EXPLORING OCEANS 1a ocean landforms

Continental shelf

The shallow, gently sloping submerged edge of a continent

PSD:

• average width 70km

• Slope angle of just under 2m per km

• Siberian shelf in the Arctic Ocean is the widest continental shelf in the world

Continental slope

The edge of a continental shelf, where the slope angle increases to a much faster rate

PSD

• 70m per km

• Extends from around 200m to 3000-4000m in depth

• West coast of South America is an active continental slope- can drop at angles of up to 20 degrees or more. Techtonic plate meets edge of continent

Abyssal plain

Deepest part of the ocean

(Deepest general floor)

PSD

• Foundations made of dark oceanic crust

• Near freezing temperatures 0-4 degrees, pressures up to 600 times atmospheric pressure

• Hatteras Abyssal Plain (North Atlantic Ocean) covers over 100,000 square km

Sea ridges + rifts

Ridges:

• long underwater mountain chains with central rift valley in some areas

• Mid Atlantic Ridge PSD—> constructive plate margins= 2 plates pull away from each other (North American and Eurasian plates). As plates move apart, magma rises from the mantle to fill gaps, creates new oceanic crust. Ridge is 3km high from ocean floor in places with transform faults occurring at right angles to the main plate boundary.

Rifts

• form at constructive/divergent plate boundaries where magma rises as plates move apart and creates new oceanic crust.

• Mid Atlantic Rift Valley is 15km wide in places

Paleomagnetism

Evidence that earths magnetic field changed over time, tectonic plates moved

• magnetic minerals in cooling magma/rock align with direction of magnetic field

• Once rock cools, direction locked into the rock

• Earths magnetic field flips, alignment of minerals shift

• New magma rises and cools at mid ocean ridges

• Matching bands of normal and reversed magnetism on both sides of ocean ridge form- new oceanic crust continually forming and moving away from the ridge

Ocean trenches

Long narrow and very deep depressions in the ocean floor

• subduction important in forming these

• Form at destructive/convergent plate boundaries, one oceanic plate subducted beneath another or a continental plate

PSD

• Mariana Trench is the deepest known trench, found in the pacific

Seamounts

Chains of volcanic mountains and isolated peaks, some rising over 3000m above the abyssal plain. These dont reach the ocean surface

• magma rises through oceanic crust, volcanic mountains created. If volcano grows enough to reach surface- volcanic island

PSD

• New England seamount contains more than 30 peaks along its 1600m length, located in North Atlantic Ocean

Guyots

Flat topped peaks that once rose above the ocean

• seamounts that extended above sea level, wave erosion flattened the ridge

• Underlying tectonic plate moves away from hotspot and cools, (volcano not fed by mantle plume), oceanic crust becomes denser, mount subsides below sea level

• High levels of biodiversity

PSD

• Emperor seamount chain in Pacific Ocean, 2000-2500km