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Volcanowatch

June 24, 2010

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Asphalt Volcanoes: Where the Lava Meets the Road?

On the Island of Hawai`i, the only time people think of volcanoes and asphalt together is when an active lava flow crosses the road (cue "Why did the lava cross the road?" joke).

But in California, people are not too concerned about lava from recently discovered volcanoes spreading across highways. That is because the volcanic flows are made of asphalt, or tar, instead of lava, and are located under about 210 m (700 ft) of water on the ocean floor. These underwater domes are situated 16 km (10 mi) from the Santa Barbara shoreline and are made of crude oil similar to that used to pave roads and parking lots.

It is important to note that these are not true volcanoes. By definition, a volcano is an opening in the earth's crust out of which gas, rock, and lava is erupted. Hawaiian volcanoes, such as Kīlauea and Mauna Loa, are perfect examples of the stereotypical volcano. The discovery of eruptions and flows involving materials other than lava has led to the coining of terms like "mud volcano" and "asphalt volcano."

Asphalt volcanoes are located all over the world in correlation with oil seeps and reservoirs, but not all oil seeps and reservoirs coincide with asphalt volcanoes. These volcanoes are quite rare and occur where there is a reservoir of especially heavy, viscous petroleum than can seep up onto the overlying sea floor and solidify as asphalt. An on-land example is the La Brea Tar Pits in Los Angeles, California, which were originally named Los Volcanes de Brea, or Tar Volcanoes.

Unlike underwater lava flows (or any lava flows, really), the erupting petroleum is not especially hot or molten. Instead, the lukewarm petroleum erupts onto the sea floor rich in gases. The flow becomes more dense and viscous as the gases leak out until, eventually, it slows to a halt.

Each underwater volcano has a single petroleum source and a limited number of exit points, meaning that, over time, these volcanoes will (and do) die out. Successive eruptions occur in the same place, and older flows are covered by new flows. The California volcanoes erupted between 44,000 to 31,000 years ago.

Asphalt flows exhibit a variety of textures. The blocky and ropey morphologies observed on those near Santa Barbara look like the `a`a and pahoehoe lava flows observed around Hawai`i. Flow thickness was difficult to estimate but may be several meters (yards).

The Californian asphalt volcanoes, formed from ancient seepage, built underwater domes of tar through a process similar to how lava domes are created. Due to the high viscosity of the petroleum, the oil did not flow far. Instead, it piled up on itself, creating a dome around the eruption site that increased in height over time.

Lava domes can grow to be several hundred meters high. The height of the largest Californian asphalt mound, named Il Duomo, is relatively puny by comparison, at only 20 meters (65 feet) high, with a volume of 50,000 cubic meters (315,000 barrels) worth of crude oil.

For oil companies, that is still an impressive amount of oil. But before you start planning your next "get rich" scheme around mining asphalt volcanoes, it is important to note that it is not good-quality oil. This is bottom-of-the-barrel "my car will kick and moan" oil.

While asphalt volcanoes may not represent the largest volcanoes or be a potential source of oil, they represent a newly discovered, intriguing geologic process that scientists are still trying to figure out. And, thanks to extensive study of lava volcanoes, researchers are having an easier time understanding and quantifying these volcano-like features.

Kīlauea Activity Update

Over the past week, activity on the east rift zone flow field remained focused on the construction of low lava shields high above the pali. Some lava flows shed from the shields advanced a short distance toward the south and southeast, but none were reported to have crested the top of the pali. No surface flows have been reported on the pali or coastal plain for the last few weeks. The lava pond within Pu`u `Ō `ō crater is also no longer active, though a new incandescent vent had opened midway up the vertical eastern wall of the crater by last weekend.

At Kīlauea's summit, a circulating lava pond deep in the collapse pit within the floor of Halema`uma`u Crater was visible via the Webcam throughout the past week. The baseline lava level dropped over the past several days and was punctuated sporadically by short-lived lava-level increases. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

One earthquake beneath Hawai`i Island was reported felt during the past week. A magnitude-2.9 earthquake occurred at 1:01 p.m. on Sunday, June 20, 2010, and was located 9 km (6 miles) northwest of Kailua-Kona at a depth of 43 km (27 miles).

Visit our Web site (http://hvo.wr.usgs.gov) for detailed Kīlauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kīlauea activity summary; email questions to askHVO@usgs.gov.

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Updated: July 25, 2010 (pnf)