A week before massive fires began scorching California, scientists from around the world gathered to talk about an entirely different kind of danger that might be lurking beneath the Golden State. A little-known volcano near the town of Mammoth Lakes was the central focus. The volcano does not have the classic look of Mt. Fuji or the characteristic shield-shape of Kilauea or Mauna Loa. Indeed, it’s significant not for its peak or edifice, but rather for the opposite: Long Valley is a topographic depression that formed during an enormous eruption about 760,000 years ago.
This depression, which geologists refer to as a “caldera,” was created when a large chamber of molten rock suddenly erupted onto the surface. The void left by the exiting magma had no strength to hold up the roof rock. As the eruption progressed, the roof rock foundered relentlessly into the former magma chamber, and a caldera was born.
The scale of this eruption is difficult to comprehend. Based on studies of the ash deposits, which piled to thicknesses well over 100 m (330 ft) near the caldera and reached as far east as Nebraska, geologists estimate the total amount of magma erupted to have been about 700 cubic kilometers (170 cubic miles). The eruption is thought to have lasted only a few days. Kilauea would take 7,000 years to erupt an equivalent amount of magma.
The material erupted was quite different from Hawaiian lava. Our hometown volcanoes erupt a very fluid, magnesium- and calcium-rich, silica-poor lava known as basalt. In contrast, the Long Valley eruption of 760,000 years ago consisted of rhyolite, a much more silica-rich magma. The excess silica makes rhyolite very viscous and resistant to flowing.
High-viscosity magma can trap large quantities of volcanic gases. Eventually, these gases become so pressurized that they can force a very high-energy, explosive eruption. The magma expelled from an eruption like this consists largely of shattered volcanic glass, pumice, and bits of the pre-existing, or so-called “country,” rock that had surrounded the volcano.
Geologists are very fond of these bits of country rock, since they can use them to understand how a particular eruption evolved. In the case of the Long Valley eruption, certain telltale bits of country rock turn up in the lowest-, and hence earliest-erupted, parts of the ash deposits. Country rock of this type occurs only in a certain area. Hence, geologists have inferred that this is where the eruption must have begun.
Could it happen again? Well, that is the question on everyone’s mind, and that question is the main reason for the recent conference at Long Valley. The residents of Mammoth Lakes should be happy to know that it is extremely unlikely that another gigantic eruption will occur any time soon. Geologists, using a variety of geophysical techniques, have concluded that there is almost certainly not a sizeable magma chamber beneath Long Valley at present.
However, the news is not all good. There have been small eruptions in the Long Valley area at least three times during the last 5,000 years, with the youngest occurring only 600 years ago. Of course, “small” is a relative term. These eruptions were only about one-thousandth the size of the 760,000-year-old eruption but they were still big enough to have been destructive and dangerous. The fact that these eruptions occurred shows that even though magma is not present in large quantities, there is still enough left to cause trouble.
The residents of Mammoth Lakes do not need to be convinced that they live in a volcanically active region. There have been tens of thousands of earthquakes in the region over the last two decades, several as large as magnitude 6. Carbon dioxide, which is percolating up from a magmatic source region, is killing trees over wide swaths of Mammoth Mountain. Visitors to these tree-kill areas get a stark visual reminder that this volcanic area is not dead.
The Long Valley Observatory (LVO), a sister to HVO, is operated by the U.S. Geological Survey to monitor this restless caldera. LVO geologists use a variety of different monitoring methods, including continuous GPS, electronic tiltmeters, strainmeters, and seismometers. We at HVO can take some small pride in knowing that many of these techniques and instruments were pioneered right here on the Big Island.
Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. During darkness, an occasional glow is visible from renewed activity within Pu`u `O`o crater and the West Gap area of the cone. Except for small patches of molten material, the Mother’s Day flow is entirely crusted over from the vent to Paliuli. The distal end of the Kohola arm of the Mother’s Day flow is starting to cascade down Paliuli, and the lower 200-300 meters (yards) of the flow above the pali is incandescent. The east side of the Mother’s Day flow, and the coastal flat below Paliuli, continue to be dark. No lava is entering the ocean.
One earthquake was reported felt in the week ending on October 30. Residents of Pahala and Kealakekua were awakened by the ground shaking at 3:46 a.m. on Thursday, October 30. The earthquake responsible was a magnitude-3.8 event located 8 km (4.8 mi) north of Pahala at a depth of 11 km (6.6 mi).
Mauna Loa is not erupting. The summit region continues to inflate. Seismic activity remains low, with three earthquakes located in the summit area during the last seven days. Visit our website (hvo.wr.usgs.gov) for daily volcano updates and nearly real-time earthquake information.
This article was written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory and is republished by HawaiiNews.com with permission.