All the islands are of volcanic origin, and have been built up by the eruptive process from a base about 15,000 ft. below the sea to a maximum height (Mauna Kea) on the largest island (Hawaii) of 13,823 ft. above the sea; altogether there are forty volcanic peaks. Evidence of slight upheaval is occasionally afforded by an elevated coral-reef along the shore, and evidence of the subsidence of the S. portion of Oahu for several hundred feet has been discovered by artesian borings through coral-rock. In some instances, notably the high and nearly vertical wall along the N. shore of the E. half of Molokai, there is evidence of a fracture followed by the submergence of a portion of a volcano. With the exception of the coral and a small amount of calcareous sandstone, the rocks are entirely volcanic and range from basalt to trachyte, but are mainly basalt. Cinder cones and tufa cones abound, but one of the most distinguishing features of the Hawaiian volcanoes is the great number of craters of the engulfment type, i.e. pit-craters which enlarge slowly by the breaking off and falling in of their walls, and discharge vast lava-flows with comparatively little violence. The age of the several inhabited islands, or at least the time since the last eruptions on them, decreases from W. to E., and on the most easterly (Hawaii) volcanic forces are still in operation. That those to the westward have long been inactive is shown by the destruction of craters by denudation, by deep ravines, valleys and tall cliffs eroded on the mountain sides, especially on the windward side, by the depth of soil formed from the disintegrated rocks, and by the amount as well as variety of vegetable life.
Hawaii Island, from which the group and later the Territory was named, has the shape of a rude triangle with sides of 90 m., 75 m. and 65 m. Its coast, unlike that of the other islands of the archipelago, has few coral reefs. Its surface consists mainly of the gentle slopes of five volcanic mountains which have encroached much upon one another by their eruptions.
Mauna Loa (“Great Mountain”), on the S., is by far the largest volcano in the world; from a base measuring at sea-level about 75 m. from N. to S. and 50 m. from E. to W., it rises gradually to a height of 13,675 ft. On its E.S.E. side, at an elevation of 4000 ft. above the sea (300 ft. above the adjoining plain on the W.) is Kilauea, from whose lava-flows the island has been extended to form its S.E. angle. To the N.N.E. of Mauna Loa, and blending with it in an intervening plateau, is Mauna Kea (“White Mountain,” so named from the snow on its summit), with a much smaller base but with steeper slopes and a crowning cinder cone 13,823 ft. above the sea, the maximum height in the Pacific Ocean; blending with Mauna Loa on the N.N.W. is Mauna Hualalai, 8269 ft. in height; and rising abruptly from the extreme N.W. shore are the remains of the oldest mountains of the island, the Kohala, with a summit 5505 ft. in height. On the land side the Kohala Mountains have been covered with lava from Mauna Kea, and form the broad plains of Kohala, having a maximum elevation of about 3000 ft.; on the ocean side, wherever this lava has not extended, erosion has gone on until bluffs 1000 ft. in height face the sea and the enormous gorges of Waipio and Waimanu, with nearly perpendicular walls as much as 3000 ft. high and extending inland 5-6 m., have been formed. Mauna Kea is not nearly so old as the Kohala Mountains, but there is no record of its eruption, nor have its lavas a modern aspect. The last eruption of Mauna Hualalai was in 1801. Mauna Loa and Kilauea are still active. Cinder cones are the predominant type of craters on both Mauna Kea and the Kohala Mountains, and they are also numerous on the upper slopes of Mauna Hualalai; but the more typically Hawaiian pit or engulfment craters also abound on Mauna Hualalai and Mokuaweoweo, crowning the summit of Mauna Loa, as well as Kilauea, to the S.E. of it, are prominent representatives of this type. Kilauea is the largest active crater in the world (8 m. in circumference) and is easily accessible. Enclosed by a circular wall from 200 to 700 ft. in height is a black and slightly undulating plain having an area of 4.14 sq. m., and within this plain is a pit, Halemaumau, of varying area (about 2000 ft. in diameter in 1905), now full of boiling lava, now empty to a depth of perhaps 1000 ft. When most active, Halemaumau affords a grand spectacle, especially at night: across the crust run glowing cracks, the crust is then broken into cakes, the cakes plunge beneath, lakes of liquid lava are formed, over whose surface play fire-fountains 10 to 50 ft. in height, the surface again solidifies and the process is repeated.[1] According to an account of the natives, a violent eruption of Kilauea occurred in 1789, or about that time, and deposits of volcanic sand, large stones, sponge-like scoria (pumice) and ashes for miles around are evidence of such an eruption. Since the Rev. William Ellis and a party of American missionaries first made the volcano known to the civilized world in 1823, the eruptions have consisted mainly in the quiet discharge of lava through a subterranean passage into the sea. In the eruptions of 1823, 1832, 1840 and 1868 the floor of the crater rose on the eve of an eruption and then sank, sometimes hundreds of feet, with the discharge of lava; but since 1868 (in 1879, 1886, 1891, 1894 and 1907; and once, before 1868, in 1855) this action has been confined to Halemaumau and such other pits as at the time existed.
