# Volcano Watch — Lava slowly fills Puu Oo and builds broad ridge

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Lava flows stopped pouring into the sea on the south shoreline of Kīlauea 6 months ago. Few imagined that it would mark the beginning of a progressive collapse of the main lava tube system and the slow retreat of lava from the coast back to its source — the Puu Oo cone.

Even fewer would have guessed that, by the beginning of the new year, lava would fill large deep pits on the cone's west flank, build a nearby broad ridge of overlapping rootless shields, or start filling the Puu Oo crater once more. That's the nature of erupting volcanoes and in particular the long-lived eruption of Kīlauea, changing all the time and creating new landforms that dazzle and sometimes confuse even the most frequent visitors.

In the past two weeks, all seven vents in the crater of Puu Oo have at one time or another shot lava spatter into the air and roared with gas escaping from the lava below. Lava pouring intermittently from these vents has paved nearly the entire crater floor with new flows. Today, the west crater rim stands only about 2 m (7 feet) above the crater floor, and the east rim, about 8 m (26 feet) above the floor.

Since October, however, even more vigorous activity has occurred from two new spatter cones located just west of West Gap, a low area on the west crater rim that collapsed in 1997. From the West Gap vents, lava has spread several hundred meters north, west, and south. These flows have completely filled large pits on the west and south flanks of Puu Oo, built spatter cones 10-15 m (30-50 ft) tall, and significantly added new bulk to the cone in the form of the West Gap shield, which extends westward from Puu Oo itself.

In the past few days, lava from the West Gap shield has poured through the gap into the Puu Oo crater. If this activity continues, the west crater rim will disappear soon beneath flows fed from the growing shield, the vents on the west crater floor, or both.

The highest lava discharge for the past several months, however, has been occurring between 1 and 4 km (0.5-2.5 miles) from Puu Oo. We have identified more than 15 locations where lava has ponded and then flowed outward in all directions. Why so many different locations?

Lava that poured into the ocean 6 months ago flowed through a series of tubes originating near Puu Oo. Tubes insulate lava from heat loss, allowing it to remain fluid and travel far without cooling. Lava commonly breaks out of a tube to form a new flow or series of flows at the surface. Such breakouts occur when lava completely fills the tube and exerts upward pressure against the overlying rock. The tubes fill because discharge of lava has increased or part of the tube collapsed downstream.

A breakdown in the tube system near the coast began in late July. Soon thereafter, breakouts began occurring within a few kilometers (miles) of Puu Oo in an area where the ground slope is less than about 5 degrees. A low, broad, shield-like structure grew as lava accumulated around each breakout point. The breadth or height of the shield increases with each new flow. We call these structures rootless shields, because they are built by lava emerging from the shallow lava tube system and not directly from a deeper reservoir.

More than a dozen such rootless shields have grown recently, ranging from 50 to 500 m (150 to 1,500 ft) in diameter and 3 to 50 m (10 to 150 ft) high. Several of these low shields have been active at the same time, sending lava flows in all directions. Most of the shields have grown together to form three broad centers of activity along an elongate area in the shape of a dog's leg about 3 km (1.8 mi) long.

Every sunrise brings something different to the Puu O`o area these days. It is too bad the area is so hard to visit, for the activity can be breathtaking.

### Volcano Activity Update

Eruptive activity for the past week is described above. One earthquake was reported felt in the week ending on January 7. Residents of West Hawaii reported the 3.4-magnitude event at 4 p.m. January 4. The earthquake took place 9 km (5 miles) north the summit of Hualalai at a depth of 25 km (16 miles).

Mauna Loa is not erupting. The summit region continues to inflate. Seismic activity remains very low, with 4 earthquakes located in the summit area during the last 7 days.