4.1 Melting, Cooling, Crystallizing!

The Rules of Molten Rock Cooling

When rock is melty and molten beneath the Earth’s surface, it is called magma. However, when it erupts above the surface, it is called lava. This is an easy distinction to make, but different rocks can still form from cooling magma AND lava. Rocks that form from magma beneath the Earth’s surface are called intrusive (think IN-side the earth) while those that form from lava are called extrusive (think on Earth’s EX-terior).

What happens when super-heated molten rock cools to form something that we can hold in the palms of our hands? We will first go over some of the concepts or “rules” of molten rock cooling and what they mean for these two main categories of igneous rocks.

Pizza Time!

Imagine two balls of pizza dough 20 cm in diameter. One is rolled out to form a crust 1 cm thick and 50 cm in diameter. Both pieces of dough have the same initial size and weight, but their shapes are now different. Both are baked at 450 degrees for 25 minutes and removed.

Coffee!

Consider how equal amounts of hot coffee are poured into paper and styrofoam cups.

Put it on Ice

For this last thought experiment, let’s picture two blocks of ice. One is a large cube, about the size of your hand. The other is a typical ice cube that you might find in a freezer tray.

Now that we have completed these thought experiments and understand the concepts behind the “rules” of molten rock cooling, let’s apply them to the rocks that form from magma and lava.

Remember: Intrusive rocks cool from magma, Extrusive rocks cool from lava. Intrusive is INSIDE the Earth. Extrusive is on the Earth’s EXTERIOR!

We have now touched on an important identifying characteristic of igneous rocks related to its cooling history--texture!