Hawaiian eruptions
Although fissure eruptions also occur on the Hawaiian volcanoes, they are really dominated by effusive lava emissions from vents clustered near a central spot. The Hawaiian islands began life as seamounts, but eruptions have built them 4000m above sea level. Hawaiian eruptions produce hot, very fluid basalts with few and rather weak explosions. They accumulate in vast, gentle shields with slopes of 3-5', culminating in a central hub that has collapsed in a shallow caldera, pock- marked by pit craters that sometimes contain molten lava lakes, such as Kilauea Iki which formed in 1959.
Most frequently, Hawaiian eruptions begin with lava-fountaining which spreads downwards along fissures as the gas is expelled. Sometimes small cones of fragments are formed as they did at Pu'u O'o in 1984, but calmer effusions usually follow, which may send long lava tongues down valley.
The Hawaiian
volcanoes are related to a mantle hotspot, and it is above similar oceanic
hotspots that comparable landforms have been developed in Reunion in the Indian
Ocean and in the Galapagos Islands in the eastern Pacific Ocean. The Hawaiian
volcanoes are the latest to be created by eruptions that have formed the
submerged Hawaiian and Emperor chains of seamounts. The oldest eruptions took
place over the Hawaiian hotspot some 75 million years ago, and the first
volcanoes that they formed have been carried northwestwards on the Pacific
Plate, and are now about to be subducted into the Aleutian Trench. In the
Hawaiian Islands, Kauai is the oldest, between 3.8 and 5.6 million years old,
whereas the youngest, Hawaii itself, emerged only about 700000 years ago, and
is, in fact, composed of five separate shield volcanoes. It is no doubt the
northwestward movement of the Pacific Plate over the hotspot that explains why
Kohala and Mauna Kea, the northwestern volcanoes on the island, are probably
extinct (although Mauna Kea may only be dormant), whereas the present activity
is concentrated on the southeast, in Kilauea and Mauna Loa and in the active
seamount Loihi that still lies 1000m below sea level. Each volcano seems to have
a life-span of about a minion years. Kilauea, which first erupted above sea
level about 1 00 000 years ago, and Mauna Loa, which emerged about 500000 years
ago and has accumulated a volume of 40000km3 of basalt, may be expected to make
further contributions to the bulk of Hawaii before they retire into repose and
are carried northwestwards beneath the Pacific Ocean like their predecessors.
They still erupt frequently: Mauna Loa has had 39 eruptions since 1830 and
Kilauea no less than 60. Magma also often rises into dykes and thus adds to the
bulk of the island without actually reaching the surface. From 1979 to 1983, for
instance, four eruptions took place on Kilauea, but 13 dyke intrusions failed to
reach its surface.
Reunion Island marks the most recent of a series of volcanoes formed as the African Plate moved over a hotspot at a speed of about 3 cm a year. Their remnants are found in the submerged Mascarene Plateau, 750km from Reunion, erupted about 40 million years ago; the now much-eroded island of Mauritius, 200km from Reunion, developed between 28 million and 18 million years ago; Reunion itself began to form on the floor of the Indian Ocean about 5 million years ago. Reunion is a massive accumulation of basaltic lava-flows that have formed two shields: one probably extinct, crowned b Piton des Neiges 3069 m, and the other, Piton de la Fournaise ("Blazing Furnace"; 2632m), is amongst the world's most active volcanoes, erupting on average every ten months. The base of the double volcano lies at a depth of 4000m and has a diameter of220 km. The total volume of Reunion has been estimated to be at least 57000km3. Piton des Neiges emerged from the Indian Ocean about 2.1 million years ago, but probably became extinct some 12000 years ago. Piton de la Fournaise rose above sea level about 500000 years ago, and has been vigorously erupting ever since. During historic time, which in Reunion is about 400 years, three-quarters of the eruptions of Piton de la Fournaise have lasted less than a month and 39 per cent have lasted less than a week. On average, Piton de la Fournaise expels about 10 million m3 per year of fluid lava-flows discharged rapidly from the summit cone or its immediate flanks. About 95 per cent of the eruptions have been confined within the Enclos Fouque, the youngest of three nested summit calderas. However, perhaps the most remarkable feature of the island is the Grand Brule, caused by vast landslides extending deep into the Indian Ocean.