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The Rock Cycle

© Geoff Habiger

Before we can get into the finer details of geology, there are some basic items we need to cover first. I feel it is important to cover some general information before the next few articles that will go into greater detail on the basic rock types, sedimentary rocks, igneous rocks, and metamorphic rocks. Why not just jump right into discussions of earthquakes or volcanoes? Simple. Any understanding of geology comes from understanding how the rocks are formed and what minerals form the rocks. We can’t start learning about volcanoes without first learning about the igneous rocks that form volcanoes.

One of the most basic items to understand about all rocks is that no matter how they are formed, whether from the fiery mouth of a volcano or the slow accumulation of microscopic animals that have died, they are all built out of the same basic building blocks. Nearly all rocks (about 98 percent of the Earths crust by weight) are formed from the combination of eight basic elements, aluminum, iron, calcium, sodium, potassium, magnesium, oxygen, and silicon. These last two elements, silicon and oxygen, occur in some form in about three quarters of the earths rocks.

The basic elements, plus many other elements, combine to form about 2000 different kinds of minerals. Minerals, like quartz, are the basic building blocks of rocks. We will cover minerals in depth at a later date. For now, it is important to know that minerals are what the rocks you see around you are composed of.

The other basic concept to learn before we get into more detail is that all rocks are interconnected in a basic pattern known as the Rock Cycle. The rock cycle is a simple view of the complex interactions that occur between rocks. At the top of the page is a simple diagram of the rock cycle. Looking at it you can get a feel of how the different rock types are related to each other and how they interconnect.

Some basic items we can gather from looking at the rock cycle. Sedimentary rocks can only come from the erosion and lithification of other rocks, either igneous, metamorphic, or other sedimentary rocks. Metamorphic rocks are formed when intense heat and/or pressure are applied to sedimentary or igneous rocks. Igneous rocks are formed from the melting of metamorphic rocks.

The rock cycle is a basic concept that a geologist can use to help understand how rocks of different types interact with one another. Understanding how the rocks will interact is essential to understanding how the different rock types, sedimentary, igneous, and metamorphic, form. In the next three articles I will cover each of the basic rock types in more detail. We will first look at sedimentary rocks and how they are formed. We will then cover the igneous rocks and learn the two primary ways they are formed. Finally we will uncover how metamorphic rocks are formed from the intense heat and pressure of the earth’s interior.

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