.Geologia : PlateTectonics As you can see in the above map, many of the spreading boundaries arelocated deep in the ocean on the sea floor. The entire plate is moving northand westward (up and to the left) as the top edge converges with theNorth American and European plates. You can see the left side of the Pacificplate is converging with the Indianplate. Deep down where the leading edge of the Nazca plate is diving down underthe South American plate it's making contact with the molten magma of theearth's mantle. The long cordillera, orchord-like chain of volcanic mountains known as the Andes, are a resultof the rumpling of the South American plate where the Nazca plate crashes intoit, AND the volcanoes that have formed from the melting Nazca plate margin deepdown.No subduction happens along these margins, just massive deformation ofthe edges of the plates. The resulting strain from the sliding action of theplates causes cracks in the crust called faults. Oneof the most famous faults in the world is the San Andreas fault, which runsalong the west coast of California. The instruments that seismologists use to measure earthquake magnitudesare designed to detect the amount of energy released by the movement of theground during a quake. In the case of the Chile earthquake, the amount of energy releasedduring the quake, not the number of human deaths and damage to structures,earned it the title of greatest. The epicenter of the earthquake (the point on theearth's surface directly above the focus of an earthquake) was 60 meters downbelow the ocean floor about100 miles off the coast of Chile out in the Pacific. The land along the coastof Chile, particularly in the Port city of Peurto Montt, subsided (sunkdownward) as a result of the movement of the ground during the quake and thecoastal city was flooded with ocean water. The damage from the quake was not limited to the nearby shores of Chile.
The tsunamis were created by the shifting of the sea floor that generated thehuge temblor. Sometimes faults are just cracks inthe earth caused by buckling and stress from the movement of the tectonic plates andsometimes the faults are plate boundaries (where the edges of the tectonicplates meet) See the page on Ocean's Deep for more information. The subduction (downward movement) of the Nazca plate under the theSouth American continent is what caused the major quake back in 1960 (seethe page on Plate Tectonics). The more gallons ofwater you put between you and the surface of the ocean, the greater thepressure is on your body because of the weight of the water over the top ofyou. The oceanic crust is much heavier than the continental crust so when theplates crash into each other, the oceanic plateplunges downward toward the molten mantle, while the lighter, continental platerides up over the top. The forces driving the two plates together are reallyintense so the underlying oceanic plate (the subducted plate) creates a trench where it drags the edge of thecontinental crust down as it descends underneath (check out the picture atleft).This is what's happening on the bottom of the Pacific Ocean off theMarianas islands. The really deep part of the ocean is in the bottom of thetrench created by the subducting oceancrust.The echo sounder measures precisely how long it takes for the soundwaves to be returned to the surface and determines the depth based on the rateof return.