Where is crust neither formed nor destroyed

Geologists believe that, if spreading continues, the three plates that meet at the edge of the present-day African continent will separate completely, allowing the Indian Ocean to flood the area and making the easternmost corner of Africa the Horn of Africa a large island. The size of the Earth has not changed significantly during the past million years, and very likely not since shortly after its formation 4.

The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created, as Harry Hess surmised. Such destruction recycling of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks is subducted under another.

The location where sinking of a plate occurs is called a subduction zone. The type of convergence -- called by some a very slow "collision" -- that takes place between plates depends on the kind of lithosphere involved. Convergence can occur between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates.

If by magic we could pull a plug and drain the Pacific Ocean, we would see a most amazing sight -- a number of long narrow, curving trenches thousands of kilometers long and 8 to 10 km deep cutting into the ocean floor.

Trenches are the deepest parts of the ocean floor and are created by subduction. Off the coast of South America along the Peru-Chile trench, the oceanic Nazca Plate is pushing into and being subducted under the continental part of the South American Plate.

In turn, the overriding South American Plate is being lifted up, creating the towering Andes mountains, the backbone of the continent. Strong, destructive earthquakes and the rapid uplift of mountain ranges are common in this region. Even though the Nazca Plate as a whole is sinking smoothly and continuously into the trench, the deepest part of the subducting plate breaks into smaller pieces that become locked in place for long periods of time before suddenly moving to generate large earthquakes.

Such earthquakes are often accompanied by uplift of the land by as much as a few meters. On 9 June , a magnitude This earthquake, within the subduction zone between the Nazca Plate and the South American Plate, was one of deepest and largest subduction earthquakes recorded in South America.

Fortunately, even though this powerful earthquake was felt as far away as Minnesota and Toronto, Canada, it caused no major damage because of its great depth. Oceanic-continental convergence also sustains many of the Earth's active volcanoes, such as those in the Andes and the Cascade Range in the Pacific Northwest. The eruptive activity is clearly associated with subduction, but scientists vigorously debate the possible sources of magma: Is magma generated by the partial melting of the subducted oceanic slab, or the overlying continental lithosphere, or both?

As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed.

The Marianas Trench paralleling the Mariana Islands , for example, marks where the fast-moving Pacific Plate converges against the slower moving Philippine Plate. The Challenger Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's interior nearly 11, m than Mount Everest, the world's tallest mountain, rises above sea level about 8, m.

Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano.

Such volcanoes are typically strung out in chains called island arcs. This is a special type of convergent boundary called a collisional boundary. The Himalayas in India are the result of two continental plates the Indo-Australian and Eurasian plates colliding head on.

The third type of plate boundary is called a conservative or transform boundary. It is called conservative because plate material is neither created nor destroyed at these boundaries, but rather plates slide past each other.

The classic example of a transform plate boundary is the San Andreas fault in California. The North American and Pacific Plates are moving past each other at this boundary, which is the location of many earthquakes. These earthquakes are caused by the accumulation and release of strain as the two plates slide past each other. Another example of a transform boundary is seen at the mid-ocean ridges, where the spreading centers are offset by transform faults anywhere from a few meters to several kilometers in length.

Plate tectonics Activities. Plate boundaries Plate boundaries are found at the edge of the lithospheric plates and are of three types, convergent , divergent and conservative. Wide zones of deformation are usually characteristic of plate boundaries because of the interaction between two plates.

A chain of volcanoes often forms parallel to convergent plate boundaries and powerful earthquakes are common along these boundaries.

The Pacific Ring of Fire is an example of a convergent plate boundary. At convergent plate boundaries, oceanic crust is often forced down into the mantle where it begins to melt. Magma rises into and through the other plate, solidifying into granite, the rock that makes up the continents.

Thus, at convergent boundaries, continental crust is created and oceanic crust is destroyed. Most of the active volcanoes on Earth are located on this circumference. The Ring of Fire gets its name from all of the volcanoes that lie along this belt. This area is also a hub of seismic activity, or earthquakes. One such area is the circum-Pacific Ring of Fire, where the Pacific Plate meets many surrounding tectonic plates. The Ring of Fire is the most seismically and volcanically active zone in the world.

The plates which make up the Ring of Fire are so huge even the slightest shift results in massive tremors, volcano activity and tsunamis. In total, volcanoes sit upon the Ring of Fire and seismologists state it is impossible to predict when a volcano will erupt. Rising gradually to more than 4 km 2. The largest super eruption at Yellowstone 2.

New Zealand is located on the edge of a zone of intense seismic activity known as the Ring of Fire.