Plate Tectonics

You can't feel it, but the ground under you is moving. North America drifts about 2.5 cm (1 inch) a year, roughly as fast as your fingernails grow. That doesn't sound like much, but give it 100 million years and you've moved an entire ocean's width.

In 1912, a German scientist named Alfred Wegener noticed something odd: the coastlines of South America and Africa fit together like puzzle pieces. He proposed that all the continents were once joined in one supercontinent he called Pangaea ("all land"), which broke apart about 200 million years ago. He called it continental drift.

His evidence:

• Puzzle-piece coastlines.
• Matching fossils on continents now separated by oceans, like the reptile Mesosaurus, which couldn't have swum across the Atlantic.
• Matching rock layers and mountain chains that line up across oceans.
• Ancient climate clues: coal (from tropical swamps) in Antarctica, and glacier scratches in hot places like India and Africa.

Most scientists laughed at him. His problem? He couldn't explain what force could push whole continents around. He died in 1930 before he was proven right. Science lesson: good evidence matters, but so does a mechanism.

In the 1950s and '60s, new technology let scientists map the ocean floor. They found a giant underwater mountain chain, the mid-ocean ridge, running around the globe like the seam on a baseball.

Then came sea-floor spreading: at the ridges, magma rises, cools, and makes new ocean crust, pushing older crust away on both sides. The proof:

• Rock ages: crust is youngest at the ridge and gets older the farther away you go.
• Magnetic stripes: remember how Earth's magnetic field flips? As new rock cools, it locks in the field's direction. The sea floor shows matching stripes of normal and reversed magnetism on both sides of the ridge, like a barcode. That's a perfect recording of the sea floor spreading apart.
• Thin sediment near the ridge, thicker sediment farther away.

The oldest ocean crust is only about 180 million years old, while continents have rocks over 4 billion years old. So where does old ocean crust go? It sinks back into the mantle. More on that below.

Put it all together and you get plate tectonics: Earth's lithosphere is broken into about a dozen large plates and many smaller ones. They float on the asthenosphere and are moved by mantle convection, plus the pull of sinking plates (slab pull) and the push from ridges (ridge push).

Most of the action, earthquakes, volcanoes, and mountains, happens where plates meet, at plate boundaries.

1. Divergent (pulling apart)

Plates move away from each other. Magma rises to fill the gap.

• In the ocean: mid-ocean ridges. Iceland sits right on top of one, and it's splitting in half.
• On land: rift valleys. The East African Rift is slowly tearing Africa apart.

2. Convergent (crashing together)

Plates collide. What happens depends on who's crashing:

• Ocean vs. continent: the denser ocean plate dives under in a process called subduction. It melts, forming volcanoes. The Cascades (Mount St. Helens, Mount Rainier) formed this way.
• Ocean vs. ocean: one subducts, forming deep trenches and chains of volcanic islands like Japan. The Mariana Trench is the deepest spot on Earth, about 11 km down.
• Continent vs. continent: neither sinks, so the crust crumples up into giant mountains. India smashing into Asia built the Himalayas, and Mount Everest is still growing a few millimeters a year.

3. Transform (sliding past)

Plates grind sideways past each other. Crust isn't made or destroyed, but they get stuck, build up stress, then suddenly slip. Earthquakes! The San Andreas Fault in California is the famous one.

Idaho isn't on a plate boundary, but it's been shaped by them:

• Long ago, subduction along the West Coast created the magma that formed the Idaho Batholith granite.
• The Yellowstone hotspot is a plume of hot mantle rising under the plate. As North America drifts southwest over it, the hotspot has burned a trail of old volcanic craters across southern Idaho, creating the Snake River Plain. It's like moving a piece of paper over a candle.
• The Basin and Range region is being stretched apart, which is why Idaho still has active faults and earthquakes.