Formation- Case Study: Hawaiian Islands

The Hawaiian Islands form an archipelago of nineteen islands and atolls, numerous smaller islets, and undersea seamounts trending northwest by southeast in the North Pacific Ocean. The archipelago takes its name from the largest island in the group and extends some 1500 miles (2400 km) from the Island of Hawaii in the south to northernmost Kure Atoll.

The principal Hawaiian islands (all capital letters) are the exposed tops of volcanoes that rise tens of thousands of feet above the ocean floor.
Source: http://pubs.usgs.gov/gip/hawaii/fig02.gif

The chain of islands or archipelago formed as the Pacific plate moved slowly northwestward over a hotspot in the Earth's mantle at about 52 km (32 miles) per million years. This hot spot partly melts the region just below the overriding Pacific Plate, producing small, isolated blobs of magma. Less dense than the surrounding solid rock, the magma rises buoyantly through structurally weak zones and ultimately erupts as lava onto the ocean floor to form volcanoes.

Hawaii (the Big Island) is the largest and youngest island in the chain, built from five different volcanoes. Kohala, at the northwestern corner of the island, is the oldest and the smallest, having ceased eruptive activity about 60,000 years ago and go through longest exposure to erosion. The second oldest is Mauna Kea, which last erupted about 3,000 years ago; next is Hualalai, which has had only one historic eruption (1800-1801), and, lastly, both Mauna Loa and Kilauea have been vigorously and repeatedly active in historic times. Because it is growing on the southeastern flank of Mauna Loa, Kilauea is believed to be younger than its huge neighbor. Mauna Loa, comprising over half of the Big Island, is the largest shield volcano on the planet. The measurement from the base locally depressing the sea floor in the Hawaiian Trough to its peak is about 17 km.

Mauna Loa Volcano towers nearly 3,000 m above the much smaller Kilauea Volcano
(caldera in left center).
Source: http://hvo.wr.usgs.gov/maunaloa/4303062_L.jpg

Almost all magma created in the hotspot has the composition of basalt, and so the Hawaiian volcanoes are constructed almost entirely of this igneous rock and its coarse-grained equivalents, gabbro and diabase. The majority of eruptions in Hawaiʻi are Hawaiian-type eruptions because basaltic magma is relatively fluid compared with magmas typically involved in more explosive eruptions, such as the andesitic magmas that produce some of the spectacular and dangerous eruptions around the margins of the Pacific basin.