Volcanic Landforms

Volcanic Landforms Volcano is essentially a fissure or vent (opening) which serves as an outlet for hot magma from beneath the Earth’s crust. The hot liquid magma coming out in the form of lava (most commonly molten basalt) is potential of shaping different landforms on earth crust. The most common landforms that are results of volcanic activity include cones, craters, calderas, domes etc. This article gives a description of these volcanic landforms. SHIELD VOLCANOES- These are characterised by gentle upper slopes. The slope varies from only 5 degrees to 10 degrees.The shield volcanoes are made up of mainly thin lava flows around central vent. The low viscosity lava (low silica content) travels down gently and as it cools down it creates less steeper lower slope. These are also found at the flanks of bigger volcanoes like Kilauea is situated on the Hawaiian shield volcano. STRATOVOLCANOES- These are characterised somewhat steeper slopes (30 degrees to 40degees) near the vent a nd the summit due to highly viscous lava flows are not able to travel long distances before they cool down.The less steeper slopes at the base is due to erosional activity. The show inter layering of pyroclastic material and thus sometimes also called composite volcanoes. Pyroclastic material make upto 50% of a stratovolcano. They show a long time of repose which makes them quite dangerous. CINDER CONES- They are small volume cones build up by fall deposites around the eruptive vent. There slope is around 25-30 degrees. During later stages of eruptions the lava may come out from the flanks. Cinder and tephra cones usually occur around summit vents and flank vents of stratovolcanoes.MAARS- The magma heats up the groundwater and converts it into vapours which builds up pressure and results in eruptions containing water and pre-existing rocks. Walls of crater fall back into crater and fills it with loose material. If the crater depth reaches below the water table of the region, the maa rs often fill up with water. LAVA DOMES- The viscous lava or rhyolitic lava is not able to flow far away from the vent, it rather accumulates over the vent. The forecoming lava from the vent pushes and the upper solidified outer surface rolls down to the flank.The slope is very rough due to spines pushed up by the magma below. CRATERS AND CALDERAS- Craters are the depressions which are the results of explosions emitting gases and tephras. They can range upto 1km in diameter. Calderas are large depressions, circular or elliptical in shape with diameters ranging from 1 km to 50 km. Calderas form as a result of collapse of a volcanic structure. The collapse results from evacuation of the underlying magma chamber. The evacuation process of lava chambers in stratovolcanoes in rapid and as the lava goes out the walls collapse and form a large crater.They are enclosed depressions which often collect rain water and snow melt and may form lakes. Like Crater Lake in southern Oregon is about 8 km in diameter and dated to be formed around 6800years ago. FUMAROLES- A fumarole is vent where gases, either from a magma body at depth, or steam from heated groundwater, emerges at the surface of the Earth. Since most magmatic gas is H2O vapor, and since heated groundwater will produce H2O vapor, fumaroles will only be visible if the water condenses. H2O vapor is invisible, unless droplets of liquid water have condensed). HOT SPRINGS- Hot springs or thermal springs are areas where hot water comes to the surface of the Earth. Cool groundwater moves downward and is heated by a body of magma or hot rock. A hot spring results if this hot water can find its way back to the surface, usually along fault zonesGYESERS- A geyser results if the hot spring has a plumbing system that allows for the accumulation of steam from the boiling water.When the steam pressure builds so that it is higher than the pressure of the overlying water in the system, the steam will move rapidly toward the surfac e, causing the eruption of the overlying water. Some geysers, like Old Faithful in Yellowstone Park, erupt at regular intervals. The time between eruptions is controlled by the time it takes for the steam pressure to build in the underlying plumbing system. |