OCEAN BASINS
Grade 8 Science 2012
OCEAN BASINS
Oceans form the largest ecosystem on Earth.
Much remains to be discovered about what lies
below the ocean’s surface.
Only about 1% of the ocean floor has been
mapped
Most of the ocean (100 metres and below) is
pitch-black.
 Continental Shelf – the gradual slope between the coastline and the
edge of the ocean basin
 Continental Slope – a steep drop dividing the continental slope and the
ocean basin
 Abyssal Plain – wide, open, flat plains that stretch out along the ocean
basin
 Oceanic Trench – narrow, deep, and steep sided canyons running along
some ocean floors. Some of the deepest places on Earth

A JOURNEY ON THE OCEAN FLOOR
The and formation of the ocean landscape are due
mainly to the movements of Earth’s tectonic
plates.
Features in the ocean basins are much bigger than
on land.
There are mountain ranges taller than the
Himalayas.
Steep valleys deeper than the Grand Canyon.
Plains wider than the Canadian Prairies.
CONTINENTAL SHELVES & SLOPES
 Ocean basins do not begin at the coastline. They begin may km out
at sea.
 The area between the coast and the edge of the ocean basin is
actually a submerged part of the continent, called the continental
shelf.
 Continental Slopes exist at the edge of the shelves and plunge at
steep angles to the sea floor.
SEAMOUNTS
Large underwater
mountain peaks called
seamounts exist at
the edges of midocean ranges.
Seamounts are most
often found in clusters
and are most common
in the Pacific Ocean.
TRENCHES
 Along the sea floor are narrow,
steep-sided canyons, called
trenches.
 They are formed where the
edge of an ocean plate pushes
against the edge of a continental
plate. As the plates move
together, the ocean plate is
forced to bend steeply down
beneath the heavier continental
plate.
TRENCHES (CONT.)
 The deepest trench,
Marianas Trench, extends
11 km below sea level.
 Nearly SEVEN times
deeper than the Grand
Canyon and deep enough
to submerge an object as
tall as Mount Everest.
ABYSSAL PLAINS
 Between the high mountains
and the deep trenches, the
ocean floors are very flat.
 These wide open features are
called abyssal plains.
 They are formed of thick
deposits of sediment, up to 1
km deep in places.
 As tectonic plates move
apart or together, some
oceans expand, while
other shrink. Precise
measurements show that
the Atlantic Ocean is
expanding, carrying North
America and Europe
farther part at a rate of 3
cm per year.
MID-OCEAN RIDGES
 Long undersea mountain chains
called mid-ocean ridges run along
the ocean floor
 These ridges are the youngest areas
of the sea floor and are still being
formed by volcanic eruptions.
 Molten lava flows from these ridges,
quickly hardening into new plate
material that pushes tectonic plates
further apart.
 Mid-ocean ridges are more than
1000 km wide and rise over 10003000 m above the sea floor.
VIDEOS
MAPPING OUT THE OCEAN FLOOR
Please get a textbook and open to page 361.