Mauna Loa is the largest of the five volcanoes that comprise the island of Hawaii. It is not only the largest volcano on the island, but the largest volcano on the planet. Mauna Loa has a volume of approximately 19,000 cubic miles (80,000 cubic kilometers), and its summit is 13,680 feet (4,170 meters) above sea level. The name Mauna Loa translates to “Long Mountain.” The name is quite fitting, as the part of the mountain that is above sea level is approximately 75 miles (120 kilometers) long, from the southern tip of Hawaii to Hilo on the island's northeast coast. The mountain's length is doubled if the areas offshore are included. It makes up fully one half of the island of Hawaii, and is as large as 85% of all of the other Hawaiian Islands combined. Mauna Loa is a relatively young volcano, although it is believed that it first rose above the surface of the ocean nearly 400,000 years ago. The volcano still is within the shield-building phase of its life cycle. Mauna Loa erupts frequently, with 33 well-documented eruptions since the first westerners observed an eruption in 1843. It is certain that Mauna Loa will erupt again, and the mountain is carefully monitored by the staff of the Hawaiian Volcano Observatory (an installation that the U.S. Geological Survey oversees).
Mauna Loa has a summit caldera named “Mokuaweoweo,” which translates loosely to “Islet of the Aweoweo.” “Aweoweo” is a red fish found in Hawaiian waters. The red fish is suggestive of lava as it moves and undulates through the landscape. The summit caldera is 2 miles by 3 miles (3 kilometers by 5 kilometers), with the long dimension oriented northeast-southwest. Mauna Loa has two major rift zones, the Southwest Rift Zone and the Northeast Rift Zone. Mauna Loa also has two minor rift zones; one extends to the north and the other to the northwest. Eruptions can occur along any of these rift zones or at the summit. Eruptions seem to be much more common along the major rift zones, however, than along the minor rift zones.
Mauna Loa also has a number of major faults that are believed to be locations of gravitational sliding. The Honuapo-Kaoiki Fault Zone is located just upslope of the border between Mauna Loa and the volcano Kilauea, which lies to the southeast. A curious feature lies to the west of this fault system near its southern extent. The Ninole Hills are a group of three towering plateaus. The hills contain some of the oldest rocks on the island. Some geologists think that the hills are formed as a result of movement along these faults—that the movement has brought a block of old Mauna Loa rocks to this high elevation, creating these hills. Other scientists think that the Ninole Hills are the only remnants of a large shield volcano that existed before Mauna Loa. In any case, the Ninole Hills are a prominent landmark on the southeast slope of Mauna Loa.
Another major fault is the Kahuku Fault, which lies parallel to the Southwest Rift Zone on the southern part of the island. Recent lava flows from Mauna Loa parallel the fault and reach the ocean near South Point, the southern tip of the island. One lava flow entered the ocean near South Point and created a beautiful littoral (coastal) cone. The cone has been eroded on the ocean side, and forms a cove with a beautiful green sand beach. The green of the sand is from the mineral olivine, which is a bright apple-green color. It was a component of the lava flow that fed the cone's eruption. As the hot lava hit the cold ocean, the lava exploded into small fragments that created the cone. As wind, waves, and rain erode the cone, these small crystals of olivine created the beach sand in that area. Approximately 60% of the sand is olivine, with remaining components composed of rock fragments and solid debris from the ocean, such as tiny shell fragments.
On the west side of the island, the Kealekekua-Kaholo Fault System is located near—and runs parallel to—the shoreline. The fault system, again related to gravitational sliding, is thought to be related to large landslides that have shaped the coastline in this area. Kealekekua Bay is a relatively small cove with a steep cliff at the rear of the bay. The bay is famous for excellent snorkeling, but also has historical significance. British Captain James Cook—the first western explorer to visit the Hawaiian Islands—made his first landfall here, and was killed in this location a few years later. A monument to Captain Cook stands in the water toward the northern end of the bay.
Scanning of the ocean floor of the west coast of Hawaii near Kealekekua Bay has led to an understanding that big landslides that involve gravitational sliding of immense blocks from the edges of the large Hawaiian volcanoes are a primary way that the islands have attained the shapes they have today. Volcanic activity is responsible for building the islands, but landslides and movement along faults alter the shape of the islands once the volcano creates them.
The most recent eruption of Mauna Loa occurred in March and April of 1984. The eruption began about 1:25 a.m. on March 25 in Mokuaweoweo, the volcano's summit caldera. Lava fountains erupted across the caldera floor and extended a short distance into the Southwest Rift Zone. A few hours later, the eruption migrated into the Northeast Rift Zone. Over the next several days, new cracks opened at progressively lower elevations along the Northeast Rift Zone. At one point, lava advanced to within 4 miles (6.5 kilometers) of the city of Hilo. Thankfully, the lava channels that were carrying lava rapidly toward Hilo degraded, starving the flow fronts near Hilo of their lava supply, so lava never entered the city. The eruption ended on April 15. The eruption caused quite a bit of distress within the city of Hilo, because people were understandably concerned about the possibility of lava reaching homes and businesses within the city limits. At some point in the future, this is a likely scenario. There is very little that can be done to prevent lava from reaching Hilo if an eruption of just the right duration in just the right location occurs on Mauna Loa.
See also: Hawaiian Islands; Hawaiian Volcano Observatory; Kilauea Volcano, Hawaii, United States
During one of the longest eruptions on Hawaii, Mauna Loa was active at the same time as the younger volcano Kilauea. Here, fire fountains...
\lō-ä\ 1 Mountain, W Molokai I., Hawaii; 1382 ft. (421 m.). 2 Volcano, on S cen. Hawaii I., Hawaii, in Hawaii...
pronunciation volcano 13,680 ft (4170 m) Hawaii in S cen Hawaii (island) in Hawaii Volcanoes National Park