Mauna Kea
Mauna Kea ( or , ) is a dormant volcano on the island of Hawaii. Its peak is above sea level, making it the highest point in the state of Hawaii. Most of the mountain is under water, and when measured from its oceanic base, Mauna Kea is the tallest mountain in the world measuring over . Mauna Kea is about a million years old, and has thus passed the most active shield stage of life hundreds of thousands of years ago. In its current post-shield state, its lava is more viscous, resulting in a steeper profile. Late volcanism has also given it a much rougher appearance than its neighboring volcanoes due to construction of cinder cones, decentralization of its rift zones, glaciation on its peak, and weathering by the prevailing trade winds. Mauna Kea last erupted 6,000 to 4,000 years ago and is now considered dormant. The peak is only about higher than its more massive neighbor, Mauna Loa.
Savai'i
Savai'i is the largest shield volcano in the South Pacific and only 3% is above water.
It is an active volcano, which last erupted in 1905-1911 with lava flows that destroyed villages on the central north coast. The island is formed by a massive basaltic shield volcano which rises from the seafloor of the western Pacific Ocean. A possible model for the formation of the volcanic Samoa island chain is explained by the Samoa hotspot situated at the east end of the Samoa Islands. In theory, the Samoa hotspot is a result of the Pacific Tectonic Plate moving over a 'fixed' deep and narrow mantle plume spewing up through the Earth's crust. The Samoa islands generally lie in a straight line, east to west, in the same direction the Plate is moving. In the classic hotspot model, primarily based on studies of the Hawaii hotspot, the volcanic islands and seamounts further away from the Samoa hotspot should be progressively older. However, Savai'i, the most western of the Samoa island chain, and Ta'u Island, the most eastern of the Samoa islands, both erupted in the last century, data which is an enigma for scientists. Ta'u last erupted in 1866. Another discrepancy in the data from the Samoa islands is that subaerial rock samples from Savai'i were too young by several million years to fit the classic hotspot model of age progression in an island chain, raising arguments among scientists that the Samoa islands does not have a plume origin. The nearness of Savai'i and the Samoa island chain to the Tonga Trench at the south became a possible explanation for these discrepancies as well as the possibility that the islands were formed by magma seeping through cracks in stressed fracture zones. However, in 2005, an international team gathered further submarine samples from the deep flanks and rifts of Savai'i. Tests on these later samples showed much older ages, about five million years old, that fit the hotspot model. The discovery in 1975 of Vailulu'u Seamount 45 km east of Ta'u in American Samoa has since been studied by an international team of scientists and contribute towards understanding of the Earth's fundamental processes.