The point where a suddden break in the river's gradient occurs. See Knickpoints
A volcano is formed when magma finds a weak spot in the Earth's crust and burst through. Usually magma is concentrated along plate boundaries, but in continental interiors hot spots can generate volcanoes.
Distinction of Volcanoes
As the name suggest, these volcanoes are always active and erupt more often e.g Mt Fuji in Japan. These volcanoes can erupt violently or just release heat or gases on a yearly or monthly bases. Usually these volcanoes are found along plate boundaries where a lot of collision occurs or along hotspots in earth's interior such as in Hawaii. Active volcanoes require regular monitoring as they can erupt at any time.
Extinct volcanoes have not erupted for a very long time and when they were periodically anticipated to erupt they remained quiet and are now considered non-threat. Usually these volcanoes have been eroded for years or covered with vegetation.
Dormant volcanoes have been quiet for a while but have just recently erupted. They die and resurrect again. When these volcanoes erupt most people are unaware of it as they believe that the volcano is extinct and no longer a menace. For example Mt Pinatubo erupted in 1991 after more than 600 years when it erupted in 1380. Mt St.Helens in the western end of USA erupted in 1980 after more than 120 years.
Types of Volcanoes
Composite Cones are one of the most destructive volcanoes. They are symmetrical in shape and mainly extrude viscous(thick) magma from a central vent which is composed of toxic gases such as carbon dioxide and argon. When these volcanoes erupt they release tonnes of ash of in the atmosphere which can reduce sunlight for months or years e.g 1816 year without summer, caused by 1815 Mt Tambora eruption in Indonesia. A fiery flow of ash mixed with rock fragments known as a Pyroclastic flow/ Nuee Ardente race down the mountain side at high speeds engulfing anything in the path. When these volcanoes erupt they leave a huge void (blow out) known as a crater. Mt Aconcagua in S.America, Mt Fuji (Japan) and Mt Rainier in Washington are examples of composite cones.
Pyroclastic Materials/Tephra vary in Size. Ash is less than 4mm Lapilli are small fragments ranging between 4-30 mm in size and Bombs are huge rock blocks above 30 mm in size.
Cinder cones are small volcanoes that eject fragmented lava. They are less violent and less gases compared to composite cones. These volcanoes usually develop as child volcanoes near major parent volcanoes.
Shield Volcanoes are wide dome shaped Volcanoes that mainly extrude fluid basaltic magma. The fluid lava is ejected regularly and flow long distances before congealing. These volcanoes are less violent. Mauno Loa in Hawaii is a good example.
Cumulo-domes are dome shaped Volcanoes mainly made from viscous acidic lava that travel a few kilometres before solidifying. They resemble a tortoise's back.
These are broad, shallow craters left when a volcano erupts.The hole can be filled with water to form a lake.
A caldera is an enormous depression that result from extreme explosions which collapses the whole flanks of the mountain into the depression. In simple terms a caldera is a widened crater. Generally the volcano is left unrecognisable.