Ocean Floor Physiology

Question 1

What is the continental margin?

Answer 1

Submerged edges of the continent. Composed of: Continental Shelf + Continental Slope +Continental Rise.

Some authors exclude the continental rise since under it there is often oceanic crust present

Question 2

What type of sediment dominates the continental margin?

Answer 2

Terrigenous sediments

Question 3

What is the continental Terrace?

Answer 3

• Continental Slope + Continental Rise

Question 4

What do the morphological characteristics of the continental margin depend on?

Answer 4

• Tectonics
• Sea level changes
• River inputs
• Tidal range
• Wave & current climates

Question 5

What is an active and passive margin when referring to continental margins? What is the morphological characteristics the shelf of both?

Answer 5

Active Margin: continental crust crashing into an oceanic plate. High tectonic activity (earthquakes, volcanos, formation of igneous rock). The shelf is narrow to non-existent (due to sediments sinking into mantel)

Passive Margin: No subduction or Collision. Minimal tectonic activity. Leads to accumulation of sediments, therefore wide continental shelf

Question 6

Define the Continental Shelf. What are the 3 divisions of the shelf?

Answer 6

*Shallowest part of the continental margin which ranges from low tide to the shelf break ~ 140-200m.
*Characterized by gentle slopes.
*Rich in biological diversity compared to other marine habitats
*Inner shelf (<40m), middle shelf (40-80m), outer shelf (>80m)

Question 7

Describe the continental shelf morphology associated with Glaciation and Interglacial periods?

Answer 7

Glaciation/Low sea levels: shelf is deeply dissected by rivers ,which deposit sediments near or on the continental slope. AKA water recedes from the shelf and rivers run on the shelf itself.

Inter-glaciations/High sea level: Present day. Rivers deposit sediment on the inner shelf and decreased sediments reaching the slope and rise. Depressions that may have been on the shelf fill with sediment (this causes the minimal slope)

Question 8

Define the Shelf Break

Answer 8

Corresponds to a first order gradient change in the transition between the shelf and slope.

Generally energetic due to upwelling, contour currents, downwelling and internal waves.

World average ~75-140m deep.

Many shelf breaks had sub-arial exposure during ice ages which can cause the development of terraces due to low sea level.

No agreement on where exactly to measure/define

Question 9

Define the Continental Slope

Answer 9

The part of the continental margin that i s characterized by a steep slope (average ~4 degrees) after the break, down to the continental rise

Usually around depths of 1500-3500m. where the slope then smooths that is the transition to the rise

High slopes = low stability which causes ravines (gullies) and submarine canyons to develop from turbidity currents

Sea level greatly affects how much sediment makes it to the slope

Question 10

Define the Continental Rise

Answer 10

Where the steep slopes of the continental slope start to gentle to ~<0.5 degrees. Huge accumulation of sediments here. Most of the worlds terrigenous sediments end up here

Very poorly defined

Not large at active margins due to subduction

Slope progressively flattens and transitions to the abyssal plains

Question 11

Define a submarine canyon

Answer 11

Gullies that dissect the continental margin. Steep V-shaped profile that grow perpendicular to the shelf break

They transport sediments from the shelf to deep sea, creating submarine deltas where they deposit.

Often have turbidity currents causing landsides and mass movements

Question 12

What are the two types of origin of submarine canyons?

Answer 12

Sub-Arial Origin: by stream erosion. Associated with important rivers

Tectonic Origin: from plate movement. Although often associated with rivers.

Can be both

Question 13

What are the types of submarine canyons?

Answer 13

Shelf-Indenting (Gouf Type): carve deep into the shelf up to the coast. Often interrupt littoral drift of sediments, leading to deep areas with a heavy coarse sediment load (sediments taken directly to the deep from the coast). These are relatively rare

Slope-Confined: only cut the outer shelf or near platform edge. Most canyons are these

Question 14

What is an Abyssal Fan? What are the four largest fans?

Answer 14

Large scale fan/delta like sediment deposit formed by a turbidity current and associated with large scale sediment inputs such as rivers

Laurentian Fan, Indus Fan, Amazon Fan, Bengal Fan

Question 14

Define the Abyssal Plains

Answer 14

Extensive flattened areas on oceanic crust at 3000-6000m depth.

Flat from years of sediment accumulation (mainly terrigenous)

Depth variation of ~10-100cm/km

Largest feature on the planet >50% of earths surface

Question 15

What is an abyssal hill?

Answer 15

Small elevation less than 200m in the abyssal plains.

Similar origin and composition to basaltic flanks of oceanic ridges

Question 16

What is a submarine knoll?

Answer 16

An elevation between an abyssal hill and seamount. 200-1000m

Question 17

What is a seamount?

Answer 17

Large elevation on the seafloor of >1km in height
Very steep slopes 20-25 degrees
Most (not all) are caused by volcanic activity

Question 18

What is the difference between a basaltic and limestone seamount?

Answer 18

Basalt seamounts are directly made at the ridge by volcanic activity causing magma to rise (most common).

Limestone seamounts are near continental margins and are caused by crushing of plates forcing sediment deposits to rise up.

Question 19

What is a Guyot?

Answer 19

A guyot is a flat-topped seamount that once was an island that sunk when it became inactive and was errored to below sea level.

Generally found only in the west-central pacific.

AKA table mounts

*An atoll top is at sea level, but a tablemount top is below sea level.

Question 20

Define an oceanic ridge

Answer 20

Mountain ranges where diverging plate boundaries causes seafloor spreading

Crests can be locally displaced by transform faults

Young basalt rock, highly seismic, rise from 5km deep

Question 21

What are Flanks when referring to oceanic ridges?

Answer 21

The flanks of an oceanic ridge are marked by sets of mountains and hills that are elongate and parallel to the ridge trend.

Question 22

2What is the central part morphology associated with slow, intermediate and fast expansion rates of ridges?

Answer 22

Slow: deep valleys(rifts) ~20-40km wide

Intermediate: central ridge is also high, sometimes have small tectonic valleys

Fast: no rift only defined by axial elevation

Question 23

What is an Aseismic Ridge vs a Seismic ridge?

Answer 23

Aseismic ridges: associated with hot spots (high volcanic activity, low seismic activity)
Best example is the emperor seamounts in Hawaii where the pacific plate moved over a stationary hotspot.Hotspot provides a persistent source of magma by partially melting the overlaying plate

Seismic ridge is a geologic feature that is created when two tectonic plates collide.

Question 24

What is a fracture zone?

Answer 24

Areas where ridges, aseismic or seismic, are dislocated by transform faults

Question 25

What is a Submarine/Abyssal Gap?

Answer 25

steep-sided furrow that cuts transversely across a ridge or rise; such a passageway has a steeper slope than either of the two abyssal plains it connects

Caused by a transform fault.

Sides are relatively steeper and are typically at different depths than the surrounding abyssal plains

Question 26

27. What is an Oceanic Trench?

Answer 26

Arc shaped depressions in deep ocean floor caused by converging oceanic plates being sub-ducted with a continental or oceanic plate.

Highly seismic with steep slopes

Often associated with island arcs

Question 27

What is an island arc?

Answer 27

Aka volcanic arcs: curving chain of volcanic islands & seamounts found parallel to the concave edges of Tenches

Formed by an oceanic plate being subducted at a convergent boundary causing melted materials from the subducted plate to be extruded on the subducting plate

Highly seismic areas