HAWAII NATURE NOTES
THE PUBLICATION OF THE
NATURALIST DIVISION, HAWAII NATIONAL PARK
AND THE HAWAII NATURAL HISTORY ASSOCIATION

MAUNA LOA, FIERY COLOSSUS OF THE PACIFIC

DESCRIPTION. Mauna Loa is the world's largest active volcano and probably the largest single mountain of any sort on earth. It rises 13,680 feet above sea level, and approximately 30,000 feet above its base at the ocean floor. Its volume is of the order of 10,000 cubic miles, as compared to 80 cubic miles for the big cone of Mount Shasta in California. This huge bulk has been built almost entirely by the accumulation of thousands of thin flows of lava, the individual flows averaging only about 10 feet in thickness.

In form, Mauna Loa is a very broad flat dome, the slopes of which nowhere are steeper than about 12°. Similar slopes extend outward beneath the water all the way to the sea floor. This type of volcano is known as a shield volcano. At the summit of Mauna Loa is an oval depression 3 miles long, 1.5 miles wide, and as much as 600 feet deep (Plates 8 and 9). This depression, commonly called "the crater" but more properly termed a caldera, was formed by insinking of the summit of the mountain. Its name is Mokuaweoweo. At the northern and southern ends Mokuaweoweo coalesces with smaller nearly circular pits formed in a similar manner (see Figure 3). Southwest of the caldera there are two more of these pit craters, named Lua Hou (New Pit) and Lua Hohonu (Deep Pit).

TABLE 2
Eruptions of Mauna Loa¹
(After Stearns and Macdonald, 1946)

From the caldera at the summit of the mountain there extend outward two prominent zones of fracturing—rift zones. The rift zones are marked at the surface by many open fissures and cinder and spatter cones built during eruption by the accumulation of spatter and fragments of lava thrown into the air as fountains of liquid lava at the source of lava flows. Some of the gobs of liquid lava solidify in the air, and pile up into loosely cemented cinder cones upon striking the ground. Others that are still liquid when they strike the ground stick together and form spatter cones. One rift zone extends southwestward from Mokuaweoweo caldera, and the other northeastward toward the city of Hilo. A much less definite rift zone extends northward toward the Humuula Saddle, between Mauna Loa and Mauna Kea. On a clear day the profile of the northeast rift zone of Mauna Loa can be seen from the vicinity of Park Headquarters as a succession of hills (cinder cones). The most prominent of these is Puu Ulaula (Red Hill), at an altitude of 10,000-feet, which is 200 feet high on its downhill side. A rest house, located in one side of this cone, provides shelter for travelers enroute to and from the summit.

FIGURE 3. Map of cross-section of Makuaweoweo caldera after the 1949 eruption. Lua Poholo, East Pit, and Lua Hohonu are pit craters formed since 1941. (Modified after Stearns and Macdonald, 1946, click on image for an enlargement in a new window).

The fresh flows of aa lava extending downslope from the rift zones appear black. The fresh pahoehoe flows may also appear black, but when light is reflected from them they appear silvery gray from a distance. Older lavas are dark gray, and still older ones reddish-brown. Lava flows commonly divide, leaving within their boundaries small "islands" of older land not covered by the new lava. These islands are known in Hawaii as kipukas. From the vicinity of Kilauea caldera the slopes of Mauna Loa show many variations in color, depending on the age and surface characteristics of the different flows. On the upper slopes of the mountain the newer black flows surround kipukas of older gray or brown lava. On the lower slopes the kipukas show as clumps of large trees, such as Kipuka Ki on the Mauna Loa truck trail, or Kipuka Puaulu, in which Bird Park is situated.

PLATE 3. A stream of liquid rock gliding smoothly into the sea is recorded by a photographer who faced extreme heat to come within 100 feet of shore. Mauna Loa eruption, June 1950. (Hawaii Air National Guard).

The growth of Mauna Loa was not entirely uninterrupted. On its south eastern slope, a short distance west of Hawaii National Park, is an area that received no new lava flows for many thousands of years. There stream erosion carved big valleys into the mountainside. The remains of these big valleys and the ridges that separated them can still be clearly seen inland from Pahala and Punaluu, although the valleys have been partly filled by new lava flows.

As the great cone of Mauna Loa approached its present size it appears to have become somewhat unstable. There is a tendency for the rocks to break along certain lines and for the blocks seaward from the breaks to slide outward and downward toward the ocean. These breaks are known as faults and the cliffs formed on the land surface by the movements as fault scarps. A prominent series of fault scarps, partly buried by later lava flows, lies just northwest of the road from Park Headquarters to Pahala. The faults are still active, and are the source of many earthquakes recorded on the seismographs of the Hawaiian Volcano Observatory.

Extending inland from South Point, the Kahuku Pali is a large fault scarp marking the edge of a downsunken area on the southwest rift zone.

PLATE 4. 1950 lava flow of Mauna Loa approaching the sea. Many acres of ohia forests and several buildings were destroyed by this eruption, the first to strike the sea since 1926, when the village of Hoopuloa was covered (Plate 10). (Hawaii Air National Guard).

ACTIVITY OF MAUNA LOA. Throughout the past century Mauna Loa has been one of the most active volcanoes on earth. It has erupted on an average of once every 3-1/2 years, and its eruptions during that period have poured out a total of more than 3-1/2 billion cubic yards of lava. The eruptions can be classified as summit eruptions—those that occur in and near the summit caldera and flank eruptions—those that occur lower on the flanks of the mountain, generally on one of the rift zones. Probably all flank eruptions begin with brief activity at the summit, followed after a few hours of quiet by the outbreak on the flank.

A typical eruption of Mauna Loa begins with the opening of a fissure or series of fissures, as much as 13 miles long (Plate 5). From this fissure liquid lava squirts in a nearly continuous line of low fountains, from a few feet to 50 feet high. This very spectacular manifestation has been called the "curtain of fire." From the fountains pour copious fast-moving floods of lava, and along the fissure lava spatter builds up a nearly continuous wall, a few feet high, known as a spatter rampart. The "curtain of fire" is short-lived. generally lasting less than a day. The upper and lower ends of the fissure then become inactive, and eruption is restricted to a few hundred yards of the central part of the fissure. The lava fountains increase in height, reaching as high as 800 feet (Plate 2), and debris from the fountains accumulates around them to form a cinder cone. Pumice and Pele's hair (natural spun glass) rain down on the country to leeward of the vents. A great cloud of yellowish-brown gas rises several thousand feet above the fountains. The principal flows of lava issue during this stage, which may continue for weeks or even months. Some of these flows reach the shore, and may be destructive (Plate 10). Eventually the abundant gas liberation and high lava fountains come to an end, and the final phase of the eruption consists of a relatively quiet outpouring of lava. It usually is short, lasting only a few hours, but it may continue for several weeks, as in the 1949 eruption.

PLATE 5. Lava fountains up to 300 feet high along Mauna Loa's southwest rift, June 1950. Most of the 1950 lava was produced by these fountains. (United States Air Force).