<P> The hotspot has since been tomographically imaged, showing it to be 500 to 600 km (310 to 370 mi) wide and up to 2,000 km (1,200 mi) deep, and olivine and garnet - based studies have shown its magma chamber is approximately 1,500 ° C (2,730 ° F). In its at least 85 million years of activity the hotspot has produced an estimated 750,000 km (180,000 cu mi) of rock . The chain's rate of drift has slowly increased over time, causing the amount of time each individual volcano is active to decrease, from 18 million years for the 76 - million - year - old Detroit Seamount, to just under 900,000 for the one - million - year - old Kohala; on the other hand, eruptive volume has increased from 0.01 km (0.002 cu mi) per year to about 0.21 km (0.050 cu mi). Overall, this has caused a trend towards more active but quickly - silenced, closely spaced volcanoes--whereas volcanoes on the near side of the hotspot overlap each other (forming such superstructures as Hawai ʻi island and the ancient Maui Nui), the oldest of the Emperor seamounts are spaced as far as 200 km (120 mi) apart . </P> <P> Tectonic plates generally focus deformation and volcanism at plate boundaries . However, the Hawaii hotspot is more than 3,200 kilometers (1,988 mi) from the nearest plate boundary; while studying it in 1963, Canadian geophysicist J. Tuzo Wilson proposed the hotspot theory to explain these zones of volcanism so far from regular conditions, a theory that has since come into wide acceptance . </P> <P> Wilson proposed that mantle convection produces small, hot buoyant upwellings under the Earth's surface; these thermally active mantle plumes supply magma which in turn sustains long - lasting volcanic activity . This "mid-plate" volcanism builds peaks that rise from relatively featureless sea floor, initially as seamounts and later as fully - fledged volcanic islands . The local tectonic plate (in the case of the Hawaii hotspot, the Pacific Plate) slowly slides over the hotspot, carrying its volcanoes with it without affecting the plume . Over hundreds of thousands of years, the magma supply for the volcano is slowly cut off, eventually going extinct . No longer active enough to overpower erosion, the volcano slowly sinks beneath the waves, becoming a seamount once again . As the cycle continues, a new volcanic center manifests, and a volcanic island arises anew . The process continues until the mantle plume itself collapses . </P> <P> This cycle of growth and dormancy strings together volcanoes over millions of years, leaving a trail of volcanic islands and seamounts across the ocean floor . According to Wilson's theory, the Hawaiian volcanoes should be progressively older and increasingly eroded the further they are from the hotspot, and this is easily observable; the oldest rock in the main Hawaiian islands, that of Kaua ʻi, is about 5.5 million years old and deeply eroded, while the rock on Hawai ʻi island is a comparatively young 0.7 million years of age or less, with new lava constantly erupting at Kīlauea, the hotspot's present center . Another consequence of his theory is that the chain's length and orientation serves to record the direction and speed of the Pacific Plate's movement . A major feature of the Hawaiian trail is a sudden 60 ° bend at a 40 - to 50 - million - year - old section of its length, and according to Wilson's theory, this is evidence of a major change in plate direction, one that would have initiated subduction along much of the Pacific Plate's western boundary . This part of the theory has recently been challenged, and the bend might be attributed to the movement of the hotspot itself . </P>

What kind of mantle activity causes volcanism on hawaii