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Chapter 23 Section 2
THE OCEAN FLOOR
PLATE TECTONICS REVIEW

Earth’s lithosphere is divided into large tectonic
plates. They are constantly in very slow, motion.
PLATE TECTONICS REVIEW


____________ pushes the tectonic plates apart in sea
floor spreading. This causes the motion of the plates.
This is a ____________ boundary (plates move apart.)
MID-OCEAN ________

Sea-floor spreading produces large
mountain ranges under the ocean.
CONVERGENT BOUNDARIES

As plates separate in
the ocean, they
collide in other
areas.

When plates collide,
the more dense plate
is ________ under
the lighter one.
OCEAN FLOOR


New rock is constantly forming from __________ as
the sea floor spreads.
The ocean crust is primarily _________ rock, and it
is newer than the continental crust.
SEDIMENT



Rivers constantly erode
the continents. They
carry rock along with
them to the ocean.
The solid particles of
sediment fall to the
ocean floor.
The dissolved rock
makes the ocean water
salty.
SEDIMENT ON THE OCEAN FLOOR
OCEAN FLOOR


The ocean floor is made of continental margins and
deep-ocean basins.
Sediment is thicker on the margins.
SHORELINE

Ocean water meets land
at a shoreline or beach.

This is NOT the dividing
line between the
continental and oceanic
crusts.
CONTINENTAL SHELF




Ocean water covers
the edges of the
continents.
Average depth: 60 m.
The shelf slopes
slightly downward:
0.12 m drop per 100
m of land.
This slope would be
24 m in 20 km.
CONTINENTAL MARGINS



At the edge of the continental shelf, the ground slopes
more quickly, in the continental slope.
The slope drops several thousand meters in 20 km.
Sediment on the edge of the ocean floor forms the
wedge-shaped continental rise.
DEEP OCEAN BASIN

Formed of ocean crust and thin sediment.
ABYSSAL PLAINS

The flattest regions on
Earth.

Level drops about 3 m
across 1300 km.
ABYSSAL PLAIN

These plains
cover about
half of the
deep ocean
basin.
ABYSSAL PLAINS ARE COVERED WITH A THIN
LAYER OF FINE SEDIMENT


The sediment is brought from the continental margins
by wind and water.
Some sediment is also from decomposed organisms.
SEDIMENT THICKNESS VARIES

Old rock has thickest
sediment.

Thicker near the
continental margins.

Sediment gets thinner
approaching an ocean
trench.
OCEAN TRENCHES

A trench is a long,
deep depression in
the ocean floor,
shaped like a trough
or ditch.
DEEPEST POINT OF EARTH’S CRUST

Mariana
Trench

11,000 m (11
km) deep

Almost 7 miles
MARIANA TRENCH

Only four people have
ever been there.

In 1960, two men in a
submersible spent 20
minutes at the
Challenger Deep.

The sediment was
stirred up so much,
they could not take
photos.

Trenches are caused
by the subduction of
an ocean plate.

This is a convergent
boundary.
MID-OCEAN RIDGES

Form at divergent
boundaries.

Produced by sea-floor
spreading as plates
move apart.
MID-OCEAN RIDGES


All mid-ocean ridges are connected.
This makes the longest mountain range in the
world.
ICELAND
ABYSSAL HILLS FORM

Divergent
boundaries
make faultblock
mountains.

Smaller than the ocean ridges, abyssal hills are
produced on the flanks.
SEAMOUNTS

Underwater volcanos
taller than 1 km are
called seamounts.

Seamounts form when
hot spots of magma rise
through the ocean crust.

If the seamount rises
above the ocean surface,
it is an island.
EROSION

Waves erode the new island, wearing it down
into a flattened cone called a guyot.