To many of us, Hawaiian volcanoes loom sleepily overhead, occasionally stirring from their slumber and oozing lava flows down their flanks toward the sea. Deep within the towering volcanoes, however, lies the potential for powerful explosions of volcanic ash and blocks.

These explosive eruptions usually occur when magma moves upward from far below the surface and encounters groundwater. As the lava vaporizes the water, the expanding steam creates enough pressure to break the ground, ejecting volcanic debris high into the air and littering the eruption site with thousands of rocks. The skies darken with ash, and the countryside tens of kilometers (miles) downwind becomes coated in grime.

Kīlauea had violent steam-driven eruptions as recently as 1924, but Mauna Loa has not produced any major explosions in recorded history. Even though an explosive eruption from Mauna Loa has never been witnessed, ample geologic evidence for pre-historic explosive eruptions exists high atop Mauna Loa.

At the summit of Mauna Loa, around Moku`aweoweo caldera, lie several deposits of ancient explosive debris. These deposits rest atop pahoehoe lava flows that spilled out from the top of the volcano around 1,000 to 1,500 years ago. The largest blocks in these deposits are up to 2.2 m (7.2 ft) in diameter and weigh more than 17,000 kg (38,000 lbs).

Compositionally, the explosive debris varies widely. Some rocks are dense, crystalline, and devoid of bubbles, but others are dense, lack large visible crystals, and contain moderate amounts of bubbles. The many different compositions of the debris give the blocks a variety of different colors, from light green to dark red to salt and pepper. Along with differences in composition, chemical interactions with extremely hot fluids during the eruption affected the color and appearance of the debris.

Some of the debris is a type of rock called gabbro, which is entirely made of crystals. Gabbros are usually composed of the mineral plagioclase (a white mineral, at least 50 percent), pyroxene (a black mineral), and olivine (a green mineral). These rocks are also referred to as xenoliths ("xeno" meaning foreign and "lith" meaning rock), in that they did not form during the explosive eruption but instead had already formed and were brought to the surface by the explosions. Since all of the blocks have been exposed to the howling winds and driving snows for hundreds of years, their surfaces are eroded and dull.

The debris lies in four distinct areas around Moku`aweoweo—two around the north and northwest sides of the caldera, one on the southeast rim that envelops the cabin, and a smaller area down the `Ainapo trail. Within each area, the debris is widely scattered, and it becomes less abundant with increasing distance from the rim of Moku`aweoweo. Most of the larger blocks rest on or near the rim of the caldera, but some of the debris lies as far away from Moku`aweoweo as 4 km (2 miles). Preliminary analysis shows that at least three separate explosive events occurred!

Ongoing study of these ejected rocks and ash by HVO scientists will lead to a better understanding of what caused the explosive eruptions—was it really magma interacting with groundwater, or could volcanic gas have been the driver? In addition, we hope to learn how to predict future explosive eruptions, though this is tough business. Detailed analysis of the minerals in the rocky debris will yield clues as to where and how they formed. In addition, a continuing search for any remaining ash around Moku`aweoweo, if successful, could reveal information about the mechanism of the eruptions. Although Mauna Loa silently rests for the time being, it will certainly awaken again and remind us all that the largest volcano in the world dominates our Big Island home.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing southeast through a tube system down to the flats below Pulama pali and beyond to the ocean. Breakouts from the tube system are feeding 4 small flows at the top of Pulama pali. Lava is entering the ocean at Kamokuna located 1.6 km (1 mi) west-southwest of Waha`ula. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

Three earthquakes were reported felt during the week ending on October 26. Residents of Hilo and Pahoa reported feeling an earthquake at 2:25 p.m. on Saturday, October 21. The magnitude-3.2 earthquake was located 8 km (4.8 mi) east of Makaopuhi Crater at a depth of 4 km (2.4 mi). A magnitude-2.9 earthquake located 6 km (3.6 mi) northwest of the summit of Kīlauea Volcano at a depth of 13 km (7.8 mi) was reported felt by residents of Volcano village. The earthquake occurred at 1:24 p.m. on Sunday, October 22. A late-working HVO chemist felt an earthquake at 9:26 p.m. on Tuesday, October 24. The magnitude-2.5 earthquake was located 2.4 km (1.4 mi) beneath the summit of Kīlauea Volcano.