|MadSci Network: Earth Sciences|
Some kinds of lava are very liquid and runny when they are erupted, and flow very easily. Other kinds of lava are “sticky” and do not flow very well. Liquid lava is called basalt, and basalt is the kind of lava that can form plateaus. Sticky lavas build up to form cone-shaped volcanoes. A good example of a cone-shaped volcano made from sticky lavas is Mount Fuji in Japan. A good example of liquid basalt lava is the kind of lava that erupts in Hawaii. You may have seen pictures of rivers of molten lava in Hawaii. Liquid lava can flow for many miles before it finally cools and stops. As it flows it spreads out, rather like water, instead of building up into a cone-shaped mountain. Liquid lava can form a plateau when many eruptions take place, one after another over a very long time, perhaps thousands or even millions of years. Each lava flow will flow until it cools and hardens, often forming a flat plain of rock. Hawaii has many of these large, flat lava plains. The next eruption will flow over the cold rock of the previous eruption, adding another layer. Over many eruptions the thickness of the stack of lava flows will increase, and a plateau may be the result. There are many volcanoes like Hawaii that have liquid basalt lava. There is also a rare type of eruption called a “flood basalt” eruption. A flood basalt eruption has the same type of liquid basalt lava as Hawaii, but is very much larger. It can happen either on land or in the oceans, and it will erupt such huge amounts (floods) of basalt lava that the lava flows will cover vast areas of land or of the ocean floor. Flood basalt eruptions take place, in fact, as many eruptions, one after another over a very long time in the same area. Luckily for us flood basalt eruptions do not happen very often – the last one was about 17 million years ago in what is now the states of Oregon and Washington, and lasted, on and off, for about 2 million years. A flood basalt eruption can build up a huge plateau. The flood basalts in Oregon and Washington are called the Columbia River Volcanic Province because the Columbia River flows through them. The lava flows cover an area of about 165,000 square kilometers (about 64,000 square miles). Many lava flows are stacked up one on top of the other, so that if you visit the area you can see the different lava flows as layers in the mountain sides. Have a look at: http://volcano.und.nodak.edu/vwdocs/volc_images/north_america/crb.html Some of the individual lava flows are huge, up to 20 to 30 meters (about 60 to 100 feet) thick. The Columbia River Volcanic Province has been eroded by rivers, rain, wind and ice for so long that it is now worn down and divided up into hills and valleys. Sticky lavas do not flow like basalt. Instead they flow very slowly and do not flow very far. They tend to pile up close to the crater where they are erupted, forming a cone-shaped pile of lava and rubble instead of a flat plain. There are many volcanoes of this kind, and when we think of volcanoes we often think of a cone-shaped mountain like Mount Fuji. Have a look at: http://volcano.und.nodak.edu/vwdocs/volc_images/img_fuji.html
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