The Big One  

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Earthquakes shake us up, but they’re as natural as rain. They’ve raised up Northwest mountains over millions and millions of years. What causes earthquakes? It’s the constant motion of the earth’s crust.

Earthquakes Are Inevitable

Earthquakes are the result of the structure of our planet. The earth beneath us is rock all the way through, but only the thin crust is a brittle solid. Inside the earth, the rock is so softened by heat that it slowly moves and flows.

The Earth is like an egg

The inside of the earth is layered something like an egg. Both have a thin, brittle shell. The crust of the earth is broken into pieces, like the cracked shell of a hardboiled egg. The mantle of the earth is like the egg white, and the core of the earth lies in the center, like the egg yolk.

Not as solid as it seems
The earth’s brittle crust is broken into pieces that geologists call plates.

These plates are in constant slow motion, carried by currents in the earth’s heat-softened interior. These convection currents move at about the speed that our fingernails grow.

What are convection currents?
If you’ve ever watched a lava lamp, you’ve seen convection currents at work.

  1. Heat (from the light bulb) warms up a colored liquid at the base of the lamp.
  2. The colored liquid rises, because hot liquids are less dense than cool ones.
  3. As the blobs of hot liquid move away from the heat source, they cool off, become more dense again, and sink back to the bottom. This cycle of movement is called a convection current.

Heat keeps plates in motion

The earth is something like a lava lamp. Inside the mantle, convection currents rise slowly, fueled by heat in the earth’s core. Most of the earth’s heat is left over from the Big Bang at the formation of the Universe. Ongoing radioactive decay constantly produces more heat.

The convection currents within the earth are huge. They carry the crust plates along with them, because the crust is so thin in comparison.

Convection currents circulate slowly within the earth’s mantle.

Plate motion makes earthquakes
All the plates that make up the crust are jostling against each other. Strain builds up as these plates of solid rock grind together. Eventually the rocks can’t take it any more. The plates slip and the built-up stress is suddenly released as an...Earthquake!
Because of the jostling of the plates, earthquakes usually happen at plate edges.

Motion is at the margin
The Pacific Northwest lies on the margin of the North American continental plate. Just off the coast, a dense oceanic plate is sliding—subducting—beneath us. These plate motions are continuous, and so earthquakes are inevitable.

The earth has plates
Earth’s crust is composed of eight large plates and a number of small ones. Strings of triangles and lines show the edges of crustal plates.
YOU ARE HERE detail:
At the edge of the Pacific Northwest, an oceanic plate is sliding (subducting) beneath a continental plate.

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