Look at a map of Hawaii and you'll see volcanic islands strung out like pearls on a necklace. The biggest--and the youngest--is Hawaii, in the south-east corner.
As the islands stretch away to the north-west they become older and more worn. While Hawaii is about a million years old, the extinct volcano of Kauai on the north-west end is five times older.
The Hawaiian islands were formed by what geologists call "hot spot" volcanos. These occur where hot magma from the Earth's liquid mantle is steadily forced through the Earth's crust.
Hot spots can last for millions of years, staying in the same place while the plates of the Earth's crust move gradually overhead.
The plates of the Earth's crust move at uneven speeds, so when the plate moves slowly, this allows a tremendous volcano to build up. That's why we get a string of separate volcanos, rather than a continuous line.
This is exactly what's happened with the Hawaiian islands. The hot spot stayed in the same place, pushing up a string of volcanos as the Earth's crust moved over it.
Because of this, the volcano on north-west Kauai is old and extinct, while the volcanos on Hawaii's south-east edge are the most active in the world.
The Hawaiian hot spot is now located about seventeen miles off the south-east coast of Hawaii. There's no new island there, but there is a huge, underwater volcano, rising almost three miles above the ocean floor.
This new volcano will break the ocean's surface in about fifty thousand years, giving us our next Hawaiian island.