Ch. 23THE OCEAN

FLOOR

STUDYING THE OCEAN FLOOR

Submersibles, satellites, and other technology allow scientists to study the structure and composition of the ocean floor.

a system that uses transmitt ed and refl ected sound waves to measure distances to the ocean fl oor; SONAR.measure how long it takes for signal to be emitted

and come backmulti-beam echo sounding measures area twice

as wide as shipuse info to make seafloor mapsintensity of sound beams determine seafloor

compositionrock & gravel reflect more strongly than mud

1. ECHO SOUNDING

ECHO SOUNDING

Core sampling: hollow cylinder removes long cores of material from seafl oor layers are preserved1-1500m of sediment are gatheredcan analyze past climate, life, and ocean events

2. SEDIMENT SAMPLING

greater range & speed for mappingdoesn’t reach floor, bounces off surfaceOcean surface varies based on what’s below

higher over mountains, lower over trenchesdetermines differences down to cm’smakes high-resolution seafloor image

3. SATELLITE OBSERVATIONS

Pacifi c and Western Atlanti c Oceans

HIGH RESOLU-TIONSEAFLOOR IMAGE

In order to understand the conti nental margin, we must fi rst understand the crust of the Earth.

The conti nental margin is part of the crust.

THE CONTINENTAL MARGIN

the Earth has 4 main physical layers:

LAYERS OF THE EARTH

Depth from Surface (km)

Temperature (K)

State of Matter

Composition

*Crust 0-65 <1000, increases w/ depth

Solid Rock we live onSi, 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

1. Continental Crustlighter, less dense rockMakes up all continents, but not necessarily

all islands

2. Oceanic Crustdarker, dense rockany ocean floor or ocean basin

TWO TYPES OF 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

THE MOVING CRUST

WORLD PLATE BOUNDARIES

The underwater porti on of the conti nental crust.

There are two types of conti nental margins.

THE CONTINENTAL MARGIN

conti nental 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

conti nental rise is small or nonexistent rocky, short beach w/ cliff srugged, coastal mountains on land

EX: West coasts of North America and South America

1. ACTIVE CONTINENTAL MARGINS

conti nental margins that don’t occur along plate boundaries

broad conti nental shelf long, sandy beach no trenches, mountains, or faults

EX: East coast of North America

2. PASSIVE CONTINENTAL MARGINS

1. conti nental shelf: part of the conti nent that extends from the shoreline to the conti nental slope flat, lengths vary depending on location

2. conti nental 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. conti nental rise: descends gradually from the slope to the ocean fl oor considered part of ocean basin very long & gradual

PARTS OF THE CONTINENTAL MARGIN

an undersea gully that cuts across the conti nental shelf and slope

rivers erode the conti nental shelf and deposit sediment on conti nental slope

gravity and powerful turbidity currents carry sediment down to the conti nental riseVery powerful agents of erosion

coarse parti cles sett le fi rst, then fi ne such as clay

SUBMARINE CANYON

Abyssal Plain – flatt est of all Earth’s surfaces, composed of sediment from continentsOccur in all oceansMore in Atlantic Ocean where there are fewer

trenchesAbyssal hills – small hills, occur in groups next to

oceanic ridge systems

OCEAN BASIN

Deep-sea trenches – long, narrow, steep-sided troughs that run parallel to continental margins or to volcanic island chains called island arcsExist at subduction zonesCommon 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 plateUsually in western Pacific

DEEP-SEA TRENCHES

DEEP-SEA TRENCHES

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

VENTS AND RIDGES

Seamounts are cone-shaped mountain peaks that rise high above the ocean fl oorOccur in all oceans, but more abundant in Pacific OceanVolcanic in origin (ex. Hawaiian islands)

Guyots are fl at-topped seamountsWaves removed their tops when they rose above sea level

SEA MOUNTS & GUYOTS

Corals are ti ny 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.

CORAL &CORAL ATOLLS

Sediment reaches the ocean floor in several ways: turbidity currents, fall from above, sett le 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 ti ny marine animalsCalcareous ooze=calcium carbonate

Most common from shells and skeletonsDissolve as they sink below 4500 meters

Siliceous ooze=silicon dioxideMore 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

OCEAN FLOOR 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 windspattern 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.

IMPORTANCE OF SEDIMENTS

OCEAN FLOOR