|MadSci Network: Earth Sciences|
It is hard to measure the temperature inside an erupting volcano directly because it is impossible to get close enough to an active crater, and because temperatures will be so high that they would melt any instruments lowered inside. What we can do is measure the temperature of the lava that is erupted. This gives us a good indicator of the temperatures at shallow depth inside the volcano. Once again, of course, we cannot stick a thermometer into the lava because it would melt. But we can use remote sensing devices to measure temperature. There are different types of volcano that erupt different types of lava. Some volcanoes erupt lava called basalt, which is derived from the Earth’s mantle at depths typically of around 60 or more km. Basalt tends to be very fluid, and when erupted gives rise to the rivers of lava such as you may have seen in films about Hawaii. Basalt lavas are the hottest. Basalt lavas tend to erupt at temperatures of around 1150°C to 1200°C, which suggests that temperatures inside the volcanic crater and at shallow depths would be similar. In general, the deeper in the Earth the hotter the temperature. This is true not only in volcanoes but everywhere. Temperature increases about 30ºC for every kilometre depth. Volcanoes occur at locations where the thermal gradient (as we call this increase of temperature with depth) is much greater than normal. The lava erupted at a volcano migrates upward from its starting point in the mantle or deep in the crust, and usually collects in a large magma chamber (lava is simply magma which erupts at the surface), which can be many kilometres below the surface. An eruption occurs when the magma in a magma chamber rises to the surface. The temperature in a magma chamber must be hot enough to keep the magma in a molten state, and because we know what the melting temperatures of minerals are we can infer that temperatures in a magma chamber must be at least high enough to prevent the minerals from crystallizing. For example, basalt magma is rich in the chemical elements that form a mineral called olivine. The melting temperature of olivine is about 1100° C, so if the magma is to remain liquid and olivine crystals are not to start forming, the temperature must remain at least as hot as this. An interesting clue that geologists can sometimes use to estimate temperature in a magma chamber is provided when some crystals do form in magma in a magma chamber. Lavas erupted from volcanoes sometimes contain early-formed crystals (called phenocrysts) embedded in them. Knowing the crystallization temperature of the minerals from which these phenocrysts are formed, geologists can work out the temperature at which they formed. Therefore, sometimes a magma chamber will cool enough at depth for some crystals to start to form, but may then erupt before it can cool any more, or even receive a fresh injection of hot magma from below. In the Earth’s core the temperature is estimated to be about 8000° C. This is the highest temperature to be found inside the Earth, and much hotter than in any volcano. The outer part of the Earth’s core is permanently liquid at these high temperatures. A website you might like to visit is Volcanoes Online, at http://library.thinkquest.org/17457/english.html. Volcanoes Online has many good diagrams and explanations about how volcanoes work. I hope this answers your question. Best wishes, David Scarboro
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