Introduction to Geology

Geology is the study of the Earth, its substances, shapes, processes and history.

Minerals
 

Minerals are natural, inorganic solids with a consistent composition—a chemical formula. Each mineral also has a unique arrangement of atoms, expressed in its crystal form (or habit) and its hardness, fracture, color and other properties. Organic natural substances, like petroleum or amber, are not called minerals.

Minerals of exceptional beauty and durability are called gemstones (as are a few rocks). Other minerals are sources of metals, chemicals and fertilizers. Petroleum is a source of energy and chemical feedstocks. All of these are described as mineral resources.

Rocks

Rocks are solid mixtures of at least one mineral. While minerals have crystals and chemical formulas, rocks instead have textures and mineral compositions. On that basis, rocks are divided into three classes reflecting three environments: igneous rocks come from a hot melt, sedimentary rocks from accumulation and burial of sediment, metamorphic rocks from altering other rocks by heat and pressure. This classification points to an active Earth that circulates matter through the three rock classes, on the surface and underground, in what is called the rock cycle.

Rocks are important as ores—economic sources of useful minerals. Coal is a rock that is a source of energy. Other rock types are useful as building stone, crushed stone and raw material for concrete. Still others serve for toolmaking, from the stone knives of our prehuman ancestors to the chalk used by artists today. All of these, too, are considered mineral resources. 

Fossils 

Fossils are signs of living things, found in many sedimentary rocks. They may be impressions of an organism, casts in which minerals have replaced its body parts, or even remnants of its actual substance—ancient remains. Fossils also include tracks, burrows, nests and other indirect signs. Fossils and their sedimentary environments are vivid clues about the former Earth and what living there was like. Geologists have compiled a fossil record of ancient life stretching hundreds of millions of years into the past, of which human time is a brief moment in the latest chapter.

Fossils have practical value because they change throughout the rock column. The exact mix of fossils serves to identify and correlate rock units in widely separated places, even in the grit pumped up from drillholes. The geologic time scale is based almost entirely on fossils supplemented with other dating methods. With it we can confidently compare sedimentary rocks from everywhere in the world. Fossils are also resources, valuable as museum attractions and as collectibles, and their commerce is increasingly regulated.

Landforms, structures and maps 

Landforms in all their variety are products of the rock cycle, built of rocks and sediment and shaped by erosion and other processes. Landforms give testimony of the environments that built and altered them in the geologic past, such as ice ages. From uplifted mountains to water bodies to caves to the sculpted features of the beach and seafloor, landforms are clues to the Earth beneath them.

Structure is an important part of studying rock outcrops. Most parts of the Earth's crust are warped, bent and buckled to some extent. The geologic signs of this—jointing, folding, faulting, rock textures and unconformities—help in assessing structure, as do measurements of the slopes and orientation of rock beds. Structure in the subsurface is important for water supply.

Geologic maps are an efficient database of geologic information on rocks, landforms and structure. They are useful to many people beside geologists and are beautiful too.

Geologic processes and hazards 

Geologic processes drive the rock cycle and create landforms, structures and fossils. They include erosion, deposition, fossilization, faulting, uplift, metamorphism, volcanism and many others with less familiar names.

Geologic hazards are powerful expressions of geologic processes. Landslides, volcanic eruptions, earthquakes and tsunamis, climate change, flooding and cosmic impacts are extreme examples of ordinary things. Understanding the underlying geologic processes is a key part of mitigating geologic hazards. Likewise, studying catastrophic hazards today helps us to see their signs in the rocks.

Tectonics and hearth history
 

Tectonics is geologic activity on the largest scale. As geologists mapped the world's rocks, untangled the fossil record and studied geologic features and processes, they began to raise and answer questions about tectonics—the life cycle of mountain ranges and volcanic chains, motions of continents, the rise and fall of the ocean and the doings of Earth's rocky mantle and iron core. Plate-tectonic theory, which explains tectonics as the motions in Earth's outer broken skin, has revolutionized geology, enabling us to study everything on Earth in a unified framework.

Earth history is the intricate story that minerals, rocks, fossils, landforms and tectonics tell. Fossil studies, in combination with gene-based techniques, yield a consistent evolutionary history of life on Earth. The Phanerozoic Eon (age of fossils) of the last 550 million years is well mapped as a time of expanding life punctuated by mass extinctions. The previous 4 billion years—Precambrian time—is being revealed as an age of enormous changes in the atmosphere, oceans and continents.