Whilst most volcanic activity happens at plate margins, there are cases of volcanoes erupting in the middle of plates. The Hawaiian Islands are formed by volcanic activity, despite the nearest plate margin being 3,200 km away.
Some geologists have suggested that a 'hot spot' in the mantle, which remains stationary as the Pacific Plate moves over it, explains the existence of the island chain. The hot spot may represent the top of a mantle plume which originated deep down at the outer core - lower mantle boundary. The plate moves in a north westerly direction due to sea floor spreading along the East Pacific Rise. As oceanic lithosphere moves away from the hot spot, volcanic activity ceases and it cools, becomes denser, and slowly subsides. As new oceanic lithosphere is positioned over the hot spot, a new island will begin to form above.
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The islands extend for around 2,400 km, forming a chain that is progressively older from the south east end to the north west end. The volcanoes are often very wide, with gently sloping sides comprising many thin (1 to 5 metres thick) basaltic lava flows. These are referred to as 'shield volcanoes'. Kilauea and Mauna Loa on Big Island are currently active examples.
The next island to appear in the Hawaiian chain has already been identified, and named as Lo’ihi. It is currently 975 metres below sea level, and is estimated to emerge above sea level in the next 10,000 to 100,000 years.