Volcano Watch — Lava unlocks subterranean mysteries

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 zenoliths. 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 Kīlauea Volcano, from perhaps one hundred miles deep, arrives at a temperature of about 1,360° Centigrade (about 2,480°F). However, lavas with eruption temperature this high have been found only along the submarine part of Kīlauea's East Rift Zone. The lavas erupted near the summit and along the subaerial part of the rift zones commonly have temperatures of about 1,160°C (2,120°F) and range from only about 1,115°C (2,040°F) to about 1,200°C (2,190°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 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 Kīlauea, such as those in 1790 and 1924, are caused by addition of water to the magma stored inside the volcano.

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 Kīlauea 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 the mixing predates the eruption by more than a few days, the evidence for mixing will have been erased.

Lava from the ongoing eruption, like nearly all lavas erupted at Kīlauea, has cooled and formed crystals of olivine before transport through the rift system to the eruptive vents. Most of these crystals have apparently settled through the magma, because of their greater density, and accumulated inside the volcano, as they are not present in the erupted lava. The earliest lavas erupted in 1983 also had abundant evidence of magma mixing. The magma erupting today has also lost some of its gas content through the summit and much more through the Pu`u `O`o vent prior to eruption.

Volcano Activity Update

The nearly 10-year-long eruption on Kīlauea's East Rift Zone continues to start and stop, as it has for the last six months. The eruptive interval that began on August 16 shut down early in the morning on August 29. The pause in activity was brief as the episode 51 vents became active once again about 1:00 a.m. on September 2. The flows from the August 16–August 29 interval advanced to about the 2,080-foot elevation, just west of the flow that cascaded over the pali in late July. Due to poor visibility most of this week, we have little information about the extent of the flows since activity restarted on September 2. The pond inside Pu`u `O`o rose about 50 feet to a level about 120 feet below the rim of the cone between September 1 and 2, as the eruption began.

In the past two weeks, Hawai`i has had four earthquakes with magnitudes greater than 3.0. The first of these was located just west of Pahala and occurred at 3:19 p.m. on August 18. It was located about 6 miles deep and had a magnitude of 3.1. The next two were located on the south flank of Kīlauea Volcano, south of Napau Crater. The first of these occurred at 2:58 p.m. on August 24 and the second at 1:43 a.m. on August 28. Each was magnitude 3.0 and located at a shallow depth (less than 4 miles deep). The last earthquake, with a magnitude of 3.5, occurred on August 30 at 6:57 p.m. and was located beneath the Hamakua coast at a depth of about 8 miles. These small felt earthquakes serve as a constant reminder that Hawai`i, particularly the Big Island, is prone to earthquakes that can be much larger and more damaging.