Bumps on the Sea Surface: What a Mapping Satellite Shows About Moving Plates

Use seafloor depths, rock ages, fracture zones and trenches as evidence of plate motion.

Across the Mid-Atlantic Ridge

Read the mission briefing and Table 1, then answer the questions.

Mission Planning Briefing: A Map of the Seafloor From Orbit

1A space agency is getting a new satellite ready that will measure the height of the ocean surface to within a few centimeters. At a mission planning briefing, oceanographer Dr. Marguerite Okonjo explained one of its jobs to the team and to two seventh graders in the agency's summer intern program, Ines Valderrama and Kwabena Thorsen. The ocean surface is not perfectly flat. Above an undersea mountain, the extra mass of rock pulls water toward it, so the sea surface stands up to a few meters higher. Above a deep trench, it sags. Computers turn these bumps and dips into a map of the seafloor.

2To test the map, the team will compare it with depths measured by ships using sonar along a line across the South Atlantic Ocean, from Brazil toward southern Africa (Table 1). Halfway across, the line crosses the Mid-Atlantic Ridge, an underwater mountain range. Along its crest, lava erupts from long cracks and hardens into new seafloor rock. Rock that has just formed is hot, and hot rock takes up a little more space than the same rock does after it cools. Drill cores collected along the line give the age of the seafloor rock.

3The map will also show fracture zones, long, straight valleys in the seafloor. Each one cuts across the ridge and breaks it into pieces that are offset from one another, like steps. Between two offset ridge pieces, the seafloor on one side slides past the seafloor on the other side, and small earthquakes are common there. Beyond the ridge pieces, each fracture zone continues as a quiet scar for more than 1,000 km toward each continent. In the Atlantic near the equator, the fracture zones run nearly east to west. Their western ends point at bends in the coast of Brazil, and their eastern ends point at bends of a matching shape in the coast of West Africa.

4Later, the satellite will map the Pacific Ocean (Table 2). Along much of the Pacific's edge are trenches, narrow valleys in the seafloor more than 8 km deep, with chains of volcanoes and many deep earthquakes on the land beside them. Drilling has found no ocean floor anywhere that is older than about 200 million years, but some rocks on the continents are about 4,000 million years old.

Table 1. Ship depths and drill-core ages along the line
Site	From ridge (km)	Depth (km)	Rock age (million yr)
W3	1,200 west	5.2	60
W2	800 west	4.8	40
W1	400 west	4.1	20
Ridge crest	0	2.7	0
E1	400 east	4.1	20
E2	800 east	4.7	40
E3	1,200 east	5.2	60

Which statement best describes the pattern in Table 1?

A. Moving away from the ridge in either direction, the seafloor gets deeper and older.
B. The seafloor is about the same depth everywhere, but its age goes up and down.
C. The seafloor gets deeper and older going west, but shallower and younger going east.
D. The seafloor is deepest and oldest at the ridge crest and gets shallower toward each coast.

Use Table 1. The seafloor at Site E2 is km deeper than the seafloor at the ridge crest.

Use paragraph 2. Which explanation of the depths in Table 1 fits the briefing best?

A. Rock forms hot at the ridge, then moves away and cools, so it takes up less space and sits lower.
B. Seafloor is squeezed together from both sides at the ridge, which pushes the middle upward.
C. Mud piles up thickest on the ridge crest, which raises it above the seafloor on each side.
D. The two continents press down on the old seafloor near them, which pushes it deeper.

The team plans one more ship check 1,000 km west of the ridge. If the pattern in Table 1 continues, what should the ship find there?

A. Seafloor about 5 km deep, with rock about 100 million years old
B. Seafloor about 5 km deep, with rock about 50 million years old
C. Seafloor about 3 km deep, with rock about 10 million years old
D. Seafloor about 6 km deep, with rock about 50 million years old

Use Sites W3 and E3. Their rock formed at the ridge crest 60 million years ago. 1,200 km is 120,000,000 cm.

On average, the rock at each site moved away from the ridge about cm per year. So Sites W3 and E3 moved apart from each other about cm per year.

Fracture zones and trenches

Use paragraphs 3 and 4 and Table 2.

Use paragraph 3. Between the offset ridge pieces, the seafloor on one side of a fracture zone slides past the other side. In the Atlantic near the equator, which way have the plates on each side of the ridge most likely moved?

A. Toward the ridge from both sides, along the fracture zones
B. Up and down, as each ridge piece rose higher than the next
C. East and west, away from the ridge and along the fracture zones
D. North and south, straight across the fracture zones

Circle the two observations that are evidence that South America and Africa have moved apart.

The sea surface stands a few meters higher above undersea mountains.Fracture zones run from bends in Brazil's coast to bends of a matching shape in West Africa's coast.Trenches more than 8 km deep line much of the Pacific's edge.Rock is 0 million years old at the ridge crest and 60 million years old 1,200 km toward each coast.Some rocks on the continents are about 4,000 million years old.

Table 2. Two oceans compared
Feature	Atlantic Ocean	Pacific Ocean
Mid-ocean ridge	Yes, near the middle	Yes, toward the east side
Trenches at the edges	Very few	Along most edges
Volcano chains near coast	Very few	Along most coasts
Oldest seafloor (million yr)	about 180	about 180

No ocean floor is older than about 200 million years, but some continental rocks are about 4,000 million years old. Which explanation fits the briefing best?

A. Old seafloor rock is buried under so much mud that the drills stop before reaching it.
B. Seafloor rock wears away in seawater much faster than rock on land wears away in air.
C. New seafloor forms at trenches, and the first trenches opened about 200 million years ago.
D. Old seafloor sinks back down into Earth at trenches, while continents stay at the surface.

Use Table 2 and the briefing. Which statement about the two oceans today is best supported?

A. The Pacific is likely getting wider, while Atlantic seafloor sinks back into Earth along its ridge.
B. The Atlantic is likely getting wider, while Pacific seafloor sinks at trenches as new floor forms at its ridge.
C. The floors of both oceans likely stopped moving, since their oldest seafloor is the same age.
D. Both oceans are likely losing seafloor at the trenches that line most of their edges.

Planning the Pacific map

Use the whole briefing and both tables.

10.

Before the satellite maps the Pacific, the team lists predictions. Sort each one using the evidence from the Atlantic and the briefing.

Seafloor that gets deeper with distance from the Pacific ridgeSeafloor rock 500 million years old beside a trenchSeafloor rock that gets older with distance from the Pacific ridgeThe oldest Pacific seafloor lying along the ridge crestA sea surface that sags above each deep trenchA sea surface that sags above each undersea volcano

Expected on the Pacific map	Not expected on the Pacific map

11.

Kwabena says the matching bends in the coasts of Brazil and West Africa are just a coincidence. Which response is best supported by the briefing?

A. The bends line up with fracture zones that mark where the coasts split.
B. Lava from the ridge flowed to both coasts and hardened into the same bends.
C. Trenches along the Atlantic's edges carved both bends at the same time.
D. Waves from the same storms carved both bends during the last 100 years.

12.

Write the summary for the mission briefing. Make a claim about whether South America and Africa have always been where they are today. Use evidence from Table 1 and from at least one other ocean feature (fracture zones or trenches), and explain your reasoning.