1. The Seafloor (69)

2. Contiental Shelf
Ocean basins, which are low areas of Earth that are filled with water, have many different features.
The continental shelf is the gradually sloping end of a continent that extends under the ocean as deep as 350 meters.

3. Beyond the shelf, the ocean floor drops more steeply, forming the continental slope.
The continental slope extends from the outer edge of the continental shelf down to the ocean floor.

4. Beyond the continental slope lie the trenches, valleys, plains, mountains, and ridges of the ocean basin.
In the deep ocean, sediment, derived mostly from land, settles constantly on the ocean floor.
These deposits fill in valleys and create flat seafloor areas called abyssal plains.
Some areas of abyssal plains have small hills and seamounts.
Seamounts are underwater, inactive volcanic peaks.

6. Ridges and Trenches
Mid-ocean ridges can be found at the bottom of all ocean basins. They form a continuous underwater ridge approximately 70,000 km long.
A mid-ocean ridge is the area in an ocean basin where new ocean floor is formed

7. As crustal plates move, the ocean floor changes.
When ocean plates separate, hot magma from Earth’s interior forms new ocean crust.
New ocean floor forms along mid-ocean ridges as lava erupts through cracks in Earth’s crust.
When the lava hits the water, it cools quickly into solid rock, forming new seafloor.

8. Subduction Zone
On the ocean floor, subduction zones are marked by deep ocean trenches.
A trench is a long, narrow, steep-sided depression where one crustal plate sinks beneath another.

9. Life in the Ocean
Marine organisms such as plants and algae use energy from the Sun to build their tissues and produce their own food.
This process of making food is called photosynthesis.
Chemosynthesis involves using sulfur or nitrogen compounds as an energy source, instead of light from the Sun, to produce food.
Bacteria that perform chemosynthesis using sulfur compounds live along mid-ocean ridges near hydrothermal vents where no light is available.

10.
Marine organisms that drift with the currents are called plankton.
Plankton range from microscopic algae and animals to organisms as large as jellyfish.
Most phytoplankton plankton that are producersare one-celled organisms that float in the upper layers of the ocean where light needed for photosynthesis is available.

11. Plankton
One abundant form of phytoplankton is a once-celled organism called a diatom.
Diatoms and other phytoplankton are the source of food for zooplankton, animals that drift with ocean currents.

12. Coral Reefs
Corals thrive in clear, warm water that receives a lot of sunlight.
Each coral animal builds a hard capsule around its body from the calcium it removes from seawater.
Each capsule is cemented to others to form a large colony called a reef.
A reef is a rigid, wave-resistant structure built by corals from skeletal material.

14. What is the continental shelf?
The continental shelf is the gradually sloping end of a continent that extends under the ocean.

15. Which structure extends from the outer edge of the continental shelf to the ocean floor?
A. abyssal plain
B. continental slope
C. oceanic trench
D. seamount

16. What is the area in an ocean basin where new ocean floor is forming?
New seafloor forms at mid-ocean ridges as lava erupts through cracks in Earth’s crust.

17. Which process involves using sulfur or
nitrogen compounds as an energy source to produce food?
A. chemosynthesis
B. nitrosynthesis
C. photosynthesis
D. sulfurosynthesis