6. Hotspots, the Pace of Change & Reading Tectonic Data

There's one famous exception to 'everything happens at boundaries': hotspots. A hotspot is a plume of unusually hot mantle rising in the middle of a plate, far from any edge — and it stays roughly fixed in place. As the plate slides over it, the plume punches through to make a volcano. Over time this leaves a chain of volcanoes that get older with distance from the active one.

The Hawaiian Islands are the classic example: the Big Island sits over the hotspot now and is still erupting, while the islands to the northwest are older and extinct. The age-versus-distance pattern even lets scientists measure how fast and in which direction the plate is moving.

How fast does the map change? Plates move about 2–10 cm per year. That's slow, but it adds up: the Atlantic Ocean widens a few centimeters a year, and India's slow-motion crash into Asia is still pushing the Himalayas higher. Today GPS instruments measure this motion directly.

Reading tectonic data (a GED® skill). Many GED® items give a table or graph — island age vs. distance from a hotspot, or GPS plate-velocity data. Read the title, axis labels, and units first; a steady rise of age with distance supports a moving plate; and you can find rate = distance ÷ time.

Worked example: an island 500 km from a hotspot is about 10 million years old. Speed ≈ 500 km ÷ 10 million yr = 50 km per million years = 5 cm per year.

⚠️ Common misconception: in a hotspot chain it is the plate that moves, not the hotspot. The plume stays put; the conveyor-belt plate carries the old volcanoes away.

💡 Tip: to get a plate's speed from a hotspot chain, divide the distance by the age (watch your units — aim for cm/year).

📚 Sources: USGS, This Dynamic Earth (Kious & Tilling) and Understanding Plate Motions; Tarbuck & Lutgens, Earth Science; Wegener (1915); Hess (1962); Vine & Matthews (1963).