To the non-geologist, lava is hot, orange, molten rock or, when cooled, smooth and gray or black and jagged rock. However, to the geologist or volcanologist, lava contains a set of clues to decipher processes occurring in the interior of the Earth and the volcano. [These processes, once understood, form the basis of eruption forecasting, including eruptive style (passive or explosive), magnitude, and timing.] Magma (lava before it erupts) consists of molten rock, crystals, dissolved gases, and bubbles. Sometimes there are also fragments of the rocks through which the magma moved, called xenoliths. Many lavas have had complex histories that include cooling and forming crystals, loss of gases, remelting of rock that surrounds the magma body inside the volcano, and mixing of different magmas. In order to understand the initial formation of the magma deep inside the Earth, one needs to be able to "see through" the effects of all these processes.
Magma that moves up into Kilauea Volcano, from perhaps one hundred miles deep, arrives at a temperature of about 2,480 degrees F. However, lavas with eruption temperatures this high have been found only along the submarine part of Kilauea's East Rift Zone. The lavas erupted near the summit and along the subaerial part of the rift zones commonly have temperatures in the range from only about 2,040 to about 2,190 degrees F. Clearly, these lavas have had to cool by several hundred degrees within the volcano before they erupt. Cooling of magma causes crystals to form. In Hawaiian lavas, the first crystal to form as magma cools is olivine, the mineral that forms green sand beaches.
As magma is stored inside the volcano in a magma reservoir, it also loses some of the gases that are dissolved in the melt. The main gas components in magma are carbon dioxide, water, and sulfur gases. Most of the carbon dioxide, and some of the water and sulfur dioxide, leaks out of the summit of the volcano as the magma is stored in the magma reservoir about 3 miles below. During this period of storage, the magma may also melt the rocks adjacent to the magma reservoir, thereby changing the chemistry of the magma. Sometimes groundwater or hydrothermal fluid is added to the magma with dramatic results. Explosive summit eruptions at Kilauea, such as those in 1790 and 1924, were caused by addition of water to the magma stored inside the volcano (this phenomenon is rare however).
There are numerous separate batches of magma stored
inside the volcano at any time. Each has slightly different
chemical characteristics that can be used to trace mixing
of the different batches. Most eruptions at Kilauea
appear to involve several of three magma batches that
have mixed with each other. Commonly, the magmas
that mix have different temperatures and contain distinctive
assemblages of crystals. Upon mixing, some of these
crystals dissolve back into the melt while other crystals
grow rapidly. If the magma is erupted soon after the
magma batches mix, these crystals are quenched before
they can dissolve. However, if mixing predates the
eruption by more than a few days, the evidence for
mixing will have been erased.
The preceding article was published in the April 1993 issue of Lithosphere, the official bulletin of the Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch (Editor).
The material is in the public domain, and may be republished freely.
Last updated: 18 September 2002