Rocks!

Rocks are what you usually pick up from the ground. They are made of more than one mineral - click here if you haven’t read about minerals yet!

Because there are over 5000 different kinds of minerals, it makes for A LOT of different kinds of rocks. Thankfully, we only need to know a handful of types to get a good grip on the geology around us.

Rocks can be divided into three basic types - igneous, sedimentary, and metamorphic.

Igneous rocks are made from cooling magma or lava. The types of minerals and sizes of the crystals in igneous rocks depend on the composition of the magma and how fast the molten rock cooled.

Here’s some basalt. This is a dark, dense rock that formed when lava cooled quickly above ground - either on land or in the ocean.

In contrast, here’s some granite. Like basalt, it cooled from molten rock. However, it cooled underground much more slowly. Because of this, you can actually see the minerals that make up the rock with the naked eye!

Sedimentary rocks are made from a bunch of particles (which we call sediment) that have stuck together. Some typical sedimentary rocks include sandstone, limestone, and conglomerate.

Categorizing sedimentary rocks is easy for the most part - it’s all based on the size and type of sediment.

A typical piece of sandstone. This type of sedimentary rock is made of sand. As you can see, the grains are all around the same size.

In contrast, here’s some conglomerate. This type of sedimentary rock is made of gravel and if you look closely you’ll find that there’s a wide variety of grain sizes.

Metamorphic rocks are made from rocks that have undergone a lot of heat, pressure or both. The heat or pressure causes the minerals in the rocks to change shape and sometimes also composition!

A classic piece of gneiss showing the typical wavy patterns. This rock has been crushed and literally folded.

This is a piece of mica schist. There are two garnets embedded inside, a common occurence.