NJSLA Review 34

The Atlantic Ocean between the east coast of the United States and the west coast of Africa is about 6,000 kilometers (km) wide. The age of the oceanic lithosphere between them is 280,000,000 years old. What is the average rate of seafloor spreading? Hint: rate of movement (or velocity) is calculated as distance/time.

• 2 km/year

• 280 km/year

• 0.00002 km/year

• We cannot calculate the rate from these data.

How do mid-ocean ridges form?

• Plates drift apart and form volcanoes.

• Plates collide and push Earth's crust upward.

• Plates drift apart, and new crust forms from magma.

• Plates collide, and one plate is forced down under the other plate.

What do the shapes of the continents tell us about past plate motion?

• Plate movement eroded continents in similar ways.

• Ocean plates are forced beneath other plates when the two plates collide.

• New rock is created along the mid-ocean ridges, which has pushed continents apart.

• The continents may have been joined together, and then plate motion moved them apart.

The rocks that make up the continents are 3 to 4 billion years old. Why are the oldest rocks on the ocean floor only 280 million years old?

• Oceans were formed 280 million years ago.

• Oceanic plates are forced beneath other plates that they collide with.

• Ocean rocks are melted and destroyed at the volcanic mid-ocean ridges.

• It is more difficult to determine the age of ocean rocks than of continental rocks, so the ages of ocean rocks are less accurate.

The following table shows the ages and locations of four different fossils. Which claim do these data support?

• South America and Africa split apart 200 million years ago.

• South America, Africa, Asia, and Antarctica were joined 200 million years ago.

• South America, Africa, Asia, Antarctica, and Australia were joined 350 million years ago.

• Australia split away from the supercontinent between 300 and 250 million years ago.