Forms of Relief
Senior 1 · Geography
Forms of Relief
Lesson 3 of 4
22 min
The longest mountain range on Earth is not the Andes or the Himalayas. It runs for tens of thousands of kilometres under the oceans, and almost nobody has seen it. Rwanda has no coast, but the ocean floor matters to Rwanda more than you might think: nearly everything the country imports, and the internet you use, crosses it.
Figure 1. Relief features of the ocean floor (schematic). The vertical scale is stretched many times so the shapes can be seen.
Definition
Continental shelf: the gently sloping edge of a continent that lies under shallow sea, usually down to about 200 m deep. Its width varies from almost nothing to more than 1,000 km. Example: the narrow shelf along the coast of Kenya and Tanzania.
Definition
Continental slope: the steep slope from the outer edge of the continental shelf down to the deep ocean floor, at about 3,000 to 4,000 m.
Definition
Abyssal plain: a very flat, deep area of the ocean floor, usually 3,000 to 6,000 m below sea level, covered with fine sediment. Example: the Somali Abyssal Plain in the western Indian Ocean.
Definition
Mid-ocean ridge: a long chain of undersea mountains running through the middle of an ocean, where two plates are moving apart and new ocean floor forms. Example: the Mid-Atlantic Ridge, whose peaks rise above the sea as Iceland.
Definition
Ocean trench: a long, narrow, very deep valley in the ocean floor, often next to a chain of volcanic islands. Example: the Mariana Trench in the Pacific Ocean, whose deepest point, the Challenger Deep, is about 10,935 m below sea level.
Definition
Seamount and guyot: a seamount is an undersea volcano that does not reach the surface; a guyot is a seamount whose top has been worn flat by waves when it once stood at sea level.
| On the continents | Its "twin" under the ocean | Example |
|---|---|---|
| Plain | Abyssal plain | Eastern lowlands of Rwanda / Somali Abyssal Plain |
| Mountain range | Mid-ocean ridge | Rwenzori / Mid-Atlantic Ridge |
| Rift valley | Rift along the ridge crest | Albertine Rift / the valley down the middle of the Mid-Atlantic Ridge |
| Volcano | Seamount or volcanic island | Karisimbi / the Comoros islands |
| Deep valley | Ocean trench | Lake Tanganyika's basin / the Mariana Trench |
Worked example 1: shelf or slope?
A continental shelf falls 200 m over a horizontal distance of 70 km. The continental slope beyond it falls 3,000 m over 50 km. Compare their gradients.
Answer. The shelf is about 1 in 350, flatter than most roads; the slope is about 1 in 17, some 20 times steeper. This is why the shelf is easy for fishing boats and cables, and the slope is where undersea landslides happen.
Worked example 2: deeper than Everest is high
Mount Everest is 8,849 m high; the Challenger Deep is about 10,935 m deep. If Everest stood on the floor of the Challenger Deep, how far below the surface would its summit be?
The summit would still be about 2 km under water.
Important
Case study: a landlocked country and the ocean floor
Even Rwanda's lakes have floor relief: Lake Kivu has steep underwater slopes, deep basins and underwater volcanic features, just like a small ocean.
Ocean Floor Features, by Lincoln Learning Solutions (5 min). As you watch, sketch the profile in Figure 1 and add each feature as it is named.
Imagine a company wants to lay a new internet cable from Mombasa to India. Using Figure 1, which features of the ocean floor would the company want to avoid, and which would be easiest to cross? Explain your choices.
Tip
Walk out from the beach: Shelf, Slope, Rise, Plain. "Some Sailors Rarely Panic." Then the ridge in the middle, and the trench, the deepest of all.
Practice
Remember
Sources and credits
The lesson text, ocean-floor diagram, table, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
