Understanding how the chemical composition of magma changes before an eruption occurs is not an easy task. A sample of lava erupted at the surface provides a look at the final composition of magma; however, lava samples do not provide
Understanding how the chemical composition of magma changes before an eruption occurs is not an easy task. A sample of lava erupted at the surface provides a look at the final composition of magma; however, lava samples do not provide a complete picture of how the magma changed on the way to the eruption site.
During its ascent beneath the summit and storage within the volcano, magma often undergoes compositional changes associated with degassing, cooling and crystallization prior to eruption. When olivine crystallizes from magma, for example, the remaining magma is depleted in magnesium and is said to be more “evolved.” As shallow magmatic pathways change and develop during a prolonged eruption, the chemical composition of magma can also change as it courses through the volcano.
Magma mixing occurs when a new batch of magma flows into and mixes with a pre-existing molten magma body. The old, cold rock around the magma conduit can also be incorporated into the magma as it moves through the system. Such changes are not always easy or straightforward to identify from erupted lava alone, but there are a few tools volcanologists can employ.
Melt inclusions — small pockets of magma, including dissolved gasses, preserved inside a growing crystal — provide a means of directly sampling the composition of the magma when the host crystal started to grow. Melt inclusions are time capsules similar to the way insects can be preserved within amber.
Upon eruption, melt inclusions quench into glass and can be analyzed separately from the host crystal and the rest of the rock sample. The original composition of magma, in the melt inclusions, compared with the composition of the olivine crystals in the erupted lava, can provide unique information, such as the depth of crystallization and crystallization history.
Kilauea is an excellent setting to study melt inclusions at an active volcano. Lava flows are easily accessed, and the Hawaiian Volcano Observatory collects fresh lava samples on a regular basis. Kilauea’s most common mineral, olivine, proves additional information for understanding early magma compositions. Olivine’s chemical composition is dependent on the magma composition from which it crystallizes and provides another means of observing changing magma compositions. By analyzing melt inclusions and olivine crystals, we should be able to deduce how the magma moving within the volcano changed prior to eruption.
With a magma history through the volcano, geologists can test ideas on how the magmatic system of Halemaumau and Puu Oo are connected. One test would be to compare the olivine and melt inclusion compositions from the summit and those from the rift zone. If olivine and melt inclusion compositions from Puu Oo are both more evolved than those from Halemaumau, then the compositional change could be because of the transit time from the summit underground to Puu Oo, degassing or heat loss. If the olivine and melt inclusion compositions from Puu Oo are less evolved than those from Halemaumau, then Puu Oo draws its magma from depths deeper than the overlook vent. If the compositions from Puu Oo and Halemaumau prove to be similar, our understanding of how fast the magma moves between two vents or how quickly minerals crystallize needs refining.
Recent Kilauea melt inclusion research for this eruption revealed that magma transport from the summit to the east rift zone alters the magma to a more evolved composition; however, more work in this area would serve to reinforce our rudimentary understanding and possibly depict how the overlook vent influences magmatic changes or how the summit-east rift zone relationship has changed over the years.
Kilauea activity update
A lava lake within the Halemaumau Overlook vent produced nighttime glow visible from the Jaggar Museum overlook and via HVO’s webcam during the past week. The lake level fluctuated slightly in response to summit deflation-inflation events but was generally between about 80 and 115 feet below the floor of Halemaumau.
On Kilauea’s east rift zone, breakouts from the Peace Day tube remain active above the pali and on the coastal plain, and small ocean entries are active on both sides of the Hawaii Volcanoes National Park boundary. In addition, the Kahaualea flow, fed directly from a spatter cone on the northeastern edge of Puu Oo’s crater floor, continues to advance slowly toward the northeast across a plain of 1980s-era aa flows. This spatter cone is also the source of short lava flows repeatedly spilling down Puu Oo’s entire eastern flank over the past several days.
There were no earthquakes felt in the past week on the Island of Hawaii.
Visit hvo.wr.usgs.gov for Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.
Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.
