Question 8 (10 points) A geologist places a compass against a layer of rock. She
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Question 8 (10 points) A geologist places a compass against a layer of rock. She uses the level in the compass to make sure it is exactly horizontal. She then records the direction of the compass with respect to north. What is she measuring? a) The dip of the layer b) The strike of the layer c) The shear of the layer d) The plunge of the layer Save Question 9 (10 points) Why is there very little direct evidence for seafloor spreading prior to 200 million years ago? a) There is not enough funding to support studies of ancient seafloor spreading. b) Seafloor spreading probably did not occur until right around 200 million years ago. c) Ocean crust is recycled at subduction zones and so is not preserved in the rock record. d) Ancient ocean crust is inaccessible because it is buried deep beneath the continents.Explanation / Answer
Answer 1 : Option (b) : Strike of the Layer
Answer 2 : Option (c) : Oceanic crust is recycled at subduction zones and so is not preserved in the rock record
According to Harry Hess : The Mid-oceanic ridges are situated over the rising limbs of convection currents in the earth's mantle and that the thin oceanic crust is nothing more than a surface expression of the mantle and continuously created by the method of lateral accretion or spreading away from the ridge crests.
The ocean floors are young and ephimeral features of the earth's surface, contantly being regenerated at ridge crests and destroyed in the trench system
It is a clear that the formation of rocks in the sea floor is due to reabsorption of materials.
Hence there is very little evidence of sea floor spreading prior to 200 million years ago.
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