Mountain Building

In survival games, you build a mountain block by block. Earth uses a different method: squeezing, stretching, lifting, and crumpling the crust over millions of years. The process of mountain building is called orogeny (say "or-AH-juh-nee").

The Sawtooths, the Tetons, the Lost River Range, the Rockies. Every mountain you can see from Idaho has an origin story. Let's crack it.

Stress is force acting on rock. There are three kinds:

• Compression: squeezing together. Shortens and thickens the crust. Happens at convergent boundaries.
• Tension: pulling apart. Stretches and thins the crust. Happens at divergent boundaries and in places like Idaho.
• Shear: pushing in opposite directions, side by side. Happens at transform boundaries.

The change in shape caused by stress is called deformation.

Under slow, steady pressure, especially deep underground where it's hot, rock can bend instead of break. Push on the end of a rug and it wrinkles. Rock layers do the same.

• Anticline: an upward fold, shaped like an A. (A for Anticline, and A for "arch up".)
• Syncline: a downward fold, shaped like a U or a sink. (S for Syncline, S for "sink".)
• Monocline: a simple step-like bend in flat layers.

Near the surface, rock is cold and brittle, so it breaks instead of bending. You met the three types of faults in Chapter 14: normal, reverse, and strike-slip. When faults lift huge blocks, they build mountains too.

1. Folded mountains

Formed when continents collide and crumple rock into giant folds. Examples: the Himalayas (still rising), the Alps, and the Appalachians, which were once as tall as the Himalayas before hundreds of millions of years of erosion wore them down.

2. Fault-block mountains

Formed by tension. As crust stretches, it cracks into blocks. Some blocks rise and tilt, forming mountains (called horsts), and others drop down, forming valleys (called grabens). This is Idaho's specialty. The Basin and Range region stretches across Nevada, Utah, and southern Idaho: mountain, valley, mountain, valley, over and over like a corrugated roof. The Lost River Range, home of Borah Peak, and the Teton Range are fault-block mountains.

3. Volcanic mountains

Built by eruptions piling up lava and ash, like the Cascades and Mount Fuji. Undersea volcanoes can build mountains too: Mauna Kea in Hawaii is over 10,000 meters tall from its base on the sea floor, which is taller than Everest.

4. Dome mountains

Formed when magma pushes up rock layers from below without breaking through, like a blister. The magma cools into a huge mass of granite called a batholith. Erosion later strips off the top layers and reveals the granite. The Sawtooth Mountains are carved from the Idaho Batholith, then sharpened by glaciers into jagged peaks.

Here's a weird fact. Mountains have deep "roots" that push down into the mantle, like an iceberg with most of its mass hidden below. The crust floats on the mantle, and thicker crust floats higher. This balance is called isostasy.

When a mountain erodes and gets lighter, the crust slowly rises back up, like a boat rising when you unload it. When glaciers melt, the land underneath rebounds too. Parts of Canada are still rising today after the ice age glaciers melted.

While plates push mountains up, weathering and erosion tear them down. Young mountains, like the Tetons and Himalayas, are tall, sharp, and jagged. Old mountains, like the Appalachians, are low and rounded. Mountain building versus erosion is a battle that never ends.