All the high islands in the Hawaiian chain are of volcanic origin, but the only active craters are on Hawaii, the last island toward the southeast. With some exceptions, volcanic activity seems to have begun on the ocean floor at the northwest end of a series of roughly parallel fissures and to have moved progressively toward the southeast. Usually the completeness of erosion and its stage have proceeded in the same direction, because the volcanoes in the northwest section have been eroded below sea level, those in the middle section largely eroded with only a few surviving stacks, and in the southeast the volcanic domes have been only partially eroded with the last island, Hawaii, affected least of all.
The main islands were constructed of flow after flow of basaltic lava one above the other, with the flows first erupting from fissures and vents on the ocean floor, and later continuing from major craters above sea level until lava domes or shields rather than cones were built high above the ocean. For example, Mauna Kea on Hawaii is 13,784 feet and Haleakala on Maui is 10,025 feet above sea level. Beds of volcanic ash, pumice, and tuff, all products of explosive eruptions, may lie between some lava flows, but these have small volume compared to the masses of lava. The larger lava domes may be 10 to 60 miles in diameter at sea level, and they rise to heights of thousands of feet. Two domes rise 2 ½ miles above the sea from depths that are 3 ½ miles below the surface of the ocean within 50 miles of the coast. At least 90 per cent of the lava is hidden below the water surface.
Associated with the domes are numerous cinder cones that resulted from minor eruptions on the slopes and margins of the major volcanoes. As a result of collapse of the summit and the enlargement of the resulting crater by stoping (fusion of wall rock), huge calderas (caldrons) develop on top of the lava domes, for example, Mokuweoweo on Mauna Loa and Halemaumau, the firepit of Kilauea.
Nearly 1,500,000,000 cubic yards of basalt have erupted from Mauna Loa in little over a century, far more lava than has come from any other active volcano in the world during that time. During the last 200 years Mauna Loa has erupted on an average of once every 3 ½ years and has had a lava flow every 6 years. The flow of 1859 was 35 miles long and lasted for 10 months, and several flows have reached the ocean in historic times.