Ocean Floor

Question 1

What were the 3 old assumptions about the ocean?

Answer 1

1. oceans were basically bowl or trough like “basins”
2. following (1) the greatest thicknesses of sediment would be predicted to accumulate at centre
3. oceans + continents btwn them are permanent features that have remained unchanged since Earth formed

Question 2

How was the ocean floor mapped?

Answer 2

• depth originally measured by lowering weighted lines overboard
• echo sounder (aka sonar) was invented as a primary instrument for measuring depth by reflecting sound waves from ocean floor
• multi-beam sonar developed later (using an array of sound sources + listening devices for mapping narrow swaths of seafloor)
• later came use of satellite radar altimeters (used to record seafloor topography by bouncing micro waves off sea surface)

Question 3

what was the seafloor topography like?

Answer 3

• huge ridges (oceanic or mid-ocean ridges), originall assumed to have been very deep
• deepest areas were through like areas (deep sea trenches that bordered major landmasses
• Suken, inactive volcanoes (seamounts) in linear patterns

Question 4

What are abyssal plains?

Answer 4

huge expanses of relatively flat, sediment-covered (nearly featureless) regions of deep ocean floor (btwn ridges + trenches) called abyssal plains

Question 5

What is a continental shelf?

Answer 5

continental margins, a submerged, shallow-water platform extended seaward for significant distances
- at other margins, continental shelf appear very narrow or even non-existent (esp where bordered by a deep sea trench)

Question 6

what is the seafloor covered with?

Answer 6

sediment
- areas most distant from land, deep sea sediment is largely biochemical
- accumulation of ooze

minor:
- mud (from windblown dust)
- chemical sediment produced by precipitation of minerals from seawater

Question 7

What is ooze

Answer 7

accumulation of microscopic skeleton of planktonic organisms

Question 8

What are the sediment input from land?

Answer 8

• closer to continental landmasses, clastic sediment derived from land sources cover the shallow-water continental shelf (transported by waves + offshore currents)
• also forms thick accumulations at continental rise (at base of steep continental slope) onto parts of flat abyssal plain

Question 9

What are turbidity currents?

Answer 9

down-slope movements of dense, sediment-laden water (often triggered by earthquakes)

Question 10

Where are turbidity currents commonly focused?

Answer 10

in steep-walled submarine canyons cut into the continental slope

Question 11

What is the sediment deposited by a turbidity current

Answer 11

turbidites: layered + commonly exhibit graded bedding (decreasing in sediment grain size from bottom to top due to settling of progressively finer-grained sediment)

Question 12

what is a submarine fan?

Answer 12

the accumulation of land-derived sediment on the continental rise (thin out on the abyssal plain) and in trenches

Question 13

What are the types of continental margins

Answer 13

• passive continental margins
• active continental margins

Question 14

what is a passive continental margin?

Answer 14

• wide continental shelf
• geologically quiet (“passively” accumulate sediment)

Question 15

what is an active continental margin?

Answer 15

-continental shelf is narrow or non-existent
- associated w/ deep sea trenches
- geologically active (associated w/ earthquakes + volcanic activity)

Question 16

What surprise did the study of mid-oceans ridges bring?

Answer 16

• basalt (mafic volcanic rock) exposed at crests of mid-ocean ridges (indicate formed recently)
• sediment thickness found to increase away from crests of ridges (+thickest in trenches)

Question 17

What are seamounts

Answer 17

submerged volcanoes
- appeared to form chains

Question 18

What is the relationship of seamounts with distance?

Answer 18

• ages of seamounts increase w/ increasing distance from Hawaii
• seamounts become more deeply submerged further away from Hawaii

Question 19

What are atolls

Answer 19

• ring-shaped reefs w/ central lagoon
• commonly associated w/ submerged seamounts
• constructed primarily from skeletal remains of corals + other organisms

Question 20

Where are atolls formed

Answer 20

forms on flanks of a sinking volcanic island
(hypothesis proposed by Charles Darwin)

Question 21

What is ocean circulation

Answer 21

Idea that ocean circulations is driven by water salinity + temperature differences
Basic idea:
- warm, less salty, low-density seawater flows at surface toward poles
- cold, more salty, high-density water flows along seafloor toward equator

Question 22

What is a Thermohaline Cirulation

Answer 22

• cooler, saltier water, is dense enough to sink to seafloor
• cold, salty deep current flows southward toward equator to connect to surface water current
• conveyor works as “circuit”

Question 23

What did Alfred Wegener challenge about the configuration of oceans + continents

Answer 23

he claimed continents + ocean were distributed very differently in the deep past - continents “drifted” over time

Question 24

What did Wegener name the larger landmass of all the continents?

Answer 24

Pangaea (“all land”)