Rocks 101

Minerals and fossils, and the rocks that host them, provide by far the greatest source of information regarding how our planet has evolved through time, and the processes which continue to develop the earth’s continents and oceans.  Different rock types are expressions of the various processes which formed them.  Knowing how rock types were formed in turn tells us a great deal about the environments which hosted the life forms now preserved as fossils and the chemical and physical conditions under which minerals were produced.

Fossils literally provide us with the story of life on stone tablets.  The Earth’s fossil record stretches back at least two billion years, and fossils of marine invertebrate animals are abundant in the rock record of the past half-billion years.  Fossils are nearly always found in sedimentary rocks, most commonly those formed by the movement, deposition and cementation of sediment by water.  Mapping of the sequence in which sedimentary rocks are deposited, along with use of ages determined from radioactivity for associated igneous rocks, provides us with the chronology of sedimentary rock types across much of the world.  Identification of fossils associated with different ages of sediments tells us the sequence in which various life forms appeared, flourished and often disappeared.  In turn, comparison with modern plants and animals allows us to determine much about the environmental conditions which formed the rocks hosting the fossils, and often a close estimate of the age of the rocks’ formation.

LEARN MORE…Specific Fossil Types

Minerals are naturally-occurring inorganic compounds with a fixed chemical composition, which leads to a characteristic internal structure.  This fixed composition and internal structure gives each mineral its physical properties, such as hardness, specific gravity (“weight”), and color.  The internal structure is often expressed by an external crystal form, but under differing physical conditions this form can often vary within certain bounds. For example, pyrite can occur not only in its basic cubic form but also as diamond-shaped octahedrons.  The chemical and physical properties of minerals can provide us with valuable information about the specific conditions which formed the host rock or later disrupted or altered its original state.  Minute amounts of radioactive elements within minerals allow the ages of rocks formed from magmas (“igneous rocks”) to be determined with surprising precision.

LEARN MORE…Specific Mineral Types