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October 31, 1997

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

How does pahoehoe flow?

How do pahoehoe lavas flow? Over the years, many scientists have watched and measured active lava flows, and now we have a pretty good idea of the process in Hawai`i.

When lava first comes out at the vent, it is highly charged with gas - so much so that the lava is more than 85 percent bubbles. The lava is more like foam, and the pahoehoe that forms when the lava cools is very "shelly"; so-called because walking on it is like walking on large egg shells.

The lava then tends to form channels that carry the fluid lava away from the vent, thereby beginning a lava flow. It may take a while for a channel to develop or it may happen almost immediately. In either case, a roof eventually forms over the channel, making a lava tube.

A tube insulates the lava inside so that it can stay hot and fluid as it flows away from the vent. The lavas are far less bubbly at this stage than when they first came out at the vent, because a lot of gas has escaped into the air, but they can still be more than 30 percent bubbles.

Sometimes we can look into skylights (openings) in the roof of a tube and see large bubbles breaking on the lava surface. The lava in the tubes remelts the old surface, creating large tunnels that carry the lava stream towards the coastline.

When the lava reaches the coastline and enters the ocean, it often forms benches where lava builds out on top of coarse black sand beyond an older coastline. The benches are not well supported and can fall off into the ocean. Sometimes these collapses expose a lava tube out of which lava pours like water from a hose. Other times, the bench slowly sinks, submerging lava tubes formed above sea level. When the tube is invaded by seawater, spectacular lava fountains play.

Whenever the advancing lava flow encounters large, flat areas, a different kind of tube can develop. The lava slows down, rapidly forms a crust, and thickens by continued injection of lava into the cooled shell, thereby forming a tumulus. The lava flows also spread out creating broad areas covered by large tumuli, which are hill- or plateau-shaped pahoehoe features that crack and bulge upward as the flow thickens. These can continue to grow as long as lava is being injected into them. There are many examples of these features in Kalapana, on the north side of Kiholo Bay (mile-markers 79 and 80), and just south of the Saddle Road near the Pohakuloa Training Area, to name a few.

These tumuli have grown over 10 m (30 ft.) thick after starting as a 30 cm (1 foot) thick advancing lava flow. Probably the largest known tumulus in Hawai`i is the so-called Uwekahuna laccolith exposed in the northwest wall of Kilauea Crater. A laccolith is formed when magma rising toward the Earth's surface stalls and solidifies at a shallow depth rather than erupting. However, our current knowledge suggests that this feature is a tumulus that once stood over 20 m (60 ft.) high and was subsequently buried by younger lava flows.

Because the lava actually lifts the cooling and thickening upper crust of tumuli, it is under pressure, and some of the gases become re-dissolved, lowering the volume of bubbles in the lava. We see this in a very characteristic glassy, steel-bluish variety of pahoehoe that only comes out of the bases of tumuli several weeks after they start to form.

Kilauea Eruption Status--October 31, 1997

The Kilauea Volcano east rift zone eruption at Pu`u `O`o continued unabated during the past week with lava from the crater vent often lighting the night sky. Lava entered the ocean at two locations - Waha`ula and Kamokuna. A minor bench collapse at the ocean entry led to a series of explosive phreatic activity on October 31. Lava viewers are reminded that this activity is very hazardous, and the ocean entry area should be avoided.

Recent Big Island Earthquakes

Seven earthquakes were reported felt during the past week. At 3:36 a.m. on Sunday morning, residents of Ka`u, Puna, Hilo, and Hamakua were awakened by a magnitude 4.0 earthquake originating from 16 km south of Kilauea summit at a depth of 31 km. Six earthquakes from the north flank of Mauna Kea near Mana were reported felt by a resident of Ahualoa. The largest of the series at 9:17 on Sunday morning was also felt in Pa`auilo and Waimea. The magnitudes of the earthquakes ranged from 3.6 to 1.6, and their depths varied from 10 to 15 km.

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