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rock size
The rock cycle: How different types of rock are formed
Diagram of the rock cycle showing transitions between rock at Earth's surface, sediment, igneous rocks, metamorphic rocks, and sedimentary rocks, with arrows indicating processes like erosion, deposition, burial, melting, and uplift. Eight labeled rock samples are shown, including black glassy obsidian, speckled porphyry, striped calico sandstone, shell-filled coquina limestone, coarse breccia, layered banded gneiss, pale green fibrous talc schist, and greenish serpentine. A chart categorizes common textural terms for rocks by grain size in millimeters, dividing them into igneous and metamorphic, sedimentary, and pyroclastic types, with terms like phaneritic, sand, silt, breccia, and ash shown in relation to size. Two diagrams compare sediment sorting: the left shows poorly sorted sediment with mixed sizes and shapes, while the right shows well-sorted sediment with uniform, similarly sized grains. A line graph displays the distribution of dry bulk densities for various rock samples, with density values on the x-axis and percent of total samples on the y-axis. Most samples cluster around 2.63 grams per cubic centimeter. An inset graph shows cumulative percentage distribution, and a text box summarizes statistics such as mean, median, mode, and sample size. Diagram showing how rock deformation changes with increasing confining pressure, progressing from extension fracture and brittle fracture to brittle faulting, ductile faulting, and finally plastic flow. Graph showing typical stress-strain curves for brittle, moderately ductile, and ductile rock materials, with X marks indicating fracture points for each type. Axes display stress versus strain, and annotations highlight yield points, permanent deformation, and recoverable regions. A magnetic hysteresis curve graph with magnetization J on the vertical axis and external field Hex on the horizontal axis, showing a looped curve with key points labeled for saturation magnetization Js, remanent magnetization Jr,sat, and coercive fields Hc and Hc,r.
At a Glance
Slate, a metamorphic rock, showing typical splintery fracture and thin layering (slightly larger than life-size).
rock summary
Quizzes
Cross section of Earth showing the core, mantle, and crust
Everything Earth
Basalt sample returned by Apollo 15, from near a long sinous lunar valley called Hadley Rille. Measured at 3.3 years old.
(Bed) Rocks and (Flint) Stones
ore. iron ore minerals, rock, metal, metallic iron
Rocks, Minerals, and More Quiz
Cross section of Earth showing the core, mantle, and crust
The Solid Earth Quiz
Encyclopaedia Britannica thistle graphic to be used with a Mendel/Consumer quiz in place of a photograph.
27 True-or-False Questions from Britannica’s Most Difficult Science Quizzes
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rock size
rock size Rocks can be any size. Some are smaller than these grains of sand. Others, like this large rock that was dropped as a glacier melted, are as large as, or larger than, small cars.

rock

geology
Written by
Robert S. Carmichael
Professor of Geophysics and Geology, University of Iowa, Iowa City. Editor of Handbook of Physical Properties of Rocks (3 vol.).
Robert S. CarmichaelAll
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Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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rock, in geology, naturally occurring and coherent aggregate of one or more minerals. Such aggregates constitute the basic unit of which the solid Earth is composed and typically form recognizable and mappable volumes. Rocks are commonly divided into three major classes according to the processes that resulted in their formation. These classes are (1) igneous rocks, which have solidified from molten material called magma; (2) sedimentary rocks, those consisting of fragments derived from preexisting rocks...

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