Seafloor Spreading (3.3)

Question 1

Who was Harry Hess?

Answer 1

• Discovered many submarine features, including flat-topped seamounts.
• Recognised they were subsided, eroded volcanoes.
• Realised ocean floor is geologically active.

Question 2

What were some early seismic investigations (Post WWII)?

Answer 2

• Pioneering ‘active-source’ seismic refraction experiments at sea.
• Discovered uniform ocean crust thickness and internal velocity structure.
• Discovered sediment layer in Pacific thin.

Question 3

What was the key report to US navy (1960)?

Answer 3

• Hess articulated concept of mid-ocean ridges as locus of generation of new ocean crust.
• Proposed lateral motion above upward mantle convection currents was mechanism for continental drift.
• Still no proof, called his ideas ‘an essay of geopoetry’.

Question 4

What was the paper in Nature by Bob Dietz (1961)?

Answer 4

• Coined term ‘seafloor spreading’; envisaged mid-ocean ridges as sites of eruption of lavas.
• Hess initially proposed ‘ocean crust’ made of serpentinised mantle peridotite.
• Abandoned when fresh glassy lavas dredged from ridge crests.
• Still no proof of seafloor spreading concept.

Question 5

What is marine magnetic anomalies?

Answer 5

• Marine surveys measure total magnetic field (B).
• MMA calculated by removing local value of Earth’s ambient field Ba from B.
• Surveys ocean floor found systematic positive and negative anomalies.
• Form anomaly ‘strips’ parallel to ridge axis.

Question 6

What was the marine magnetic anomalies in relation to proof of seafloor spreading?

Answer 6

• MMA maps revealed magnetic stripes. Authors had no idea what it meant.
• Cambridge undergraduate student Fred Vine made link between Hess’s seafloor spreading hypothesis and magnetic anomaly patterns.
• Calculated effects of alternately polarised magnetic blocks added equally on either side.
• Accepted as hard evidence for seafloor spreading.
• Able to calculate rates of seafloor spreading at different mid-ocean ridges.

Question 7

What is the basic mechanism of seafloor spreading?

Answer 7

• Symmetrical magnetic anomalies in newly-formed ocean crust record lateral movement away from ridge as polarity of Earth’s magnetic field reverses with time.
• Presence of young basalt implies partial melting of upwelling mantle beneath mid-ocean ridge rises to seafloor, solidifies to form ocean crust.