August 20, 1998

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

How High is Mauna Loa?

Groups of students of all ages frequently visit the U.S. Geological Survey's Hawaiian Volcano Observatory while they are on an excursion to Hawaii Volcanoes National Park. When prior arrangements are made, we escort them around our facilities and give a short talk on our mission and program.

On clear days we are able to see Mauna Loa and often include information about this majestic volcano in our talk. Most of the students are aware that Mauna Loa is the largest volcano in the world, but most are surprised that it is also the highest from base to summit. Its gentle slope and shield-like shape belie the greatness of this edifice.

The heights of mountains are generally given by their elevation above a datum, such as sea level. The highest point on Mauna Loa is 4,170 m (13,680 ft) above sea level. But the flanks of Mauna Loa continue another 5,000 m (16,400 ft) below sea level to the sea floor. The massive central portion of the volcano has depressed the sea floor another 8,000 m (26,000 ft) in the shape of an inverted cone, reflecting the profile of the volcano above it. Thus, the total relief of Mauna Loa, from its true base to its summit, is about 17,170 m (56,000 ft).

How do we know that the sea floor is depressed below Mauna Loa? The sea floor is part of the Earth's uppermost zone, or crust. Below the crust is the mantle. The boundary between the crust and the mantle separates regions with clearly different seismic wave speeds. It is called the Mohorovicic discontinuity, or Moho, after the Croatian seismologist, Andrija Mohorovicic, who discovered it in 1909.

The difference in seismic wave speeds in the Earth's crust and mantle is the result of differences in rock properties between these zones. Crustal earth materials transmit seismic energy at speeds up to 7 kilometers per second (4.2 miles per second), whereas mantle seismic wave speeds are 8 kilometers per second (4.8 miles per second) or greater. This difference in seismic velocities can be easily detected. Through seismic refraction studies, USGS seismologists discovered that the Moho was bowed downward by as much as 8 km (4.8 mi) beneath Mauna Loa.

This flexure of the crust beneath the volcano is due to the weight of the mountain. On top of the downwarped crust is a layer of oceanic sediments. This layer is the plane where most large earthquakes on the island take place. Friday, August 21, was the 47th anniversary of the third largest Big Island earthquake - the magnitude 6.9 Kealakekua earthquake. The fault plane defined by the aftershocks of this destructive earthquake was the oceanic sediment layer beneath Mauna Loa.

As school children and visitors to HVO are learning, Mauna Loa is not only the world's largest volcano, but it is also one of the highest. The last eruption of Mauna Loa was in 1984, and surface deformation measurements indicate that the volcano has risen nearly 50 centimeters (20 inches) since then.

Eruption and Earthquake Update

The pause in eruptive activity reported last week ended on Friday morning, August 14. The continuous recording tiltmeter at the summit of Kilauea detected a subsidence of the volcano at 09:30 a.m. Shortly thereafter, five lobes of lava were observed flowing over the pali toward the ocean. The surface flows have ignited small brush fires in the area, but they pose no threat to any structures at this time. Later in the week, the old tube system was reoccupied, and lava entered the ocean at Kamokuna. The public is reminded that the ocean entry area is extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam cloud is highly acidic and laced with glass particles.

There were no earthquakes reported felt during the past week.