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Name
CHAPTER 23
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Date
The Ocean Basins
2 Features of the Ocean Floor
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• What are the main features of the continental margins?
• What are the main features of the deep-ocean basin?
What Are the Parts of the Ocean Floor?
The ocean floor can be divided into two main parts:
the continental margins and the deep-ocean basin. The
continental margins are the shallow parts of the ocean
floor that are made of continental crust. The deep-ocean
basin, which is made of oceanic crust, begins at the edge
of the continental margin. The deep-ocean basin is the
deepest part of the ocean.
Continental
margin
Deep-ocean basin
READING TOOLBOX
Organize When you
finish reading this section,
create a concept map that
contains the following terms:
continental shelf, deep-ocean
basin, continental margin,
continental rise, abyssal
plain, trench, mid-ocean
ridge, continental slope, and
seamount.
Trench
Mid-ocean
ridge
Abyssal
plain
LOOKING CLOSER
1. Identify What are two
features found in the deepocean basin?
The two parts of the ocean floor are the deep-ocean basin and the continental
margins.
What Are the Parts of the Continental
Margins?
The continental crust and the oceanic crust do not
meet at the shoreline. Instead, they meet beneath the
ocean, at the continental margin. The continental margin
is made of three main parts: the continental shelf, the
continental slope, and the continental rise.
The continental shelf is the part of the margin that
begins at the shoreline and slopes gently toward the open
ocean. It continues until the ocean floor begins to slope
more steeply. The average depth of the water covering a
continental shelf is about 60 m.
READING CHECK
2. Describe Where do
continental crust and oceanic
crust meet?
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The Ocean Basins
Name
SECTION 2
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Date
Features of the Ocean Floor continued
CONTINENTAL SLOPE
Talk About It
Learn Word Roots Use a
dictionary or the Internet to
learn the meaning of the
prefix sub-. With a partner,
talk about why submarine
canyons are called
submarine canyons.
The continental slope is the steepest part of the continental margin. It begins at the edge of the continental
shelf and continues down to the flattest part of the ocean
floor. The boundary between the continental crust and
the oceanic crust is at the base of the continental slope.
Submarine canyons can form in the continental shelf
and continental slope. Submarine canyons are deep,
V-shaped valleys. Some of these canyons form near the
mouths of major rivers. Others form because of turbidity
currents. Turbidity currents are very dense currents that
carry a lot of sediment down the continental slopes.
CONTINENTAL RISE
The continental rise is the base of the continental
slope. It is made of large piles of sediment, which form
from turbidity currents. The continental rise covers the
boundary between the continental and oceanic plates.
Continental shelf
Continental slope
Continental rise
LOOKING CLOSER
Submarine
canyon
3. Identify Which part of the
continental margin has the
steepest slope?
The continental margin is made of three parts: the continental shelf, the continental
slope, and the continental rise.
What Are the Parts of the Deep-Ocean Basin?
Deep-ocean basins also have distinct features. The
deep-ocean basin includes the abyssal plain, mid-ocean
ridges, seamounts, and ocean trenches.
ABYSSAL PLAINS
READING CHECK
4. Define What are abyssal
plains?
The flat areas of the deep-ocean basin are called the
abyssal plains. Abyssal plains cover about half the deepocean basins, and they are Earth’s flattest regions. Layers
of sediment cover the abyssal plains. Ocean currents and
wind carry some sediment from the continental margins.
Other sediment forms when organisms that live in the
ocean settle to the ocean floor when they die.
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Holt McDougal Earth Science
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The Ocean Basins
Name
SECTION 2
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Date
Features of the Ocean Floor continued
MID-OCEAN RIDGES
A mid-ocean ridge is a long mountain chain that forms
on the floor of the ocean. Mid-ocean ridges form where
tectonic plates move apart. A narrow rift, or crack, runs
through the center of the ridge. Magma rises through the
rift and cools to form new rock. The ridge is made of this
new rock. As the new rock cools, it becomes denser and
begins to sink.
As the direction of the oceanic plate changes, the
ridges break into segments. These segments are called
fracture zones.
Critical Thinking
5. Compare How are
mid-ocean ridges similar to
volcanoes?
SEAMOUNTS
A volcanic mountain on the ocean floor is called a
seamount. Some seamounts form near hot spots. As
lava continues to erupt at a seamount, the mountain gets
taller. Seamounts that rise above the ocean’s surface form
volcanic islands. As the oceanic plate moves away from
the hot spot, the islands sink and are eroded by waves.
They become guyots, which are underwater seamounts
with flat tops.
OCEAN TRENCHES
Talk About It
Evaluate With a partner,
discuss the different features
of the deep-ocean basin.
Which features do you think
are most interesting? Why?
Long, narrow valleys in the deep-ocean basins are
called trenches. Trenches form when one plate subducts
under another plate. Earthquakes often happen near
trenches, and volcanoes often form near trenches as well.
Mid-ocean
ridge
Abyssal plain
LOOKING CLOSER
Abyssal plain
Guyot
Trench
6. Infer Which is probably
older—the guyot or the
seamounts near it? Explain
your answer.
Seamounts
Most of the deep-ocean basin consists of abyssal plains. Several other features are
common in the deep-ocean basin as well.
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The Ocean Basins
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Section 2 Review
SECTION VOCABULARY
abyssal plain a large, flat, almost level area of
the deep-ocean basin
continental margin the shallow sea floor that
is located between the shoreline and the
deep-ocean bottom
deep-ocean basin the part of the ocean floor
that is under deep water beyond the continent
margin and that is composed of oceanic crust
and a thin layer of sediment
trench a long, narrow, and steep depression
that forms on the ocean floor as a result
of subduction of a tectonic plate, that runs
parallel to the trend of a chain of volcanic
islands or the coastline of a continent, and that
may be as deep as 11 km below sea level; also
called an ocean trench or a deep-ocean trench
1. Identify What are the two main divisions of the ocean basin?
2. Compare Describe one similarity and three differences between the continental
slope and the continental rise.
3. Explain How are turbidity currents related to submarine canyons?
4. Describe Complete the table below to describe features of the deep-ocean basin.
Feature
Description
large, flat region
long, underwater mountain chain; where new crust forms
Seamount
Trench
5. Apply Concepts Which part of the continental margin is the first to be exposed
when sea level drops? Explain your answer.
6. Explain How are seamounts and guyots related?
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
354
The Ocean Basins