The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

Of these mountains, Mt. Loa and Kilauea are the only active volcanoes, and are in frequent eruption. Mt. Haulalai was in eruption during 1804. Mt. Kea has not been active during historical times, while Mt. Kohala has been inactive for so long a time that its slopes are deeply gullied wherever the rivers flow down them.

As you can see from the map, Hawaii is very large. It has a length of ninety-three miles from north to south, and a breadth of eighty miles from east to west, its area is about 6,500 square miles. With the exception of small patches of coral reefs, Hawaii is formed entirely of lava, and is the largest pile of lava in the world with the single exception of Iceland.

Where the islands of the Hawaiian chain have coral reefs extending off their coasts, excellent harbors are found in the deep waters between the islands and the reefs. Hawaii, however, has no extended reefs of this character, and, consequently, no first-class harbors. Hilo, on the eastern coast, is the best harbor, and is, therefore, the principal settlement.

A very brief examination of the map of Hawaii will show you that there are no rivers on the island, except on the sides exposed to the wind, that is, on the northern and northeastern slopes. Since the yearly rainfall on Hawaii is large, being in the neighborhood of a hundred inches, you will understand that considerable rain water falls on the island. In those parts of the island where it does not run off the surface it must drain downward through the loose piles of broken rocks or cinders. A rainfall of one hundred inches a year means that if all the rain which falls on each square foot of surface was collected in a flat vessel one foot square with vertical sides it would fill the vessel to the depth of one hundred inches, or over eight feet. The drainage of the rainwater downwards through these parts of the island, must, therefore, be large.

Another curious fact you can notice on the map, is that the lava streams of the past fifty years from Mt. Loa indicated by heavy dotted lines, in no cases begin at the crater, but start at fairly considerable distances from it. Later on in this chapter we shall explain the reason for this curious fact.

Since practically the whole of Hawaii has been formed from the streams of lava that have flowed at one time or another, you can understand how great these flows must have been. But to do this fully you must not only take into consideration the portions of the island that lie above the ocean and reach into the air at its greatest height to 13,805 feet above its surface, you must also remember that this mountain rises from a deep ocean, so that if all the water were removed, you would see Hawaii towering up above the former level of the sea to the height of about 31,000 feet, or higher than Mt. Everest, the highest point on the earth above the present sea level. This would be, approximately, five and eight-tenths miles. You can understand, therefore, how great the flow of lava must have been.

We shall begin the description of Hawaii with the active volcano of Mt. Loa, or, as it is sometimes called in Hawaii, "The White Mountain."

You will remember that the eruption of Krakatoa was of the explosive type. Practically no melted rock or lava escaped from the crater. Indeed, if it had escaped it would not have been seen; for, not only the cone near the crater, but also much of the mountain itself was blown completely out of sight and covered by the waters of the ocean.

The eruptions of Mt. Loa are of an entirely different type. In Loa there are no explosions, the eruptions being what are called the non-explosive or quiet volcanic eruption type. It will be necessary to explain some of the peculiarities of this kind of eruptions.

There is a great difference in the liquidity or the ease with which different kinds of lava flow. Some lava is very thick or viscid, or is sticky like thick molasses or tar, and therefore flows very sluggishly. Other lava is thin or mobile, more closely resembling water in the ease with which it flows. Now, in the case of a volcanic mountain of fairly considerable height, where the lava possesses marked liquidity, the lava as it rises from great depths in the tube of the volcano seldom flows over the top or rim of the crater. This is not because the force that brings the lava up is unable to carry it a few thousand feet higher, so that it can run over the brim of the crater, but because the walls of the volcanic mountains are unable to stand the great pressure which the mass of liquid lava exerts against their sides.