Chapter 23 Reading Guide
The Ocean Floor
23.1 Studying the Ocean Floor
Summarize the main ideas of this section by naming and describing three important ways
that scientists study the ocean floor.
1. Echo Sounding:
Scientists track the amount
of time it takes for single or
multiple beams of sonar to
reach sea floor and echo
back.
Studying the Ocean Floor
2. Sediment sampling:
Scientists take core
samplings of ocean
sediment in order to study
sediment layers.
3. Satellite observations:
Satellites measure the time
it takes a signal to bounce
off ocean surface and
determine ocean floor
changes from differences in
surface levels.
23.2 The Continental Margin
In the spaces provided, write brief descriptions of active continental margins and passive
continental margins. Discuss the continental shelf, continental slope, continental rise,
continental and oceanic crusts, and ocean floor.
1. Passive Continental Margin: Located away from plate boundaries, passive margins
feature broad continental shelves and fairly gradual gradients across the continental slope
and rise.
2. Active Continental Margin: Located where continental and oceanic plates meet, active
margins feature narrow continental shelves, precipitous continental slopes, and nearly
nonexistent continental rises.
Describe how submarine canyons form.
Underwater canyons either date back to rivers cut through the land in earlier times when
sea levels were lower or form from erosion effects of turbidity currents.
23.3 The Ocean Basins
Each feature of the ocean floor develops in response to particular factors. Define each
feature and explain its cause.
1. Abyssal Hill: Small rolling, sediment-covered hills formed as the sea-floor surface
near mid-ocean ridges.
2. Abyssal Plain: Flat of sea floor created by sediment brought from continents by
turbidity currents and then spread out by the sea.
3. Deep-Ocean Trenches: Long, steep-sided troughs parallel to meeting of lithospheric
plates, formed when one plate subducts beneath another.
4. Mid-Ocean Ridges: Undersea mountain ranges formed when lithospheric plates move
apart, allowing magma to rise into the space and cool.
5. Seamounts/guyots: Cone-shaped mountain peaks formed originally by volcanoes;
when the sea erodes away the top, seamounts become guyots.
Name a specific geographic location where you might find each undersea feature:
a) Abyssal Plain: Atlantic Ocean
b) Abyssal Hills: North Atlantic Ocean
c) Deep-ocean Trenches: Indian Ocean, western Pacific Ocean; Java Trench, Manilla
Trench, Philippine Trench
d) Seamounts: Hawaiian Islands
23.4 Ocean Floor Sediments
Describe each of the four types of sediments in the appropriate box. List the origin,
textures, and typical locations of each sediment type.
1. Terrigenous Sediments:
2. Hydrogenous Sediments:
Bits and pieces of continents; carried by
rivers or icebergs out to sea, particles range
from gravel and sand to tiny grains and
flakes, can travel great distance before
settling to the ocean floor.
3. Calcareous Oozes:
Chemical reactions cause minerals to
crystallize from seawater, form very slowly
on sea floor in lumps; found in all oceans
and in some lakes.
Shells and skeletons of marine animals or
plants ; containing at least 30% calcium
carbonate; dissolving into water as they
fall; most common sediment – found on
half of sea floor.
Skeletons of marine life containing silicon
dioxide; most common near equator and
around Antarctica.
4. Siliceous Oozes:
Explain why the study of ocean sediments is important.
Ocean sediments contain information about Earth’s history. Sea creatures’ remains tell
scientists about past changes in climate, water temperature, wind patterns, and glaciation.
Understanding the past can help scientists predict and prepare for future changes.