Lecture 6

Question 1

What is Bathymetry?

Answer 1

The measuring of the water depth in oceans, lakes and rivers

Question 2

What were the 5 key discoveries made using Bathymetry?

Answer 2

1. Most of sea-floor is flat and covered in undisturbed layers of sediment
2. Mid-ocean ridge systems
3. Fracture zones
4. Trenches
5. Rocks were mainly basalts if not marine sediments - all younger than 180Ma (young)

Question 3

What is the Mid-Ocean ridge system?

Answer 3

A continuous range of submarine mountains—> 50,000km long and up to 1500km wide and symmetrical

• Some ridges, like Pacific = smoother and no median rift
• Atlantic = rough bathymetry with median valleys

Question 4

What are fracture zones?

Answer 4

Deep, long linear gashes (>1000km) often offsetting mid-ocean ridges.
- The spread out from ridge or located where ridge doesn’t join up-so an offset

Question 5

What are trenches?

Answer 5

Long, deep slots in the sea floor, usually on the edge of ocean basins.

• water up to 11km
  e. g Mariana Trench

Question 6

Where are active basaltic volcanoes found in the ocean?

Answer 6

Mid-ocean ridge systems

Question 7

What are Marine Magnetic Anomalies?

Answer 7

A variation in earth’s magnetic field caused by magnetism of rocks of sea bed.

Question 8

How are these rocks magnetic?

Answer 8

TRM (Thermoremanent magnetisation) in magnetite in basalts—> measured in nanoteslas

Question 9

PPM=Magnetometer

- What is observed when looking at the magnetism of the ocean floor?

Answer 9

1. irregular—>no obvious pattern except at ridge axis AND THEN- remove regional field and you get a pattern!!!
2. Profiles symmetrical around ridge axis
3. Linear anomalies parallel to ridge axis
4. Similar patterns found on all spreading ridges
5. spacing of positive and negative anomalies (in relation to the regional field) is proportional to timing of magnetic reversals
6. Same reversal time scale works for all ridges.

Question 10

What is the Vine-Matthews Hypothesis?

Answer 10

when ocean crust forms at a midocean ridge, the cooling crust becomes magnetized in the direction of Earth’s prevailing magnetic field as it cools below the Curie temperature of the magnetic minerals

Question 11

What provided the timescale for the magnetic polarity

Answer 11

Measurements on land

Question 12

What does the hypothesis allow us to conclude?

x6

Answer 12

1. ocean-floor created at ridge axes by sea-floor spreading
2. Spreading produces symmetric pattern of linear magnetic strips (anomalies)
3. Rate of creation of one half of ridge is roughly 10-100mm per year
4. No ocean floor older than 180 Ma
5. History of magnetic field and reversal time scale back to jurassic
6. Explains why ocean-floor heat flow is high despite very low radioactivity

Question 13

What are transform faults?

Answer 13

the active parts of the fracture zones, between offset ridges that proved that it was they were caused by sea-spreading rather than by strike-slip fault (these would be active all along fracture zone but only earthquakes between ridges—>transform fault) know this because of pattern of seismicity

Question 14

What does the magnetic anomalies forms the basis of?

Answer 14

Continental reconstructions by removing newer rocks and aligning old.

Question 15

Why is the earth not expanding?

Answer 15

Rate of creation of new ocean floor must be balanced by destruction of the surface area at the same rate somewhere else on the planet