Hawaiian volcanism
Hawaiian volcanoes are by far the best-studied examples of basalt shields. Basaltic shield volcanoes comprise a small percentage of Earth's volcanoes. (~8%) The Hawaiian shield volcanoes are the largest volcanoes on earth (e.g. Peterson & Moore 1987) rising some 9 km above the ocean floor. Hawaiian volcanoes reach huge volumes in relatively short periods of time. Mauna Loa is thought to have begun forming on the sea floor some 500,000 years ago, although this is poorly constrained. Plate tectonics provides a modern explanation for the presence of the Hawaiian volcanoes and their age progression from young in the south east to old in the north west. The lithosphere consists of the crust and uppermost mantle, both of which are rigid, and together can be divided into sections called plates. Beneath the lithosphere is the asthenosphere, a hot plastic layer on which the lithospheric plates can slide. Somewhere beneath the asthenosphere, and possibly as deep as the core-mantle boundary, is a hotspot, and the Hawaiian volcanoes are formed because of it.
At Hawaii, the Pacific plate is moving at ~9 cm/year towards the north west. The Hawaiian volcanoes grow progressively older from the submarine volcano Lo'ihi and the island of Hawaii at the south-east end of the chain through the main islands, through the leeward islands (mostly atolls formed on old submerged volcanoes), and beyond Kure atoll to the Emperor sea-mounts, the northernmost and oldest of which (Meiji Seamount) is being sub-ducted under Kamchatka.
Uplift caused by the hotspot has bulged the Pacific plate upward over a broad region approximately 400 kilometres wide called the Hawaiian swell. This brings an excess of dense mantle material to near the earth's surface, and this extra mass actually results in gravity being slightly higher around the Hawaiian chain. At the same time, the loading of the volcanoes onto the heat-weakened swell has warped the centre downward. This combination of uplift and subsidence has formed a broad m-shaped profile; the resulting structures are the Hawaiian trough adjacent to the islands, and the Hawaiian arch outboard of that. Recently the arch has been found to be the source of very high-volume lava flows.
At Hawaii, the Pacific plate is moving at ~9 cm/year towards the north west. The Hawaiian volcanoes grow progressively older from the submarine volcano Lo'ihi and the island of Hawaii at the south-east end of the chain through the main islands, through the leeward islands (mostly atolls formed on old submerged volcanoes), and beyond Kure atoll to the Emperor sea-mounts, the northernmost and oldest of which (Meiji Seamount) is being sub-ducted under Kamchatka.
Uplift caused by the hotspot has bulged the Pacific plate upward over a broad region approximately 400 kilometres wide called the Hawaiian swell. This brings an excess of dense mantle material to near the earth's surface, and this extra mass actually results in gravity being slightly higher around the Hawaiian chain. At the same time, the loading of the volcanoes onto the heat-weakened swell has warped the centre downward. This combination of uplift and subsidence has formed a broad m-shaped profile; the resulting structures are the Hawaiian trough adjacent to the islands, and the Hawaiian arch outboard of that. Recently the arch has been found to be the source of very high-volume lava flows.