1. What lines of evidence confirm that seafloor spreading has been taking place?

Question

1. What lines of evidence confirm that seafloor spreading has been taking place? You should be able to explain both paleomagnetic evidence and that of ocean floor core samples.

2. What is the general relationship between global earthquake activity and plate boundaries?

3. Describe the fundamental differences among divergent, convergent, and transform plate boundaries.

4. Describe and explain the tectonic activity, volcanic activity, and general topographic features associated with the three kinds of convergent plate boundary: oceanic–continental plate subduction, oceanic–oceanic plate subduction, and continental plate collision. Mention at least one present-day example of each of these kinds of convergent boundary.

5.Why is there such a concentration of volcanoes and earthquakes around the margin of the Pacific Ocean (the region referred to as the Pacific Ring of Fire)?

Explanation / Answer

1. The Sea floor spreading was first proposed by Harry Hess in 1960. The two fundamental facts led to him to postulate this theory are- the mid ocean ridges occur in all major ocean basins, and the oceanic crust has a thickness of 6-7 kilometers only whereas the typical thickness of continental crust is 30-40 km.

Some evidences confirm that sea floor spreading has been taking place.

A) Drilling into the ocean floor has been done to study the sediments occurring there. Cores obtained from near the oceanic ridges have shown, that only thin layer of young sediments overlie the oceanic crust. With an increase in sediments from the ridge, the sediment generally thickens and the age of the sediment also increases.

B) Magnetic surveys taken the oceanic ridges have disclosed symmetrically distributed stripes of positive and negative magnetic anomalies. The axis of symmetry coincide with the crest of the mid oceanic ridge. These strips of magnetic anomalies provide proof for theory of sea floor spreading. At the oceanic ridge new sea floor being produced. This new sea floor records the reversal in the earth's magnetic field. The time elapsed between the reversal ranges from thousands to hundred of thousands of years. By determining the amount of new sea floor produced during a magnetic reversal of known length, the rate of spreading can be calculated.

C) The occurrence of earthquake and active volcanic island along the crest of mid oceanic ridges also provides evidences which are in favor of the theory of sea floor spreading.

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