Volcano Watch: When Rocks Fly

December 3, 2020, 1:55 PM HST (Updated December 3, 2020, 1:55 PM)

Tephra layers preserved at the summit of Kīlauea from at least three different eruptions. Deposits below the top of the scale are predominantly juvenile and deposits above it containing many lithics. Notice the larger size of the yellow pumice clasts compared to the much denser and finer grey lapilli and ash surrounding them from 7 to 18 on the scale. The scale is in cm. Photo: Jo Schmith, Nov. 25, 2020.

Have you ever wondered what it would be like to travel from the depths of Kīlauea and be hurled into the air as a tiny grain of volcanic ash? How high would you go and how far would you fly? Let’s take a look at the theory of airborne rocks to find out!

Tephra is the Greek word for ash, and it is the label we use for rocks that come flying out of the volcano during an eruption. Every feature of every single tephra grain has something significant to say about the volcanic process that created the grain and the transport journey it took afterward. Our task as tephra volcanologists is to do the detective work and find as many clues in the grains as possible to help us understand the dynamics of past explosive eruptions. Today, we focus on features that influence transport by air.

There are two ways to begin this journey. One starts from the hot molten magma itself, and the other begins in the wall of the volcanic conduit.

Tephra that starts as fluid hot magma is classified as juvenile. It is a diverse group here at Kīlauea that includes smooth-skinned droplets (Pele’s tears), frothy foams, almond-shaped projectiles, denser grains with small isolated bubbles, tiny shards of dark glass, and thin strands of glass (Pele’s hair). Different abundance and size of bubbles cause a wide range of density among various types of juvenile grains. We subdivide them into shards, scoria, pumice, and reticulite, in the order of increasing bubble content and decreasing density.


Grains from the wall rock are called lithic clasts. They are created when an explosion destroys part of the conduit wall or picks up talus blocks from a crater floor. At Kīlauea, they consist of shattered basalt and are mostly dense and blocky.

After the juvenile or lithic tephra has been expelled from Kīlauea, the journey of a single grain is determined by its aerodynamic properties. There are two basic ways to fly.

One is a simple throw from the volcano that will follow a ballistic curve in the air. This is reserved for the big and dense tephra grains that we refer to as juvenile bombs or lithic blocks. The height and length of the journey as a ballistic depends on the force of the explosion that sent it flying, as well as its size and density, but it ends with a big thud within 0.6-1.2 miles from the vent.

The other mode of flying includes the forces of air and is seen as a plume rising from the vent and spreading laterally across the sky. Tephra travelers in a plume will find that they are lifted much higher than the ballistics and can cover many more miles of distance. However, they literally have to go where the wind blows them.

Once in the plume, these little pieces of rock known as lapilli and ash start their final journey toward the ground. The larger lapilli grains fall faster and end up closer to the volcano, whereas the fine ash grains can stay in the air longer and are the most adventurous. Lithic grains are usually denser and fall faster than juvenile grains, and therefore the lithic ride is shorter compared to a juvenile of the same size. Pumice has a lower density than scoria of similar size and can stay in the air longer. That way both the size and grain type that meets the ground changes with distance as we move downwind from the vent.

Finally, when all the grains have settled there is a layer of tephra on the ground that reflects every single journey of each tephra grain and shows a nice outline of the plume. Wind and water can erode the tephra, but often a tephra layer can stay on the ground for hundreds or thousands of years, faithfully preserving the events of the eruption and waiting for a volcanologist to come along and measure grain size and density of the tephra grains to unfold how they got there.

Volcano Activity Updates

Kīlauea Volcano is not erupting. Its USGS Volcano Alert level remains at NORMAL (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kīlauea updates are issued monthly.

Kīlauea monitoring data for the month of November show variable but typical rates of seismicity and ground deformation, low rates of sulfur dioxide emissions, and only minor geologic changes since the end of eruptive activity in September 2018. The water lake at the bottom of Halema‘uma‘u continues to slowly expand and deepen. For the most current information on the lake, see https://www.usgs.gov/volcanoes/kilauea/k-lauea-summit-water-resources.

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 an eruption from the current level of unrest is certain. Mauna Loa updates are issued weekly.

This past week, about 110 small-magnitude earthquakes were recorded beneath the upper-elevations of Mauna Loa. Most of these occurred at shallow depths of less than about five miles. Global Positioning System (GPS) measurements show long-term slowly increasing summit inflation, consistent with magma supply to the volcano’s shallow storage system. Gas concentrations and fumarole temperatures as measured at both Sulphur Cone and the summit remain stable. Webcams show no changes to the landscape. For more information on the current monitoring of Mauna Loa Volcano, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.

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

  • An M3.1 earthquake four miles SW of Volcano at a one-mile depth on Dec. 2 at 5:59 p.m. HST
  • An M3.2 earthquake one mile S of Pāhala at a 20-mile depth on Dec. 1 at 9:58 p.m. HST
  • An M3.2 earthquake three miles SW of Volcano at 0-mile depth on Nov. 30 at 2:06 a.m. HST
  • An M3.0 earthquake six miles ENE of Pāhala at an 18-mile depth on Nov. 28 at 12:14 p.m. HST
  • An M2.0 earthquake six miles ESE of Waimea at a 13-mile depth on Nov. 28 at 8:58 a.m. HST
  • An M2.7 earthquake three miles SSW of Pāhala at a 20-mile depth on Nov. 28 at 6:48 a.m. HST

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


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