Geology 12Geology 12Plate BoundariesPlate Boundaries

There are three types of plate There are three types of plate boundariesboundaries

1.1. Divergent (or constructive) plate Divergent (or constructive) plate boundaryboundary

2.2. Convergent (or destructive) plate Convergent (or destructive) plate boundaryboundary

3.3. Transform (or slip & slide) plate Transform (or slip & slide) plate boundaryboundary

Good site?

Divergent Plate Boundary

Divergent Plate Boundary Plates move away from each other, often

causing mountain or ridge building (constructive)

Result is to create new ocean basins (such as the Atlantic) or huge valleys on land (called rift valleys)

As the plates move away from each other, magma flows from the mantle (same as water between two blocks of wood)

Divergent Plate Boundary Mid-ocean ridges are formed when plates

move apart. They are giant undersea mountain ranges that extend around the world. One example is the Mid-Atlantic Ridge.

Earthquakes & eruptions of rock are frequent along the ridges. Iceland formed atop a ridge.

Exotic creatures often live along ridges.

Convergent Plate Boundaries With convergent plate boundaries, plates move toward each

other There are three possible collisions:1. Oceanic crust colliding with oceanic crust causes one to dive

under the other (often forming deep trenches or island chains)

2. Heavier, denser and thinner oceanic crust colliding with the continental crust causes the oceanic crust to dive (often forming deep trenches)

3. Two colliding continental crusts cause crumpling & mountain building (ex. Himalayas & Mt. Everest; Appalachians of Eastern North America, including the mountains of Eskasoni)

Convergent Plate Boundaries

Transform Boundaries With transform boundaries, one plate slides

horizontally past another plate The result is often a fault or massive “crack”

in the Earth’s crust (ex. San Andreas fault in California; Aspy Fault in C.B. Highlands)

Earthquakes are very common (and often violent) along transform boundaries

Transform Boundaries

Major Plates of the WorldMajor Plates of the World

Model of Mantle Model of Mantle ConvectionConvection

What causes plate motions?What causes plate motions?

Scientists once thought that heat from the core Scientists once thought that heat from the core causes the plates to move. However, it is causes the plates to move. However, it is now believed that three types of forces are now believed that three types of forces are at work:at work:

1.1. Ridge Push: Plates near mid-ocean ridges Ridge Push: Plates near mid-ocean ridges are pushed apart by the rising magma. The are pushed apart by the rising magma. The plates then slide down the oceanic ridges.plates then slide down the oceanic ridges.

2.2. Slab Pull: The cold, dense leading edge of Slab Pull: The cold, dense leading edge of the sinking plate may pull the rest of the the sinking plate may pull the rest of the plate with it.plate with it.

3.3. Trench Suction: If a sinking plate falls at a Trench Suction: If a sinking plate falls at a steep angle into a trench, the rest of the steep angle into a trench, the rest of the plate is “sucked” forward.plate is “sucked” forward.

What are mantle plumes?What are mantle plumes?

Mantle plumes are narrow columns of Mantle plumes are narrow columns of hot, rising magma. These plumes hot, rising magma. These plumes form “hot spots” of active volcanism form “hot spots” of active volcanism at the Earth’s surface.at the Earth’s surface.

Mantle plumes have formed Iceland, Mantle plumes have formed Iceland, the Hawaiian Islands, and Ol’Faithful the Hawaiian Islands, and Ol’Faithful in Yellowstone Park (USA)in Yellowstone Park (USA)

Mantle PlumeMantle Plume

Results of Magma PlumesResults of Magma Plumes Formation of “hot spots” of active Formation of “hot spots” of active

volcanism around the worldvolcanism around the world May help to drive plates apart (ex. May help to drive plates apart (ex.

Mid-Atlantic Ridge)Mid-Atlantic Ridge) Formation of domes, or bulges in Formation of domes, or bulges in

the crust, which may produce the crust, which may produce volcanic eruptionsvolcanic eruptions

Continent splitting (ex. Yellowstone Continent splitting (ex. Yellowstone Park in U.S.A.)Park in U.S.A.)

Formation of seamounts and Formation of seamounts and guyots, often in chainsguyots, often in chains

Importance of Understanding Importance of Understanding Plate TectonicsPlate Tectonics

Explains many geological features, Explains many geological features, such as volcanoes, earthquakes, sea-such as volcanoes, earthquakes, sea-floor spreading, seamounts, hot floor spreading, seamounts, hot spots, and general features of Earthspots, and general features of Earth

Important to the prediction of Important to the prediction of volcanoes and earthquakes to help volcanoes and earthquakes to help protect peopleprotect people

Explains much of the geology of Explains much of the geology of Canada, including the hills of Canada, including the hills of Eskasoni and the Canadian ShieldEskasoni and the Canadian Shield

Importance of Importance of Understanding Plate Understanding Plate

TectonicsTectonics Explains the breakup of the Pangea Explains the breakup of the Pangea

SupercontinentSupercontinent Important mineral resources often Important mineral resources often

formed at plate marginsformed at plate margins BASICALLY, THE THEORY OF PLATE BASICALLY, THE THEORY OF PLATE

TECTONICS CONNECTS TOGETHER TECTONICS CONNECTS TOGETHER MOST TOPICS IN GEOLOGY, MOST TOPICS IN GEOLOGY, INCLUDING THIS COURSE!!!INCLUDING THIS COURSE!!!

SeamounSeamountsts

Define the following:Define the following:

Hot spotHot spot Volcanic domeVolcanic dome SeamountSeamount GuyotGuyot PangeaPangea