16-4 Minerals, Rocks, and the Rock Cycle

What Are Minerals and Rocks? Hard Stuff

The Earth’s crust, still forming in various places, is composed of minerals and rocks. It is the source of almost all the nonrenewable mineral resources we use: fossil fuels, metallic minerals, and nonmetallic minerals. It is also the source of soil and of the elements that make up your body and the bodies of other living organisms.

  • A mineral is an element or inorganic compound that occurs naturally and is solid with a regular internal crystalline structure.

  • Rock is a solid combination of one or more minerals that is part of the Earth’s crust. Some kinds of rock, such as limestone (calcium carbonate, or CaCO3) and quartzite.

What Are the Major Rock Types, and How Are They Recycled? Really Slow Recycling

Based on the way it forms, rock is placed in three broad classes:

  1. One is igneous rock, formed below or on the Earth’s surface when molten rock (magma) wells up from the Earth’s upper mantle or deep crust, cools, and hardens.

  2. A second type is sedimentary rock. It is formed from sediment produced when existing rocks are weathered and eroded into small pieces, and transported from their sources by water, wind, or gravity to down- stream, downwind, or downhill sites. These sediments are deposited in layers that accumulate over time and increase the weight and pressure on underlying layers.

  3. The third type is metamorphic rock, produced when a preexisting rock is subjected to high temperatures (which may cause it to melt partially), high pressures, chemically active fluids, or a combination of these agents. These forces can change or transform a rock by reshaping its internal crystalline structure and its physical properties and appearance.

The interaction of physical and chemical processes that changes rocks from one type to another is called the rock cycle. It recycles the Earth’s three types of rocks over millions of years and is the slowest of the Earth’s cyclic processes. It also concentrates the planet’s nonrenewable mineral resources on which we depend.

What Are Nonrenewable Mineral Resources? Useful Rock Resources

A nonrenewable mineral resource is a concentration of naturally occurring material in or on the Earth’s crust that can be extracted and processed into useful materials at an affordable cost. Because they take so long to produce, they are classified as non-renewable resources.

Ore is rock containing enough of one or more metallic minerals to be mined profitably. We convert about 40 metals extracted from ores into many everyday items that we either use and discard or learn to reuse, recycle, or use less wastefully. Geologists often divide nonrenewable mineral resources into four major categories:

  1. Identified resources: deposits of a nonrenewable mineral resource with a known location, quantity, and quality, or whose existence is based on direct geologic evidence and measurements.

  2. Reserves: identified resources from which a usable nonrenewable mineral can be extracted profitably at current prices.

  3. Undiscovered resources: potential supplies of a nonrenewable mineral resource assumed to exist on the basis of geologic knowledge and theory but with unknown specific locations, quality, and amounts.

  4. Other resources: undiscovered resources and identified resources not classified as reserves.