|MadSci Network: Earth Sciences|
Dear Lizelle, Your question has no simple answer. There are different kinds of lava, each with different properties depending on its chemical composition. The chemical composition of a lava determines what minerals will crystallize from it as it cools, and different minerals crystallize at different temperatures. For example, lavas erupted at mid-ocean ridges in places like Iceland, or at hot spots in places like Reunion Island in the Indian Ocean, are basaltic in composition; they contain minerals like olivine and pyroxene which have high crystallization temperatures. Basaltic lavas are typically erupted at temperatures of around 1100ºC. Other types of lava erupted at destructive plate boundaries, such as at volcanoes in Indonesia, have a chemical composition richer in silica; silica-rich minerals such as quartz and certain feldspars tend to have lower crystallization temperatures than olivine and pyroxene, and these lavas tend to erupt at lower temperatures than basaltic lavas. So, the chemical composition of a lava influences its temperature as it erupts and also the temperature at which it starts to solidify as it cools. Other factors that influence how long it takes for a lava to cool include the rate at which it erupts, the volume of lava erupted, and whether it is erupted into the air or under water. The rate of eruption is simply the volume of lava erupted per second. The eruption rate influences the rate of cooling because a high eruption rate will continually add a large volume of fresh, hot lava to a lava flow, and this will tend to encourage it to stay liquid for longer and to flow farther. A good example is the continual eruption that has been taking place on Hawaii for some years – you may have seen pictures of lava flowing from the crater for miles until it reaches the sea, or flowing like an underground river in lava tubes where the surface of the lava flow cools to form a crust that keeps the heat in for the lava flowing beneath. The volume of lava affects the rate of cooling according to a basic principle of physics which says that the ratio of surface area to volume of an object determines the rate at which heat is lost. A large object has a low surface to volume ratio, which means that it has a relatively small surface area in relation to its size from which it can lose heat. So a large object tends to cool more slowly than a small one. Think of a cup of hot water: if you leave the cup standing the water will cool eventually to room temperature, but if you tip the cup over so the water flows out onto a table top, the spilled water will cool much more quickly because you have greatly increased its surface area in relation to its volume. With lavas, this means that a large volume of lava will cool more slowly than a small one. A lava lake, where lava erupted in a crater remains inside the crater, would cool more slowly than a lava flow. If a lava erupts into water it will cool much more quickly than if it erupts on land into the air. The reason is that water is denser than air, and transports heat away from a lava much more efficiently than air. Lavas erupted under water frequently form what are called “pillows”, which are rounded blobs in which the surface of the lava in direct contact with the water is quenched within seconds to form a solid crust while inside the lava will remain liquid for longer as it cools more slowly. Lavas erupted under water tend not to flow very far before they solidify. On land, however, lavas can flow for miles before they solidify and stop. With eruptions of spectacular size called continental flood basalts, lava flows can reach thicknesses of tens of meters and flow for hundreds of miles before they cool. Thankfully no such eruption has occurred during human history, but many examples from the distant geological past are known to geologists. I have not exactly answered your question, have I? I’ve told you that lavas can take different amounts of time to cool depending on a number of factors. Lava flows like those being erupted on Mount Etna as I write are of a size and composition where they will flow some miles down the mountain before they solidify. In the case of Etna today the solidifying of such flows might take several weeks, and afterwards they would remain warm or hot to the touch for much longer, perhaps several months. I hope this answers your question. Best wishes, David Scarboro
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