Mauna Loa ("long or high, mountain"), a volcanic mountain occupying a large part of the central and southern regions of the island of Hawaii; elevation, 13,760 ft. It is entirely composed of lavas which have been thrown out in a highly fluid state, and which in consequence have flowed laterally with such freedom as to build up a mountain with extremely gentle slopes, averaging, according to Prof. Dana, but 6° 30'; 'the declivity upon the E. side is somewhat the steepest. It presents the appearance of a smooth, regular dome, usually crowned with snow, and partially forest-clad. On the east the forests cease at the elevation of 5,000 ft. Vegetation reaches to the height of 7,000 and 10,000 ft. on the leeward and windward sides respectively. The surface of Mauna Loa is composed of recent lavas in three forms: 1, the pahoihoi or " satin lava," a dense and solid rock; 2, scoriaceous lava, or " clinkers;" 3, a black slag or spongy lava, of the horrible roughness and hardness of which it is difficult to convey any idea. Its craters are numerous, occurring near the summit and on the sides; new ones sometimes open, and are the source of the grandest of the Hawaiian eruptions.

The terminal crater, Mokua-weo-weo, is circular, 8,000 ft. in diameter, with two lateral depressions which increase its dimensions in the N. and S. direction to 13,000 ft. It was about 1,000 ft. deep in 1864, with nearly perpendicular walls. Eruptions from Mauna Loa often take the form of enormous lava fountains, sponting continuously from the top of the mountain. In February, 1859, such a fountain played actively for four or five days, throwing up a sheaf of white-hot fluid lava about 200 ft. in diameter, and 200 or 300 ft. high, illuminating the horizon at a distance of 150 m. In April, 1868, the lavas forced their way 20 m. under ground, and appeared near the S. point of the island, bursting forth through a fissure 2 m. long, which ran N. and S. On the 10th Mr. H. M. Whitney observed four enormous lava fountains continuously spouting up from this opening. Two of them occasionally united laterally; and sometimes the whole four joined in one, making a continuous fountain a mile long. It boiled with the most terrific fury, throwing up enormous columns of crimson lava and red-hot rock to the height of 500 or 600 ft. The lava was ejected with a rotary motion, uniformly toward the south.

Mauna Loa has been seen at sea from a distance of 53 leagues; " the most striking example I have yet known," says Humboldt, "of the visibility of a mountain".