What Is Rock?
To geologists, rock is a coherent, naturally occurring solid, consisting of an aggregate of minerals or, less commonly, of glass. Let’s take this deﬁnition apart to see what its components mean.
- Coherent: A rock holds together, and thus must be broken to be separated into pieces. As a result of its coherence, rock can form cliff or can be carved into sculptures. A pile of unattached mineral grains does not constitute a rock.
- Naturally occurring: Geologists consider only naturally occurring materials to be rocks, so manufactured materials, such as concrete and brick, do not qualify.
- An aggregate of minerals or a mass of glass: The vast majority of rocks consist of an aggregate (a collection) of many mineral grains, and/or crystals, stuck or grown together. Some rocks contain only one kind of mineral, whereas others contain several different kinds. A few rock types consist of glass.
|Rocks, aggregates of mineral grains and/or crystals, can be clastic or crystalline.|
What holds rock together? Grains in rock stick together to form a coherent mass either because they are bonded by natural cement, mineral material that precipitates from water and ﬁlls the space between grains (figure above a), or because they i nterlock with one another like pieces in a jigsaw puzzle (figure above b). Rocks whose grains are stuck together by cement are called clastic, whereas rocks whose crystals interlock with one another are called crystalline. Glassy rocks hold together because they originate as a continuous mass (that is, they have no separate grains), because glassy grains were welded together while still hot, or because they were cemented together at a later time.
|Types of rock exposures.|
At the surface of the Earth, rock occurs either as broken chunks (pebbles, cobbles, or boulders) that have moved by falling down a slope or by being transported in ice, water, or wind, or as bedrock that is still attached to the Earth’s crust. Geologists refer to an exposure of bedrock as an outcrop. An outcrop may appear as a rounded knob out in a ﬁeld, as a ledge forming a cliff or ridge, on the face of a stream cut (where running water dug down into bedrock), or along human-made roadcuts and excavations (figure above a–d).
To people who live in cities or forests or on farmland, outcrops of bedrock may be unfamiliar, since bedrock may be completely covered by vegetation, sand, mud, gravel, soil, water, asphalt, concrete, or buildings. Outcrops are particularly rare in regions such as the midwestern United States, where, during the past million years, ice-age glaciers melted and buried bedrock under thick deposits of debris.
Credits: Stephen Marshak (Essentials of Geology)