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Red-hot giants form deep within the Earth’s crust — you could be living on top of a former one right now.

In this lesson you will read about the huge igneous formations known as batholiths and how they form.

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Definition of a Batholith

Half Dome, in Yosemite National Park, is a landmark recognized worldwide and beloved by rock climbers everywhere. Its spectacular shape is characteristic of the vast igneous formations known as batholiths.A batholith is a very large mass of intrusive igneous rock that forms and cools deep in the Earth’s crust. An igneous rock is a type of rock formed through the cooling of lava or magma.

The term ‘batholith’ comes from the Greek words bathos, meaning ‘depth,’ and lithos, meaning ‘rock.’ In order for an intrusion to be called a batholith, the exposed area showing at the Earth’s surface should be at least 100 square kilometers, though some of these formations are much larger than that.

Formation and Structure of a Batholith

Although from a distance a batholith may just look like a huge lump of rock, it has an internal structure and history that can be very complicated. A batholith is made up of many individual plutons, which are smaller bodies of rock that, while still molten, traveled up through the crust to their present position. Each pluton is typically several kilometers in diameter.If the resulting exposed rock mass is too small to be called a batholith, geologists often refer to it as a stock. Most batholiths are composed of felsic rock, such as granite, which is less dense than mafic rock, like basalt; this, along with its heat, is what allows the rock to rise.

Many batholithic mountains you may have seen are smooth and rounded – this is because the stresses within the rock cause it to erode in sheets, or exfoliate, like the skin of an onion.

Process

While they are still molten magma, the plutons are known as plutonic diapirs – a diapir in geology is any intrusive mass of flexible, moving rock that forces its way up through more brittle rock layers. In the case of a plutonic diapir, the heat of the molten pluton helps to soften the brittle rock and push it out of the way.

If the rising plutons reach the surface, a volcanic eruption starts – but most plutons tend to slow down, cool, and crystallize before that, anywhere from five to thirty kilometers below the surface. As more and more plutons come together in one place, a batholith gradually forms. Once the surface rock above it erodes away, the batholith is exposed.

Examples of Batholiths

Batholiths are found all over the world. Many are huge, like the Sierra Nevada or Canada’s Coast Range. A few are just one big mountain or isolated formation. Some are a ‘mere’ few million years old, while others date back to the Precambrian.

No two are alike, but all of these former ‘red-hot giants’ have something to teach us about the building of the continents we call home.

Lesson Summary

A batholith is a large mass of intrusive igneous rock that forms and cools deep in the Earth’s crust. Batholiths are vast, rising at least 100 square kilometers above the surface of the Earth, which is why they’re so hard to miss. They are made up of plutons, which are themselves several kilometers in diameter.

Batholiths can be found all over the planet, from Yosemite National Park to Canada’s Coast Range.

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