Transcript The CONTINENTAL MARGIN

THE OCEAN
FLOOR
Ch. 23
STUDYING
THE
OCEAN
FLOOR
Submersibles,
satellites, and
other
technology
allow scientists
to study the
structure and
composition of
the ocean floor.
1. ECHO SOUNDING
a system that uses transmitted and reflected
sound waves to measure distances to the ocean
floor; SONAR.
measure how long it takes for signal to be emitted
and come back
multi-beam echo sounding measures area twice
as wide as ship
use info to make seafloor maps
intensity of sound beams determine seafloor
composition
rock & gravel reflect more strongly than mud
ECHO SOUNDING
2. SEDIMENT SAMPLING
Core sampling: hollow cylinder removes long
cores of material from seafloor
layers are preserved
1-1500m of sediment are gathered
can analyze past climate, life, and ocean events
3. SATELLITE OBSERVATIONS
greater range & speed for mapping
doesn’t reach floor, bounces off surface
 Ocean surface varies based on what’s below
higher over mountains, lower over trenches
determines differences down to cm’s
makes high-resolution seafloor image
HIGH
RESOLUTION
SEAFLOOR
IMAGE
Pacific and
Western
Atlantic
Oceans
THE CONTINENTAL MARGIN
In order to
understand
the
continental
margin, we
must first
understand
the crust of
the Earth.
The
continental
margin is
part of the
crust.
LAYERS OF THE EARTH
the Earth has 4 main physical layers:
Depth from Temperature State of
Surface (km) (K)
Matter
Composition
*Crust
0-65
<1000,
increases w/
depth
Solid
Rock we live on
Si, O, Al
*Mantle
66-2890
1500-3200
increases
Solid w/ liquid
properties
Fe, Si, Mg
Outer Core
2891-5150
3700-5500
increases
Liquid
Fe & Ni
Inner Core
5151-6371
approx. 6000
solid
Fe & Ni
LAYERS OF THE EARTH
TWO TYPES OF CRUST
1. Continental Crust
lighter, less dense rock
Makes up all continents, but not necessarily
all islands
2. Oceanic Crust
darker, dense rock
any ocean floor or ocean basin
THE MOVING CRUST
 all of the crust is divided into ‘pieces’ or plates
 (Called lithospheric plates or tectonic plates)
 the plates move around on the semi-solid mantle
 where the plates meet or connect are called plate
boundaries
 at plate boundaries, the crust can be moving side by
side, apart, or together
 different topographic features are created, depending on
the type of boundary
 topography: the shape of the land
WORLD PLATE BOUNDARIES
THE CONTINENTAL MARGIN
The
underwater
portion of
the
continental
crust.
There are
two types of
continental
margins.
1. ACTIVE CONTINENTAL
MARGINS
 continental margins that occur along plate
boundaries
 plates are moving side by side or together
 when one plate sinks under another, a trench is formed
 when plates move past each other, a fault is formed
 continental rise is small or nonexistent
 rocky, short beach w/ cliffs
 rugged, coastal mountains on land
EX: West coasts of North America and South America
2. PASSIVE CONTINENTAL
MARGINS
 continental margins that don’t occur along plate
boundaries
 broad continental shelf
 long, sandy beach
 no trenches, mountains, or faults
EX: East coast of North America
PARTS OF THE CONTINENTAL
MARGIN
1. continental shelf: part of the continent that
extends from the shoreline to the continental
slope
 flat, lengths vary depending on location
2. continental slope: begins at the shelf edge where
depth decreases rapidly to the rise.
 20km long; descends 3.6km
 sediment builds up temporarily, then falls
3. continental rise: descends gradually from the
slope to the ocean floor
 considered part of ocean basin
 very long & gradual
SUBMARINE CANYON
 an undersea gully that cuts across the continental
shelf and slope
 rivers erode the continental shelf and deposit
sediment on continental slope
 gravity and powerful turbidity currents carry
sediment down to the continental rise
 Very powerful agents of erosion
 coarse particles settle first, then fine such as clay
OCEAN BASIN
Abyssal Plain – flattest of all Earth’s surfaces,
composed of sediment from continents
Occur in all oceans
More in Atlantic Ocean where there are fewer
trenches
Abyssal hills – small hills, occur in groups next to
oceanic ridge systems
DEEP-SEA TRENCHES
 Deep-sea trenches – long, narrow, steep-sided
troughs that run parallel to continental margins
or to volcanic island chains called island arcs
Exist at subduction zones
Common sites of earthquakes and volcanic activity
DEEP-SEA TRENCHES
A marginal trench forms if one plate is
oceanic and the other is continental.
If the oceanic plate descends below the
continental, a line of volcanoes stands on the
overriding continental plate, forming mountain
chains
If both plates are oceanic, an arc of volcanic
islands forms on the overriding plate
Usually in western Pacific
DEEP-SEA TRENCHES
VENTS AND RIDGES
 Deep ocean vent – geyser that erupts
underwater, mixing hot and cold water and
bringing up minerals from beneath the surface
Mid-ocean ridges- are
undersea mountain ranges,
form at divergent plate
boundaries where 2 plates
are moving apart and
magma is rising
SEA MOUNTS & GUYOTS
 Seamounts are cone-shaped mountain peaks that
rise high above the ocean floor
 Occur in all oceans, but more abundant in Pacific Ocean
 Volcanic in origin (ex. Hawaiian islands)
 Guyots are flat-topped seamounts
 Waves removed their tops when they rose above sea level
CORAL &CORAL ATOLLS
 Corals are tiny sea animals that live in shallow,
warm waters. Reefs form when new corals grow on
top of the dead ones.
 A coral atoll is a ring-shaped coral island. It forms
when a coral reef develops around a volcanic
island. The mountain sinks below the water,
leaving a circular reef with a lagoon where the
mountain was.
OCEAN FLOOR SEDIMENTS
 Sediment reaches the ocean floor in several
ways: turbidity currents, fall from above, settle
from glaciers, remains of microscopic shells
Terrigenous sediments – come from continental
rocks an minerals broken down from weathering
an erosion
 wash into rivers and out to sea
 may come from glaciers
breaking and dropping
into sea
OCEAN FLOOR SEDIMENTS
 Biogenous sediments come from living sources;
they are oozes made mostly of shells and
skeletons from tiny marine animals
Calcareous ooze=calcium carbonate
 Most common from shells
and skeletons
 Dissolve as they sink below
4500 meters
Siliceous ooze=silicon dioxide
 More common around
Equator and Antartica
OCEAN FLOOR SEDIMENTS
 Hydrogenous sediments form when chemical
reactions cause minerals to crystallized from
seawater.
 Manganese nodules are most common – has
manganese, iron oxide, nickel, cobalt, copper…
 They form on the sea floor from sediment that falls on
them and mixes with the sea water, a few mm every
million years
 Important to humans but hard to get
IMPORTANCE OF SEDIMENTS
 By studying the layers of sediment in the
ocean, scientists can determine:
 the extent of former polar ice sheets
 the history of water temperatures on sea floor
 the pasts behaviors of prevailing winds
 pattern of changes in Earth’s climate
The sediments, unique organisms, magnetic
records, industrial resources on the ocean
floor represent a past look at Earth’s
hydrosphere, biosphere, and geosphere.
OCEAN FLOOR