Volcano Watch — Lava flow continues; quake tremors light

The eruption at the episode 51 vents on the west flank of Pu`u `O`o continued without interruption this week. A lava pond has formed just west of the lower vent. This pond fills and overflows during periods of high-volume eruption, but most of the time flows are fed through tubes in the sides of the pond. The main flow from the pond has been toward the west.

A second vent, slightly farther up the side of the Pu`u `O`o cone, has erupted intermittently during the week. When active, this vent sends a smaller surface flow toward the south. The Pu`u `O`o cone still contains an active lava lake, about 130 feet in diameter and about 175 feet below the spillway on the east side of the cone. There are two areas within the lava lake, where active bubbling and degassing produce fountains less than 10 feet high.

Throughout the week, the amplitude of the tremor near the vent has been only low to moderate, despite fairly high eruption volumes. The lower tremor probably reflects the passivity of the vents. During the past week, the summit has deflated slightly, indicating that more magma is erupting than is being introduced to the magma reservoir from below.

At quarter to midnight on Thursday, a magnitude 3.3 earthquake about seven miles deep occurred slightly east of Waimea. Most of Kohala Volcano formed by 380,000 years ago, and the youngest lavas are about 60,000 years old, so the earthquake Thursday night was not directly or indirectly related to magma movement.

The northeast shoreline of Kohala was surrounded by an active reef until about 120,000 years ago, when it was submerged by rising sea level. Since the reef drowned, it has subsided to a depth of nearly 1,250 feet below sea level. This subsidence results from bending of the crust beneath Hawai`i because of the great weight of the island. A major landslide named the Pololu landslide occurred on Kohala some time shortly after 120,000 years ago. The entire northeast half of the volcano slipped seaward between Waipi`o and `Ako`ako`a Point. The indentation in the coastline defines its boundaries, and the large valleys along windward Kohala have been carved into rock disrupted by this landslide.

The landslide extends inland to the summit of Kohala and formed a series of pull-apart basins oriented northwest-southwest that were mapped many years ago as an elongate caldera. Offshore, the landslide destroyed the 120,000-year-old reef, and, after being ramped to the east by the submarine extension of the East Rift Zone of Haleakalā Volcano, rocky debris and large blocks were deposited in the Hawaiian Deep. Today, Kohala shows little evidence of its tumultuous past except for occasional small earthquakes.