Hot Islands

Q: What is the source of the heat and lava on the Big Island of Hawaii? Why do the islands get flatter as you go northwest?
John, Tijeras, New Mexico

A:

1. The heat source

"The lava comes from a plume of molten rock that rises from deep within the Earth," says Don Swanson, Scientist-in-Charge, Hawaiian Volcano Observatory.

The Big Island of Hawaii is centered over this plume, called a hot spot.

"As the Pacific plate moves over the hot spot, magma pushes through the oceanic crust and, where it reaches the surface, it [piles up and] forms islands," says Pete Winn, Colorado geologist.

The Big Island of Hawaii was created this way. The source of the hot spot's heat, however, is controversial.

"Probably, it is a combination of heat generated by the accretion that formed Earth, radioactivity within the Earth, self compression of the core of Earth and the dissipation of earth-tide energy," says Don Swanson.

2. Flatter islands to the northwest

According to Gary A. Smith, Professor of Geology at the University of New Mexico, the smaller lower islands of Hawaii began life centered over the hot spot and grew large as the magma pushed upward. The northwestern drift of the Pacific plate, however, slid the volcanic centers northwest off the stationary hot spot and they became extinct. The volcanoes settled to a lower position on the ocean floor as they coasted northwest, down the bulging hot-spot crust. They became lower still as their weight depressed the ocean crust. Finally, the volcanoes crumbled under pelting rain delivered by the trade winds.

"Eventually, the islands will subside to near sea level and then be planed off by waves," says Smith.