| Physical Geology 2002 |
Volcanoes
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Volcanoes consist of magma welling to the earth's surface and making landforms as it erupts. When magma reaches the surface it is called lava. Many factors contribute to the type and results of the eruption. The type of eruption can greatly change if and how it affects people.
Types of Volcanic Environments
There are several situations in which volcanoes form. The three major ones are subduction zones, rift zones and hot spots. In subduction zones oceanic crust is melted as it slides below another plate. Diagram one shows this process happening with the main parts labeled.
A hot
spot volcano is formed when rising plume of mantle goes through decompression
melting causing it to break the surface. The hotspot in Diagram 2 shows
how the hot magma rises through the crust, melting a passage as it goes.
Hotspots typically do not move. Plates do however and as a result a line
of islands typically forms as shown below. The Hawaiian chain is a series
of hotspot islands.
A rift zone is an area where plates are moving apart and new lithosphere is being created. This most commonly happens under water in oceanic ridges. These are also on-land in situations such as the East African Rift. In both situations, as shown in diagram 3, plates move apart and magma wells up cooling and forming new crust. These ridges typically create a few inches of new crust every year. As shown below the crust created holds record of the polarity of the earth. This is shown by the alignment of the metals in the rock.
Types of Eruptions
Effusive eruptions are characterized by a large outpouring of lava onto the ground. Effusive eruptions and the landforms they create are determined by the different types of lava depending on the makeup of the melt. Basaltic lava becomes aa (chunky, slow moving lava) or pahoehoe (runny, fast flowing lava), and creates thick rocky or thin ropy surface. These basaltic eruptions are typically slow moving enough for people to get out of the way. Property can be damaged if in the way but people are not usually hurt. Thin lava forms triangle shaped landforms. As the lava flowing out gets thicker the shape of the volcano becomes more steep and dome-like.
Debris flows are another much more violent type of eruption. A lahar debris flow is made up of rock and water caused by a volcanic eruption. These flows can travel very quickly and be quite deadly. They are formed by melting ice and snow or intense rainfall on volcanic debris.
Many other types of eruptions occur but the above are the
most often seen. As a result of these eruptions and people living in the
dangerous areas in volcanic zones many people are killed by these events.
Author:
Chip Yeakey
Creation/revision date: March, 10 2002
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Earlham · Geosciences Department · Geosciences 211: Physical Geology |
Copyright ©-2001 Earlham College. Revised 10 March 2002. Send corrections or comments to yeakech@earlham.edu