Mokuaweoweo, on the flat top of Mauna Loa, is a pit crater with a floor 3.7 sq. m. in area and sunk 500-600 ft. within walls that are almost vertical and that measure 9.47 m. in circumference. Formerly, on the eve of a great eruption of Mauna Loa, this crater often spouted forth great columns of flame and emitted clouds of vapour, but in modern times this action has usually been followed by a fracture of the mountain side from the summit down to a point 1000 ft. or more below where the lava was discharged in great streams, the action at the summit diminishing or wholly ceasing when this discharge began. The first recorded eruption of Mauna Loa was in 1832; since then there have been eruptions in 1851, 1852, 1855, 1859, 1868, 1880-1881, 1887, 1896, 1899 and 1907. The eruptions of 1868, 1887 and 1907 were attended by earthquakes; in 1868 huge sea waves, 40 ft. in height, were raised, and, as they broke on the S. shore, they destroyed the villages of Punaluu, Ninole, Kawaa and Honuapo. But the eruptions of Mauna Loa have consisted mainly in the quiet discharge of enormous flows of lava: in 1859 the lava-stream, which began to run on the 23rd of January, flowed N.W., reached the sea, 33 m. distant, eight days later, and continued to flow into it until the 25th of November; and the average length of the flows from seven other eruptions is nearly 14 m. The surface of the upper slopes of Mauna Loa is almost wholly of two widely different kinds of barren lava-flows, called by the Hawaiians the pahoehoe and the aa. The pahoehoe has a smooth but billowy or hummocky surface, and is marked by lines which show that it cooled as it flowed. The aa is lava broken into fragments having sharp and jagged edges. As the same stream sometimes changes abruptly from one kind to the other, the two kinds must be due to different conditions affecting the flow, and among the conditions which may cause a stream to break up into the aa have been mentioned the greater depth of the stream, a sluggish current, impediments in its course just as it is granulating, and, what is more probable, subterranean moisture which causes it to cool from below upward instead of from above downward as in the pahoehoe. The natives are in the habit of making holes in the aa, and planting in them banana shoots or sweet-potato cuttings, and though the holes are simply filled with stones or fern leaves, the plants grow and in due time are productive. Another curious feature of Mauna Loa, and to some extent of other Hawaiian volcanoes, is the great number of caves, some of them as much as 60 to 80 ft. in height and several miles in length; they were produced by the escape of lava over which a crust had formed. In the midst of barren wastes to the S.E. and S.W. of Kilauea are small channels with steam cracks, along which appears the only vegetation of the region.
Maui, lying 26 m. N.W. of Hawaii, is composed of two mountains connected by an isthmus, Wailuku, 7 or 8 m. long, about 6 m. across, and about 160 ft. above the sea in its highest part.
Mauna Haleakala, on the E. peninsula, has a height of 10,032 ft., and forms a great dome-like mass, with a circumference at the base of 90 m. and regular slopes of only 8° or 9°. It has numerous cinder cones on its S.W. slope, is well wooded on the N. and E. slopes, and has on its summit an extinct pit-crater which is one of the largest in the world. This crater is 7.48 m. long, 2.37 m. wide, and covers 19 sq. m.; the circuit of its walls, which are composed of a hard grey clinkstone much fissured, is 20 m.; its greatest depth is 2720 ft. At opposite ends are breaks in the walls a mile or more in width—one about 1000 ft., the other at least 3000 ft. in depth—through which poured the lava of probably the last great eruption. From the floor of the crater rise sixteen well-preserved cinder-cones, which range from more than 400 ft. to 900 ft. in height. Along the N. base of the mountain are numerous ravines (several hundred feet deep), to the bottom of which small streams of water fall in long cascades, but elsewhere on the eastern mountain there is little erosion or other mark of age. That the mountainous mass of western Maui is much older is shown by the destruction of its crater, by its sharp ridges and by deeply eroded gorges or valleys. Its highest peak, Puu Kukui, rises 5788 ft. above the sea, and directly under this is the head of Iao Valley, 5 m. long and 2 m. wide, which has been cut in the mountain to a depth of 4000 ft. This and the smaller valleys are noted for the beauty of their tropical scenery.
Kahoolawe is a small island 6 m. S.W. of Maui. It is 14 m. long by 6 m. wide. Its mountains, which rise to a height of 1472 ft., are rugged and nearly destitute of verdure, but the intervening valleys afford pasturage for sheep.
Lanai is another small island, 7 m. W. of Maui, about 18 m. long and 12 m. wide. It has a mountain range which rises to a maximum height, S.E. of its centre, of about 3480 ft. The N.E. slope is cut by deep gorges, and at the bottom of one of these, which is 2000 ft. deep, is the only water-supply on the island. On the S. side is a rolling table-land affording considerable pasturage for sheep, but over the whole N.W. portion of the island the trade winds, driving through the channel between Maui and Molokai, sweep the rocks bare. Kahoolawe and Lanai are both privately owned.
Molokai, 8 m. N.W. of Maui, extends 40 m. from E. to W. and has an average width of nearly 7 m. From the S.W. extremity of the island rises the backbone of a ridge which extends E.N.E. about 10 m., where it culminates in the round-topped hill of Mauna Loa, 1382 ft. above the sea. Both the northern and southern slopes of this ridge are cut by ravines and gulches, and along the N. shore is a steep sea-cliff. At the E. extremity of the ridge there is a sudden drop to a low and gently rolling plain, but farther on the surface rises gradually towards a range of mountains which comprises more than one-half the island and attains a maximum height of 4958 ft. in the peak of Kamakou. The S. slope of this range is gradual but is cut by many straight and narrow ravines, in some instances to a great depth. The N. slope is abrupt, with precipices from 1000 to 4000 ft. in height. Extending N. from the foot of the precipice, a little E. of the centre of the island, is a comparatively low peninsula (separated from the mainland by a rock wall 2000 ft. high), on which is a famous leper settlement. The peninsula forms a separate county, Kalawao.