There's an old saying in mining, if it's not grown it must come out of the earth. Each year the US produces 75 billion dollars worth of mineral resources that include exotic metals such as gold and silver and more basic materials like sand and gravel. As geologists we interpret minerals and rocks to unravel the history of our planet and find and manage mineral resources to support economic growth.
We have two learning objectives for this lesson. We are going to identify the surprisingly short list of Earth's most popular elements and introduce you to our friends the silicates, the most common mineral group in the rocks below our feet. We know that rocks are made of minerals and minerals are composed of elements. Elements are the primary ingredients of all Earth materials. We consume elements in the food we eat, the air we breathe, and the water we drink.
Elements can't be separated into simpler substances by chemical means. There are 92 naturally occurring elements on Earth. How many of these do you recognize? Rocks and minerals are composed of combinations of these elements, but don't get too worried about learning all of the names. Just eight elements make up most of the rocks in the Earth's crust, and several of these are also key ingredients in Earth's core and mantle.
Remember that Earth is differentiated into three major compositional layers. Most of these layers are made up of just four elements iron, oxygen, silicon, and magnesium in some combination. Dense elements such as iron and nickel sank to Earth's core during the early history of the planet.
The core and the mantle do contain silicon and oxygen, but these lighter elements are much more abundant in the outer layers of Earth, and especially in the crust. Scientists have discovered over 4,000 different minerals, and more are found each year. However, only a few dozen minerals are relatively abundant in the Earth. That's a lot easier to wrap your mind around.
We live on the planet's surface, so we are most interested in the rocks below our feet that we can use for resources and that break down to form our soils. When we look more closely at a characteristic rock on Earth's crust, we can see that it is composed of different colored minerals. Analyzing the elements in these minerals shows that oxygen and silicon make up 75% of the elements present. In addition to oxygen and silicon, there are only six other elements that make up the majority of the Earth's crust.
All six of these may be familiar to you. We take in iron, calcium, potassium, and magnesium in our diet. Foods like spinach are rich in all of these elements. We consume sodium as salt, and we use aluminum in a variety of forms every day. In combination with a handful of other elements, oxygen and silicon combine to form the most common mineral group, the silicates.
So, silicate minerals contain oxygen and silicon. Often with one or more of the other six elements we just mentioned. Let's consider the rock granite, a common rock in the Earth's crust. Granite contains several silicate minerals. Some, like quartz, have a relatively simple composition and are composed exclusively of oxygen and silicon.
Granites also contain one or more varieties of the mineral feldspar. They will have additional elements such as potassium, sodium, and aluminum. Most of the minerals in granite are light colored, and be white, pink, tan, or clear. However, some are darker.
We see a difference in the elements that are present in these minerals. They contain iron and magnesium, and lower proportions of silicon and oxygen. However, all of these minerals are silicates, as indicated by the presence of silicon and oxygen in their chemical formulae.
The silicates are therefore composed of two non-metallic elements. These non-metallic elements often give mineral groups their names. These elements may form bonds with metallic elements such as potassium and aluminum to create specific varieties of minerals such as feldspar or muscovite mica. Many silicate minerals begin life as elements in magma. Silicates form as the magma cools and crystallizes to form different types of igneous rocks.
The original composition of the magma and the characteristics of the physical and chemical environment will determine the type of igneous rocks that form. Magmas with higher percentages of silicon and oxygen will erupt more violently than those where these two elements make up a smaller proportion of the total elements present. Being able to identify the minerals present in igneous rocks allows geologists to unravel the volcanic history of ancient mountain ranges as well as evaluate the potential hazards of modern active volcanoes. These igneous rocks can be broken down into their constituent minerals by weathering processes at the Earth's surface. For example, we can see large feldspar crystals exposed here at the Black Canyon of the Gunnison National Park in Colorado.
These and other silicate crystals weather out of the outcrop and are then washed away into the river and carried downstream toward the coast where they may become part of a beach. Eventually, these mineral grains and fragments may be cemented together to form a sedimentary rock such as sandstone. Sand and gravel deposits are dominated by resistant silicate minerals like quartz and feldspar. The U.S. mines about $7 billion of sand and gravel each year, most of which goes to make concrete or is used for road construction. We have two principal learning objectives for today.
How confident are you that you could complete both of these tasks successfully? Okay, that's it for us. Jen, how about we go and replenish our elements with some spinach?