Most volcanoes sit at plate boundaries, but Hawaii is in the middle of the Pacific Plate, thousands of kilometers from any boundary. It sits over a hot spot: a place where unusually hot mantle rock rises and melts, staying put while the plate slides over it.
Key idea
A plate moving over a fixed hot spot carries each volcano away from the heat and builds a new one, leaving a line of volcanoes that get older with distance. The ages and distances give the plate's speed and direction.
Why it works
Think of a sheet of paper slid slowly over a candle: it gets a line of scorch marks, and the oldest mark is farthest from the flame. In Hawaii the Big Island, over the hot spot, has active volcanoes. Toward the northwest, Maui, Oahu and Kauai are older and more worn down. Kauai's oldest lavas are about 5 million years old, about 500 km from the hot spot, so the Pacific Plate has moved about 500 km in 5 million years: about 10 cm a year (using plate rates).
Worked example: Plate speed
An island 300 km from a hot spot is 3 million years old. How fast has the plate moved, in cm per year?
- Speed = distance ÷ time = $300 \div 3 = 100$ km per million years.
- 1 cm/yr = 10 km per million years, so 100 km per million years = 10 cm/yr.
Answer: 10 cm per year
Worked example: Which way?
In a chain of hot-spot islands, the active volcano is at the southeast end and the islands get older toward the northwest. Which way is the plate moving?
- The oldest islands have been carried farthest from the hot spot.
- They lie to the northwest, so the plate is moving northwest.
Answer: Northwest
Common mistake
Assuming every volcano sits on a plate boundary. Hot-spot volcanoes like Hawaii and Yellowstone are in the middle of plates. Check: a hot-spot chain gets older in a steady line away from the active end, which boundary volcanoes don't do.