Activity 2: Geologic Cross Sections

Geologic cross sections show the Earth as it would appear if it was cut vertically with a large knife and viewed edge on. The depth portrayed can vary from a few feet to the entire radius of the Earth. Sections through the upper 200 to 300 km of the Earth show the lithospheric plates and the upper part of the asthenosphere. Thus they are useful for depicting plate tectonic boundaries.
indent.gif (51 bytes)The top part of Fig. 1 shows a map view of a protion of the South Pacific Basin near New Zealand. Along the southeast (left side) of the map is a midocean ridge. A trench-transform boundary occurs to the west (right). A geologic cross section along line A-A is shown in the bottom part Fig. 1. The ridge near A' is where new crust is created, so it represents a location of volcanic activity. Because it is new and hot, the lithosphere is thin and the upper mantle elevated. As it moves away from the ridge, cooling causes the lithosphere to thicken and its density to increase. The increasing depth of the ocean floor and its thicker profile result from these changes in density. These changes cause the sea floor to sink and the oceanic crust to thicken. Close to A, the vertical line in the cross section represents a transform fault. At this boundary, plates slide past each other, perpendicular to the plane of the page, so that the two lithospheric plates do not show any change in thickness.



Fig. 1: Example of a lithospheric cross section.

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