arm of convective flow in the mantle. Slab-pull

The down-ward pull, from gravity, on the oceanic lithosphere at the ridge Causes oceanic lithosphere to slide down the sides of the oceanic ridge under the pull of gravity. It may contribute to plate motion. Ridge –push

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Keeping it straight Tectonic Boundaries Convergent Subduction Zone TrenchesMountainsDivergent Rift Valley Sea Floor Spreading TransformRidgesFault