Plate Tectonics, Earthquakes, & Volcanoes

Plate Tectonics, Earthquakes, & Volcanoes

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Multiple Select

What evidence suggests that continents move?

• Shapes of continents

• Locations of mountain ranges and rock formations

• Fossil distribution

• Weather events

Alfred Wegener proposed the hypothesis of continental drift.

• Late 1800's geophysicist and meteorologist

• "Pangea"

Pangea

• A single large landmass that existed about 245 million years ago.

• Mountains formed as the land masses collided.

About 200 million years ago Pangea divided into two continents.

• Laurasia: Moved northward , then separated again into N.A. and Eurasia.

• Gondwana: Became S.A. and Africa, Antarctica, Australia, and India.

Open Ended

Why would many scientists not accept the hypothesis of continental drift?

What discoveries support the idea of continental drift?

Theory that continents shift in position over time.

Plate Tectonics

• Describes the large-scale movements in Earth's crust (lithosphere).

• Helps also explain how and why features in Earth's crust form and geologic events occur.

The lithosphere is made up of the crust and upper part of the mantle.

Lithosphere: Includes crust and upper, solid part of the mantle.

Asthenosphere: Also part of the mantle, but is more fluid and is able to flow slowly.

Convection currents cause the movement of material due to different densities caused by differences in temperature.

What is a tectonic plate?

• Divisions of the lithosphere.

• These plates move around on top of the asthenosphere due to convection currents in the asthenosphere.

• Plates move in different directions and at different speeds.

Tectonic plates vary in size, shape, and thickness.

• Continental plates (the ones with the continents) are thicker.

• Oceanic plates are thinner, but much more dense.

Multiple Choice

Referring only to the principal of superposition, what would you expected the sea-floor to look like?

• flat and smooth

• uneven and mountainous

Sea-floor mapping helped explain continental drift.

• Scientists expected the sea-floor to be smooth and level.

• Instead, they found huge underwater mountain ranges (mid-ocean ridges).

Age of the sea-floor

• Mid-ocean ridges form along cracks in the crust.

• The youngest rocks are found nearest to the ridge.

• The age of the rocks increases with distance from the ridge.

Magnetic properties of the sea-floor

• Rocks on either side of the ridge contain magnetic patterns that are mirror images of each other.

Sea-Floor Spreading

• Molten rock from inside the Earth rises through cracks, cools, and forms new crust along the ridge.

• The old crust breaks along the mid-point and moves apart in opposite directions.

• As the sea floor is pushed apart, so are the continents on the same plate.

Open Ended

If the sea floor has been spreading for millions of years, why is the Earth not getting larger?

Ocean Trenches

• Scientists have found huge trenches (deep canyons) in the sea floor.

• Dense oceanic crust is sinking at the trench at the same rate as new crust is forming at the ridge.

The most dramatic changes in Earth's crust take place along plate boundaries.

• May be on the ocean floor, the edges of continents, or within continents.

There are three types of plate boundaries.

• Convergent: where two plates collide.

• Divergent: where two plates separate.

• Transform: where two plates slide past one another.

What causes tectonic plates to move?

• MANTLE CONVECTION: drags plates along.

• RIDGE PUSH: moves plates away from mid-ocean ridges as rock cools.

• SLAB PULL: tugs plates along as the dense edge of a plate sinks below another.