VOLCANO WATCH: A Closer Look at Kīlauea’s Newest Lava

Left: Magnified view of erupted material, including Pele’s hair and tears, as seen under a microscope (USGS photo by K. Lynn on Dec. 26, 2020). Middle: Zoomed in electron image of the tephra, where greyscale indicates the relative abundance of iron. Very small (less than 0.004 inches) clinopyroxene and plagioclase crystals are labeled next to vesicles (bubbles). Right: Another electron image of tephra that contains small (~0.2 inches) olivine crystals with spinels. Greyscale images from the University of Hawai‘i at Mānoa electron microprobe laboratory, used with permission.

Every rock on Earth is made of up a unique combination of chemical elements, and lavas/tephra formed during Hawaiian eruptions are no exception. What is the geochemistry of Kīlauea’s newest tephra, and how can it help us understand the processes driving the ongoing eruption?

New eruptions at Kīlauea provide a glimpse into what is happening inside the volcano and its magma reservoirs. The recent summit eruption, which began on the night of Dec. 20, 2020, allows us to “see inside” the volcano once more and learn more about where the magma supplying the eruption is coming from.

To think about where magma is stored and how it moves prior to eruption, geologists can measure the chemistry of erupted materials — including quenched lava (glass), minerals, and dissolved gases or gas bubbles. The chemistry can tell us a lot about how hot the magma was at depth, how long it sat inside the volcano prior to arriving at the surface, and how different magmas (old vs. fresh, cooler vs. hotter) might have mixed together before erupting.

So what does Kīlauea’s youngest tephra, which are particles of the magma thrown out of the vent that fall through the air, tell us about this eruption? A simple way to start investigating is to look at how much of the element magnesium (Mg) they contain.

We use Mg (expressed as MgO, or magnesium oxide) measured in glass (for example, quenched lava) to determine how hot a magma is, which indicates how recently it arrived at Kīlauea from its source deep below the Island of Hawai‘i. Higher MgO contents indicate “fresh” hot magmas entering Kilauea’s shallow reservoir shortly prior to eruption, whereas lower MgO contents reflect “older” and colder magmas that have been stored within the volcano for longer periods of time.

Shortly after the first tephra erupted on Dec. 20 and 21, the USGS Hawaiian Volcano Observatory (HVO) worked with the electron microprobe lab housed at the University of Hawai‘i at Mānoa to measure MgO in the new quenched lavas. The new tephras have glass MgO contents of approximately 7 weight percent, which is very similar to the composition of Kīlauea’s previous lava lake in 2018. This indicates that the current eruption did not begin with “fresh” hot magma entering Kīlauea’s shallow summit reservoir. Instead, the eruption is likely drawing magma from the same shallow source that fed the 2008–2018 lava lake.

Olivine crystals (the bright green mineral commonly found in Hawaiian lavas) also have a lot of Mg. In olivine the abundance of Mg is expressed as the forsterite content (Fo), which is a ratio of how much Mg there is compared to the iron (Fe) content [Mg/(Mg+Fe)x100]. Similar to the glasses, higher Mg in olivine (for example, higher Fo) means that the crystals grew from hotter, fresher magmas. If the olivine Fo content is low, it tells us that the crystals grew in a cooler magma. The first olivine crystals to grow in the freshest, hottest magmas rising beneath Kīlauea typically have Fo contents of 88–90.

Kīlauea’s newest olivine crystals, which are typically about 0.5 mm (0.02 inches) in diameter, have relatively low Fo values of 82. This suggests that the crystals grew in relatively cool magmas stored at shallow depths (up to a few km or less than 2 miles below the ground surface) at Kīlauea. The crystals are also chemically homogeneous, meaning that there are no changes in Fo content from the middle to the rim. This tells us that mixing between hotter (higher MgO) and colder (lower MgO) magmas has not occurred recently at Kīlauea’s summit.

Together, the low-MgO glasses and homogeneous, low-Fo olivine indicate that hot, fresh magma has not been erupted. Instead, these compositions tell us that older, “cooler” magma — possibly the left-over magma from 2018 — is erupting now at Kīlauea’s summit. HVO scientists are continuing to monitor the chemistry of newly erupted materials, and changes in glass MgO or olivine Fo could help us understand future changes in the eruption.

See the HVO website for the revised Volcano Awareness Month schedule, which will be posted later this week.

Volcano Activity Updates

Kīlauea Volcano is erupting. Its USGS Volcano Alert level is at WATCH. Kīlauea updates are issued daily.

Lava activity is confined to Halemaʻumaʻu with lava erupting from a vent on the northwest side of the crater. On the morning of Jan. 21, the lava lake was about 663 feet deep and remains stagnant over its eastern half. Sulfur dioxide emission rate measurements made on Jan. 16 were about 2,500 t/d below the range of emission rates from the pre-2018 lava lake. Summit tiltmeters are on a deflationary trend. Seismicity remains elevated but stable, with steady elevated tremor and a few minor earthquakes. For the most current information on the eruption, go online.

Mauna Loa is not erupting and remains at Volcano Alert Level ADVISORY. This alert level does not mean that an eruption is imminent or that progression to eruption from current level of unrest is certain. Mauna Loa updates are issued weekly.

This past week, about 56 small-magnitude earthquakes were recorded beneath the upper-elevations of Mauna Loa. Most of these occurred at depths of less than about five miles. The largest recorded earthquake was a M2.9, just east of the summit caldera, on Jan. 18 at 10:26 p.m. HST.

Global Positioning System (GPS) measurements show continued slow summit inflation, consistent with magma supply to the volcano’s shallow storage system. Gas concentrations and fumarole temperatures at both the summit and Sulphur Cone on the Southwest Rift Zone remain stable. Webcam views have revealed no changes to the landscape over the past week. For more information on current monitoring of Mauna Loa Volcano, go online.

There were 10 events with three or more felt reports in the Hawaiian Islands during the past week:

• An M2.7 earthquake six miles E of Pāhala at a 19-mile depth on Jan. 19 at 9:45 p.m. HST
• An M2.4 earthquake six miles NNE of Puakō at a nine-mile depth on Jan. 19 at 7:29 a.m HST
• An M3.2 earthquake four miles N of Nāʻālehu at a zero-mile depth on Jan. 18 at 10:55 p.m. HST
• An M2.7 earthquake 12 miles SW of Pa‘auilo at a 13-mile depth on Jan. 16 at 11:58 p.m. HST,
• An M2.5 earthquake four miles SE of Leilani Estates at a one-mile depth on Jan. 15 at 9:03 p.m. HST
• An M1.7 earthquake six miles ESE of Waikoloa at a negative one-mile depth on Jan. 15 at 8:26 a.m. HST
• An M2.2 earthquake five miles ENE of Pāhala at a 19-mile depth on Jan. 14 at 7:20 p.m. HST
• An M1.8 earthquake four miles ENE of Pāhala at a 20-mile depth on Jan. 14 at 6:42 p.m. HST
• An M1.8 earthquake three miles ENE of Pāhala at a 20-mile depth on Jan. 14 at 6:28 p.m. HST
• An M4.0 earthquake four miles ENE of Pāhala at a 21-mile depth on Jan. 14 at 6:15 p.m. HST

HVO continues to closely monitor both Kīlauea’s ongoing eruption and Mauna Loa for any signs of increased activity. Email questions to [email protected].