Photo courtesy of J. Griggs, USGS (1991)

Slumbering Giant

Bet you thought that the biggest mountain was Mount Everest! If you haven't already checked it out here at "Extreme Science", Mount Everest does hold a world record, but not as the biggest mountain. Everest is the highest elevation on land, but it is only a single peak in an entire mountain range. Mauna Loa is a single mountain on the island of Hawaii. The island of Hawaii is actually an island made up of five volcanoes which "blend" together because of their closeness to each other, making a single island.

 

 

Only about 13,448 ft/4100m of Mauna Loa are above sea level, so it may not seem like a very tall mountain. But, when you start measuring Mauna Loa from its true base on the bottom of the ocean, in the Hawaiian Trough, the total height exceeds that of Everest by over 3/4 of a mile. Mauna Loa is Hawaiian for "Long Mountain", probably because of its long, gently sloping shape. If you want to get really technical, Mauna Kea, a neighbor of Mauna Loa on the same island of Hawaii, is actually the tallest mountain in the world. Mauna Kea is about 350 ft/107m taller than Mauna Loa, but its mass doesn't compare to that of Mauna Loa. Mauna Loa takes up a lot of space because its mass is 9,700 cubic miles/40,000 cu km of mountain.

Mountain Born of Fire

What makes Mauna Loa such a big mountain is the way it was formed. As part of the Hawaiian islands, Mauna Loa is a volcano, just like many others found on the island chain. The volcanoes in the Hawaiian islands are different from the cone-shaped, explosive, "fire breathing" dynamos that most of us think of when volcanoes come to mind. Mauna Loa is one of many shield volcanoes that make up the Hawaiian Islands. These are volcanoes that, compared to their more violent companions, erupt slowly and quietly. What really distinguishes a shield volcano is its shape - they are usually much wider than they are tall. Shield volcanoes are created when red hot lava oozes out from cracks, or fissures in the earth's crust.

The biggest mountain in the world, Mauna Loa, is a shield volcano with gently sloping sides.

In the case of the Hawaiian islands, the fissures were in the ocean floor. The lava cools as it comes in contact with the ocean water. The newly deposited lava raises the level of the ocean floor just in the area around the fissure (see Figure 1). Over time, and many oozing eruptions, a gently sloping sea mount forms (see Figure 2). A sea mount is, quite simply, a mountain on the floor of the ocean, only the top doesn't reach above the surface. Over a million years, or more, lava slowly builds up the sea mount until the top reaches above the surface of the sea. When this happens, it finally earns the title of island (see Figure 3).

Hot Spots

About 30 years ago a Geophysicist named J. Tuzo Wilson came up with an idea to explain why there was volcanic activity out in the middle of the Pacific Ocean, in the middle of the huge Pacific Plate. At the time, scientists thought that volcanoes only happened at plate boundaries, but nobody could explain why they were happening out in the middle of a tectonic plate. The idea of plate tectonics said that the earth's crust is made up of plates that are constantly moving around. The Pacific Plate is a hunk of the earth's crust that is covered by the entire Pacific Ocean.

Dr. Wilson said that there are "hot spots", under the earth's crust in some places. These are called "hot spots" because they are places where a lot of heat is concentrated in a small area. The heat causes the overlying rock to melt, forming magma (what lava is called before it is expelled from the crust - see Figure 3). Since the magma is liquid and is lighter than the surrounding rock it "floats" to the surface and forces its way out of fissures in the crust. Over time, the continual outpouring of magma can form a sea mount or island volcano if the hostpot is under the ocean floor, as in the case of the Hawaiian Islands. There is just one hot spot that never moves. But the Pacific Plate continually (and slowly) moves north over the hot spot, forming a new volcano on the overlying plate each time.