Topic 4: Geomorphology of the Ocean Basin

Bathymetry

Study of what the ocean floor looks like

3 methods of direct observation of the ocean floor

1. Snorkelling

2. Diving

3. Manned submersibles

5 methods of indirect observation of the ocean floor

1. Soundings

2. Sonar

3. Remotely operated submersibles

4. Seismic data

5. Satellite data

Soundings

Use markings in rope to estimate depth

Multi-beam echo sonar

Use sonar (sound bounces off ocean floor) with multiple beams

Side-scan sonar

Sends sound out in a fan, gives more accurate images

Satellite

Space satellite uses microwave beam to pick up differences in water height, uses those to infer ocean floor’s appearance

3 main regions of ocean basins

1. continental margin

2. deep ocean basin

3. mid-ocean ridge

Continental margin

Submerged outer edge of a continent (often ~200km)

Deep ocean basin

Deep-sea floor beyond the continental margin

Oceanic crust composition

Basalt (denser)

Continental crust composition

Granite (lighter)

Active continental margins

Continental and plate boundaries coincide

Passive continental margins

Continental edges are located within plates

Characteristics of active margins

1. Narrow continental shelf (100km or less)

2. Steep slope

3. Thin sediment cover

4. Earthquakes, volcanoes (extrusive and intrusive), orogeny (mountain building)

Characteristics of passive margins

1. Wide continental shelf

2. Gentle slope

3. Thick sediment cover

4. Little volcanism (intrusive)

Continental shelf

Relatively flat submerged edge of continent (granitic crust)

Continental slope

True edge of continent (steep), begins at shelf break

Submarine canyons

• Narrow, steep-sided underwater valley that erodes into the continental shelf and slope

• Primary transport pathways for terrigenous sediment reaching the abyssal seafloor

Deep sea fan

When a submarine canyon ends in a fan-shaped sediment wedge

Turbidity currents

Quickly moving (> 25 km/hr); ,down slope flow of dense sediment-laden water, transport and deposit large amounts of sediment (“underwater avalanches”)

Continental rise

Transition zone between continental slope and true, deep ocean (extensive sediment accumulation, underlain by oceanic crust)

Deep ocean basins (aka abyssal plains)

Flat, featureless areas of deep ocean that accumulate fine-grained sediment

Suspension settling

Accumulation of fine-grained sediment

Factors that affect sediment thickness of abyssal plain

1. Age

2. Distance from mid-ocean ridge and continent

Abyssal hills

Small (less than 1000m high) elevated areas in deep ocean basins, isolated or in groups (often blanketed by sediment)— the most abundant geomorphic feature on earth

Seamount

Extinct volcano, underwater with pointy top

Guyot

Extinct volcano, flat-topped seamount (due to erosion), flat peak at water’s surface

Trenches

Where subduction occurs near active margins, concave shape (found at convergent plate margins)

Island arc

Arc of volcanic islands near trenches

Mariana Trench

Deepest part of ocean, home to Challenger Deep

Mid-ocean ridges

Submarine mountain chains created at a divergent boundary

3 characteristics of mid-ocean ridges

1. Form youngest (basaltic) rocks

2. Thin to no sediment cover

3. Where seafloor spreading occurs

Fast-spreading mid-ocean ridge characteristics

1. Hotter, plate responds fluidly

2. Smaller rift valley

3. Greater topographic high

Slow-spreading mid-ocean ridge characteristics

1. Slightly cooler

2. Plates more brittle, breakage

3. Forms normal faults (classic “step pattern”)

Hydrothermal vents are home to….

Many life forms (extremophiles)

2 types of hydrothermal vents

1. White smokers (30-350 degrees C)

2. Black smokers (more than 350 degrees C), contains metal sulphides

Benthic communities

Potential site of early life, community of extremophile organisms (mainly survive via chemosynthesis)