Earth Science: Mapping the Ocean Floor: Trenches and Ridges
Interpreting seafloor maps, mid-ocean ridges, and deep-ocean trenches
Earth Science: Mapping the Ocean Floor: Trenches and Ridges
Interpreting seafloor maps, mid-ocean ridges, and deep-ocean trenches
Earth Science - Grade 6-8
- 1
A map of the Atlantic Ocean shows a long underwater mountain chain running roughly north to south near the center of the ocean basin. Identify this feature and explain what type of plate boundary is found there.
Look for a feature where plates separate and magma rises.
The feature is the Mid-Atlantic Ridge. It forms at a divergent plate boundary where tectonic plates move apart and new ocean crust forms. - 2
A deep, narrow valley on the ocean floor is found next to a chain of volcanic islands. What ocean floor feature is most likely present, and what process forms it?
The feature is most likely a deep-ocean trench. It forms where one tectonic plate subducts, or sinks, beneath another plate at a convergent boundary. - 3
A sonar ship sends a sound pulse to the seafloor. The pulse returns after 8 seconds. If sound travels through seawater at about 1,500 meters per second, what is the ocean depth at that location?
Divide the total round-trip distance by 2.
The sound traveled down to the seafloor and back up, so the total distance was 1,500 meters per second times 8 seconds, which equals 12,000 meters. The ocean depth is half of that distance, so the depth is 6,000 meters. - 4
An ocean floor profile shows a seafloor depth of 3,000 meters, then a raised ridge at 1,500 meters deep, then a return to 3,000 meters deep. Explain why the ridge appears shallower than the surrounding seafloor.
The ridge appears shallower because it is an underwater mountain range. At a mid-ocean ridge, rising magma creates new crust and builds up the seafloor, making it higher than the surrounding ocean basin. - 5
Scientists find that rocks close to a mid-ocean ridge are younger than rocks farther away from the ridge. What does this pattern show about the ocean floor?
Think about where new crust is created.
This pattern shows that new ocean crust forms at the ridge and moves away over time. The farther the rock is from the ridge, the older it usually is. - 6
Compare a mid-ocean ridge and a deep-ocean trench. Describe one way they are similar and one way they are different.
A mid-ocean ridge and a deep-ocean trench are both major features of the ocean floor related to plate movement. They are different because a ridge forms where plates move apart, while a trench forms where one plate sinks beneath another plate. - 7
A student labels the deepest part of an ocean floor map as a mid-ocean ridge. Explain why this label is probably incorrect.
Ridges are raised features, while trenches are deep features.
The label is probably incorrect because a mid-ocean ridge is an underwater mountain chain and is usually higher, or shallower, than nearby seafloor. The deepest parts of the ocean are usually trenches. - 8
Use the following seafloor data from a ship traveling across an ocean basin: Point A is 4,000 meters deep, Point B is 2,200 meters deep, Point C is 1,600 meters deep, Point D is 2,300 meters deep, and Point E is 4,100 meters deep. Which point is most likely on a ridge, and how do you know?
The top of an underwater mountain has a smaller depth than the surrounding seafloor.
Point C is most likely on a ridge because it has the smallest depth value. A smaller depth means the seafloor is closer to the ocean surface, which matches a raised ridge. - 9
On a map, a deep trench is located along the edge of the Pacific Ocean near a continent. Earthquakes are common in this area. Explain how the trench and earthquakes are related.
The trench and earthquakes are related because both can occur at a convergent plate boundary. As one plate subducts beneath another, stress builds up and is released as earthquakes. - 10
Why is mapping the ocean floor important for understanding plate tectonics?
Ocean floor features give clues about plate motion.
Mapping the ocean floor is important because it shows the locations of ridges, trenches, and other features that mark plate boundaries. These maps help scientists understand where plates move apart, collide, or sink into the mantle.