lecture 4 ocean basins and seafloor spreading

Question 1

what are some oceanagraphic research methods

Answer 1

Depth sounding (e.g. HMS Challenger, 1870s)
 Echo sounding
 Seismic reflection profiling
 Dredging and dating of samples
 Drilling (e.g. Glomar Challenger, 1968)
 Submersibles (e.g. ALVIN)
 Magnetic field measurements
 Gravity field measurements (e.g. GEOSAT)

Question 2

what is echo sounding and what has it found

Answer 2

a frequency is emmited from a boat and a hydrophone picks up the seismic reflection, the times of the returning waves can be used to determine the depth of the ocean and the sub-bottom sedimentary layers

it found abyssal plains 4 km deep
trenches of up to 11 km
Network of mid-ocean ridges (MORs) –65,000 km long and ~ 2km high

Question 3

what are abyssal plains

Answer 3

abyssal plain is an underwater plain on the deep ocean floor

Question 4

what are trenches

Answer 4

a long, narrow, deep depression in the ocean bed, typically one running parallel to a plate boundary and marking a subduction zone.

Question 5

what is a mid ocean ridge

Answer 5

a long, seismically active submarine ridge system situated in the middle of an ocean basin and marking the site of the upwelling of magma associated with sea-floor spreading.

Question 6

what did Dredging, dating, drilling & submersibles reveal

Answer 6

seafloor is mostly covered in
sediments, but underneath these are volcanic
rocks
– Current/recent volcanic activity at MORs and at
end of aseismic ridges

Question 7

what are aseismic ridges

Answer 7

An aseismic ridge is a mountain ridge or chain of seamounts under the ocean. They do not produce seafloor spreading or seismic activity, except in the area of a “hotspot” at one end of the ridge.

Question 8

what are fracture zones

Answer 8

A fracture zone is a linear oceanic feature—often hundreds, even thousands of kilometers long—resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics.