Rates of PlatesReflect & Connect #1-5 p. 670-672

R&C #1 p. 670

• Question: Basins in rifts on continents, like the Rio Grande Rift, can be over 10 km deep. Why do you think they appear as broad valleys instead of deep troughs that are 10km deep? What processes can you infer are occurring in the land or mountains on the flanks of the rifts?

R&C #1 p. 670

• When a divergent boundary occurs beneath a thick continental plate, the pull-apart is not vigorous enough to create a clean, single break through the thick plate material. The thick continental plate is arched upwards from the convection current's lift, pulled thin and fractured into a rift-shaped structure. As the two plates pull apart, normal faults develop on both sides of the rift and the central blocks slide downwards. Earthquakes occur as a result of this fracturing and movement. Mountains next to the rifts are weathering and eroding, filling the rift valley with gravel and sediment. Streams and rivers may flow into the sinking rift valley to form a long linear lake.

R&C #3a p. 670

3a: The Arabian Plate is moving away from Africa with a velocity of 20 mm/yr. How far will the Arabian Plate move in 10 Myr?

20mm 20km1 yr 1 Myr=

20 km1Myr x 10Myr = 200 km

R&C #3b p. 670

3b: Where do you predict zones of rifting? Where do you predict zones of collision?

Rifting would continue between theArabian and African plates. This is the active zone of rifting. Collision would occur where the Arabian plate is colliding with the Eurasian plate, northeast of the Persian Gulf in Iran.

R&C #3b p. 670

3c: What do you predict will happen to the Red Sea? To the Persian Gulf?

The Red Sea will get wider, and the Persian Gulf will shrink.

R&C #4

4c. Assume that South America and Africa started rifting apart at 120 Mya. Estimate the velocity of South America relative to Africa.

Average distance = 5,500 kmTime = 120 MyrVelocity = distance/time

5,500 km120 Myr = 45.8 km/Myr

= 45.8 mm/yr

R&C #4d & 5

4d. What can you infer about the changes in the size of the Pacific Ocean over the same time period?

As the Atlantic Ocean opened, the Pacific Ocean got smaller between North and South America and Eurasia.

5. The North Atlantic also got bigger over the same period of time.