By looking at the first waves to reach many seismometers, one can deduce the orientation of the fault plane and whether the earthquake was triggered by compression or tension. Earthquakes near trenches are almost always due to horizontal tension failures at right angles to the trench axis . A prominent feature of all ocean floors is the oceanic ridge. One characteristic of the ridge figures prominently in the two competing theories of how trenches formed. As explained in the preceding chapter, the ridge is cracked in a strange pattern.
Some cracks are nearly perpendicular to the ridge axis, and other cracks are parallel to it.
Their shapes and orientation are best explained by the stretching of the oceanic ridge. Enormous amounts of melted basalt, called flood basalts , have spilled out on the earth's surface. They also help us test theories of trench formation. More than a dozen of these convulsions have occurred at different places on earth, dwarfing in volume the total magma used to form all volcanic cones. This site best viewed with Netscape 6 or Internet Explorer 5.5 or greater.