How did the Hawaiian islands form? How long ago did it happen?
See below after the answer, for a video about formation of the Hawaiian islands.
There are about 143 Hawaiian islands, in a chain (archipelago) 1500 miles long. They were all formed by volcanoes. A “hot spot” of lava has poured up out of the floor of the Pacific ocean off and on for millions of years, creating each island in turn, as the tectonic plate that forms that part of the floor of the ocean moves from east to west. The oldest islands are small because they are eroded, and are on the northwest end of the chain, which formed 65 million years ago. The youngest and largest island is the Big Island of Hawaii, on the east end of the chain, and it is still forming today (still enlarging as new lava pours into the ocean). The next island to form will be Loihi, southeast of the Big Island of Hawaii, still 3000 feet underwater, and estimated to grow to the surface in around 50,000 years.
Here are the approximate ages of the largest and youngest Hawaiian islands, all on the eastern end of the chain.
Big Island of Hawaii 300,000 years old (and still enlarging now)
Haleakala (east half of Maui) 750,000 years old (last erupted about 210 years ago)
West half of Maui 1,300,000 years old
Kahoolawe 1,030,000 years old
Lanai 1,300,000 years old
Molokai 1,300,000 years old
Oahu 3,000,000 years old
Kauai 4,000,000 years old
The name of the state is the same as the name of the largest island, Hawaii. For that reason, the largest island is often called “The Big Island” to distinguish it from the name of the state. The island with the most population is Oahu, which is the third largest island. Honolulu, Pearl Harbor, Waikiki, and Diamond Head, are on the island of Oahu. Maui is the second-largest island and has the second-largest population.
More interesting Hawaii questions and answers are at Maui Q&A.