Secrets of the Oceans

Question 1

Sediment on ocean-floor

Answer 1

Is undisturbed

Question 2

Mid-ocean ridge systems

Answer 2

Continuous range of submarine mountains

e.g. can be 50,000km long, 1,500km wide and are always symmetrical

Question 3

Fracture zones

Answer 3

Deep and long (>1000km) linear gashes that can ofset mid-ocean ridges.

Question 4

Trenches

Answer 4

Long and deep, water depths up to 11km

Question 5

Rocks

Answer 5

Mainly basalts and marine sediments

All younger than 180Ma

Question 6

Thermo-remnant magnetism

Answer 6

Preserves marine magnetic anomalies in basalts.
Measured in nanoteslas
Magnetometer trace is very irregular. But, when the regional field is removed, there are positive anomalies (higher than normal values) and negative values (lower than normal values).
Profiles are symmetrical about the ridge axis
Linear anomalies are parallel to the ridge axis
Similar pattern at all spreading ridges
Spacing of positive and negative anomalies is proportional to the timing of magnetic reversals found in land-based lavas (also shows rigid plates).
Same time reversal scale works for all ridges.

When lava cools, it passes through the Curie temperature and preserves Earth’s magnetic field.

Shows no ocean floor older than 200Ma

Question 7

Chrons

Answer 7

The main periods between major magnetic reversals.

Sub-chrons are shorter time scale reversals within a chron.

These can be matched with magnetic polarity timescales found on land.

Assuming constant spreading rates across ridge-systems, the ages of magnetic reversals can be estimated up to 160Ma

Question 8

Deep-sea drilling program

Answer 8

compared oldest sediments on the ocean-floor basalts with the age predicted by the magnetic reversal timescale from land-based lava flows.

Question 9

Spreading at mid-ocean ridges

Answer 9

Is symmetrical

Rate of half-spreading is typically 10-100mm. Full spreading rate is double this.

Question 10

Transform faults

Answer 10

Active parts of fracture zones, between offset parts of the ridge axis

Question 11

Movement on transform faults

Answer 11

right-lateral or left-lateral depends on which way the plate on the other side of the fault is moving

Question 12

Reconstructing ancient continents

Answer 12

Magnetic anomalies allow the past relative positions of continents the be identified.

Question 13

Why can’t the Earth be expanding

Answer 13

Constant water volume would mean that Pre-Triassic would be completely flooded Earth
Moment of inertia would be different
Length of day would change