Rocks and weathering 3.1

Question 1

What are the three mechanisms of plate movement?

Answer 1

• Ridge push: intrusion of magma into the spreading ocean ridges such as the Mid-Atlantic ridge propels plates apart.
• Convection drag: convection currents in the plastic mantle drag the overlying lithosphere.
• Slab pull: cold and dense lithosphere sinking due to gravity pulls the rest of the plate with it.

Question 2

Convergent plate boundaries: continental/oceanic

Answer 2

• Form fold mountains and ocean trenches
• Less dense, oceanic plate subducts beneath the continental plate.
• Subduction causes layers of rock to be forced upwards causing anticlines and synclines - fold mountains.
• Material at depth is also forced downwards, deepening crust in the mountain belt.
• Sediments scraped off the plate and in the trench builds up an accretionary wedge.
• Subducted plate is partially melted and small pockets of magma rise to form volcanoes - orogenesis.
• The ocean floor drops steeply to form an ocean trench.
• Sloping zone of earthquake foci is known as the Benioff zone.

Question 3

Convergent plate boundaries: oceanic/oceanic

Answer 3

• All similar to continental/oceanic except no fold mountains.
• Instead, main features are island arcs and ocean trenches.
• Example is the Philippines and the Philippine trench.

Question 4

What are island arcs?

Answer 4

• Island chains with the same convexo-concave form.

- Made of active stratovolcanoes and sediment from accretionary wedges.

Question 5

Convergent plate boundaries: continental/continental

Answer 5

• Compressional stresses lead to the formation of earthquakes but with shallower foci.
• Example: the alps.

Question 6

Divergent plate boundaries

Answer 6

• Found at great ocean ridges.
• Example: Mid Atlantic Ridge.
• The ridge has a higher relief than the rest of the ocean floor as the ridges consist of hot, expanded rock.
• Magma rises in between gaps from two plates separating, forming new land as it cools.

Question 7

What is the evidence of continental drift?

Answer 7

• Coastline fit: Africa and South America
• Fossils, flora, and fauna: similarities in animal and plant fossils in South America and Africa.
• Fit of orogenic belts (fold mountain belts): Norway and Britain
• Fit of rock types: Precambrian rocks on either side of the Atlantic.

Question 8

What is sea floor spreading?

Answer 8

• The process of creation of new oceanic lithosphere at the ocean ridges as magma cools.
• Magma extruded along the ridge as dark-colored basalt.

Question 9

What is the evidence of sea floor spreading?

Answer 9

• Age of rocks on the ocean floor: radiometric dating.
• Thickness of sediment: increases with increasing distance from the ridge as more time to accumulate.
• Direct satellite measurements of ocean width.
• Magnetic stripes

Question 10

What are hot spots?

Answer 10

• Areas of volcanic activity not related to plate boundaries.
• Hot magma plumes from the mantle rise and burns through weak parts of the crust, creating volcanoes and islands.
• The plume remains in the same place but plates continue to move which causes chains of islands.
• Example: Hawaii