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Volcano Watch — Can rocks speak to us? Yes, they can!

February 14, 2008

Oftentimes, in videos of Hawaiian eruptions, you may see people near the vent or the lava rivers sampling the molten rock. This can be dangerous work. Why bother?

Geologist Matt Patrick collects a lava sample from the front of an advancing sheet flow on the east side of the TEB flow field.

Scientists do it because lava samples can provide important information about the source of the magma, the length of time magma has been stored, the extent of mixing with previously stored magma, and the temperature of the lava. From these data they can make inferences regarding the volcano's internal plumbing.

Kīlauea and Mauna Loa are the most active volcanoes on Hawai'i island. Magma feeding these volcanoes originates from a large, persistent and stationary "hot spot" deep beneath the Earth's crust. Since they are being fed from the same hot spot people assume that they have the same chemical makeup?wrong!

We know, from analyzing the rocks from these volcanoes, that their chemical composition differs. Mauna Loa's lava has more silica and less alkalis, sodium and potassium. This difference has implications for the source material that supplies magma to the volcanoes. In addition, it tells us something about the volcano's position, relative to the hot spot. In this case, Mauna Loa is closer to the hot spot's center than Kīlauea.

Once the magma migrates from the hot spot into the volcano, the composition begins to change owing to crystal formation and loss. Olivine, a primary mineral in Hawaiian lavas, is one bellwether of several processes. We know that magma supplied from the hot spot to the volcanoes is chock-full of olivine crystals.

The growth history of olivine is recorded in its mineral structure. Olivine growth is analogous to growth rings on a tree. As the tree matures, the size of its rings reflects climatic conditions of time. Olivine also grows incrementally, and its composition records magma composition over time. By analyzing its composition from the core to the rim, we can derive the magma composition.

By analyzing minerals within lava, we can determine whether magma has been stored for years or transited quickly through the system. For example, olivines that have little-to-no variation from core to rim, in chemical composition moved rapidly through the volcano's plumbing system.

Another bit of information we can glean from lava chemistry is the amount of magma mixing. In this instance, other minerals help to "sleuth out" the magmatic history and whether or not mixing has occurred. Mineral formation in Hawaiian magmas follows an orderly sequence: olivine crystallizes first, followed by pyroxene (a dark green-to-black mineral), and finally plagioclase feldspar, a white mineral.

Lavas that contain pyroxene and plagioclase are stored magmas. These magmas are quite different than the olivine-rich magmas supplied to the volcano. The source regions that fed these flows indicate that magma was stored there for a period of time.

Finally, we can glean the magma temperature from its chemical composition. In a series of laboratory experiments, scientists were able to demonstrate that temperature is related to the amount of magnesium in magma. So the more magnesium magma has, the hotter the temperature.

The next time you are out and about, walking on Hawaii's lava, it will no longer look the same. Now you have some idea of the complex journey it took as it made its way to the surface. The evidence of this journey is recorded and documented by the minerals and gray mass within their hardened shell.

Lava compositions can give us insight into the magmas' parents, their life journey, who they commingled with, where they resided, for how long, and, finally, how hot they were when they were erupted. When the nature of the lava is revealed to you in this way, no wonder some geologists say the rocks "speak" to them. In any case, it will be GRAY MATTER NO MORE!


Volcano Activity Update

On July 21, 2007, lava began erupting from a set of fissures on the east flank of Pu`u `O`o. Eruptive activity soon stabilized at fissure D, 2.3 km (1.4 miles) northeast of Pu`u `O`o. For several months, lava was directed entirely into a perched lava channel that generally fed flows to the northeast and east. At dawn on November 21, lava began to erupt directly from fissure D, outside the perched channel, creating the Thanksgiving Eve breakout (TEB) flow. The perched channel continues to sporadically host minor activity, but most of the activity is now focused on the TEB flow.

The TEB flow has continued to build a series of low shields, extending southeast from the vent, over the last several weeks. On January 8, the front of the southeast-most shield collapsed, and a large volume of lava surged out to form a rapidly moving 'a'a flow. The flow advanced about 3.4 km (2.1 miles), reaching to within 180 m (~590 ft) of the top of the Royal Gardens subdivision before stagnating. The eruption then resumed its construction of low shields within about 2 km (1.2 miles) of the TEB vent. The same shield collapsed again on January 26, sending an 'a'a flow 650 meters (0.4 miles) into the Royal Gardens subdivision three days later, before stalling. Shield construction promptly resumed, and slowly advancing pahoehoe flows continue to be shed east and southeast from the shield complex.

Weak incandescence was briefly observed in Pu`u `O`o last week, having appeared a few times since late August. As in years past, Pu`u `O`o likely is serving as a large chimney, beneath which lava is stored briefly and degassed substantially on its way to the erupting fissure.

Mauna Loa is not erupting. One earthquake was located beneath the summit in the past week. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates, which have slowed further since May 2007.

Two earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-2.6 earthquake at 7:36 p.m., H.s.t., on Saturday, February 9, was located 15 km (9 miles) northwest of Kailua-Kona at a depth of 8 km (5 miles). A magnitude-2.3 earthquake at 5:14 p.m., H.s.t., on Sunday, February 10, was located 6 km (4 miles) east-northeast of Waikoloa at a depth of 7 km (4 miles).

Visit our Web site for daily Kīlauea eruption updates and nearly real-time Hawai`i earthquake information. Kīlauea daily update summaries are also available by phone at (808) 967-8862. skip past bottom navigational bar

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