Tectonics and the Ocean Floor
The theory of plate tectonics has revolutionized geology over the last half century, explaining many of Earth’s physical features. Tectonic plates are huge fragments of Earth’s lithosphere, which consists of the crust fused with the upper mantle. They move over a more ductile, or plastic, layer of the mantle called the asthenosphere. Plate motion builds mountain ranges, but plate-tectonic processes are perhaps most clearly seen on the ocean floor, where most of the plate boundaries are found.
Recycling Ocean Crust
The oldest rocks on the ocean floor are 180 million years old. This is young compared with the oldest continental rocks, which date from 3.8 billion years ago. While the continental crust has been steadily accumulating throughout Earth’s history, it seems the oceanic crust is created and destroyed rather quickly. It is created at the mid-ocean ridges from hot material rising in the mantle, and then spreads away from the ridges, before eventually being recycled into the mantle at subduction zones. Continental crust is always less dense and more buoyant than oceanic crust, so where they meet, it is the oceanic crust that gives way, sinking (subducting) back into the mantle.
Tectonic Plate Boundaries
The boundaries of a tectonic plate may be divergent, convergent, or transform. At divergent boundaries, the crust is extended, thinned, and fractured by the upwelling of hot mantle material. The crust buoys up, producing a mid-ocean ridge, and lava is extruded through a central rift valley to create new oceanic crust. Seamount volcanoes may also arise.
Tectonic plates collide at convergent boundaries. Where oceanic crust meets continental crust, the latter may be compressed and thickened, resulting in mountain-building. The dense oceanic plate sinks beneath the lighter continental plate, forming an ocean trench, and volcanic activity occurs as the crust descends into the mantle. Where two ocean plates meet, the oldest, most dense plate is subducted and an arc of volcanic islands is formed parallel to the trench. Transform boundaries arise where plates are moving past each other. No crust is created or destroyed, nor is there any volcanic activity. They can occur where segments of a divergent boundary are offset, and extensive fracture zones can result.
Earthquakes and Tsunamis
Earthquakes are associated with all plate boundaries, but they are particularly frequent at convergent boundaries, such as subduction zones. Stress builds up at faults in the crust until it overcomes the strength of the rock and the fault slips. When this happens, a huge amount of energy can be released in a short time. The earthquake that produced the Indian Ocean Tsunami of 2004 released 23,000 times more energy than the Hiroshima atomic bomb.
A tsunami may be triggered if an earthquake results in the uplift or subsidence of part of the sea floor. The water above suddenly rises or sinks, then flows to regain equilibrium. Surface waves radiate out at 310–497 mph (500–800 km/h) and can quickly cross an entire ocean basin.
Hotspots and Island Chains
The ocean floor between plate boundaries is far from featureless. Volcanic island chains are found far from any plate boundary due to the presence of hotspots (deep-seated and long-lived zones of volcanic activity) in the mantle. Some hotspots, such as the one beneath Iceland, are associated with divergent plate boundaries, while others lie in the middle of oceanic or continental plates. Chains of volcanoes often trail away from mid-ocean hotspots, with the oldest volcanoes, long extinct, now lying far away from the hotspot. These hotspot tracks are aligned along the direction of motion of the overlying plate. They change direction when the plate motion changes and may be interrupted when a new spreading ridge opens up, as it has between India and the Réunion Hotspot.