Exploration of the Oceans (3.2)

Question 1

Mapping seafloor: acoustics and echosounding?

Answer 1

• Can’t use satellites/lasers/radar to map seafloor directly.
• Acoustic energy transmitted efficiently through liquids.
• Echolocation found to be good at detecting seafloor.
• Depth can be determined if velocity of sound in water known.
• Transmit regular pings, record time to hear echo.
• Coverage still very incomplete, maps highly interpretative.

Question 2

What is seismic reflection?

Answer 2

Detail of sub-surface layers.

Question 3

What is swath bathymetry?

Answer 3

Modern systems utilise multiple acoustic beams to map strips of seafloor at a time. Swath width/resolution depends on water depth.

Question 4

What is the physiography of the ocean basins?

Answer 4

• Tharp’s bathymetry map of 1950s revealed continuous 65,000km long submarine mountain chain (mid-ocean ridge), in centres of most ocean basins, encircling Earth.
• Rise up to ~3km above seafloor, 100km wide.
• May be offset by fracture zones.
• Revealed deep troughs around margins of some oceans.

Question 5

Evidence for a difference between continental and oceanic basement: seismic data

Answer 5

• Discovery by Mohorovicic of ubiquitous seismic discontinuity.
• Compositional change within Earth - higher velocity and density beneath.
• Discontinuity taken as boundary between crust and mantle.
• Difference in thickness between continents and oceans: Continents (30-35km), oceans (7-8km).

Question 6

Evidence for a difference between continental and oceanic basement: global elevations

Answer 6

• Hypsometric curve.
• Bimodal distribution of elevations.
• Implies two fundamentally different types of crust.

Question 7

Evidence for a difference between continental and oceanic basement: sediment thickness

Answer 7

• Ocean floor covered by sediment.
• Thickest near continents.
• Thinnest at mid-ocean ridges.
• Ocean drilling showed oldest sediment gets older further from mid-ocean ridge.

Question 8

Evidence for difference between continental and oceanic basement: heat flow at mid-ocean ridges

Answer 8

• Higher geothermal gradients over crests of mid-ocean ridges.
• Implies hot material at depth.

Question 9

Evidence for difference between continental and oceanic basement: nature of material sampled from mid-ocean ridges

Answer 9

Fresh glassy basalt lavas routinely recovered in dredge hauls from ridge axes.

Question 10

Evidence for difference between continental and oceanic basement: earthquake locations

Answer 10

• Accurate earthquake location following establishment of global seismometer arrays to monitor nuclear tests during Cold War.
• First global map showing belt-like distribution of earthquake epicentres (1950s).
• No explanation for phenomenon when first discovered.