Geology Collection / Rock Types


There are three main classes of rocks: igneous, metamorphic, and sedimentary. The following is some general information about the different classes of rocks, including their texture, composition, and origin.


Igneous Rocks



Igneous rocks are minerals that have solidified (crystallized) from molten or partly molten material (lava or magma). These rocks are mostly silicate in composition and are formed deep in the Earth. Intrusive igneous rocks are formed by the process of emplacement of magma in pre-existing rock. Extrusive igneous rocks are rocks that have been erupted onto the surface of the earth. They include lava flows and pyroclastic material such as volcanic ash. The eight igneous rock-forming minerals make up over 90% of the earth's crust. Igneous rocks are classified according to texture (size and shape of mineral grains) and composition which leads to an understanding of the environment of formation (genesis).

Bowen's Reaction Series is one of the most powerful models we have for understanding igneous rocks. It can be used to help describe igneous rocks and is an extremely powerful tool for interpreting their origin.

Glassy extremely rapid cooling, noncrystalline
Vesicular extremely rapid cooling with rapid gas escape forming bubbles in the rock
Aphanitic fast cooling, microscopic crystal growth
Phaneritic or
very slow cooling, crystals grow to visible size, pegmatite crystals are >1 cm and can become meters in size
two-stage cooling, one slow underground (phenocrysts), the second rapid at the earth's surface (groundmass)
explosive release of gases shattering the lava and sometimes welding the fragments together

Olivine pale green, glassy
Pyroxene dark, dull greenish blocks
Amphibole shiny black elongate crystals
Biotite shiny black plates
Plagioclase (not mafic) gray, with striations
Sodic Plagioclase white/light colored with striations
Orthoclase pink/white
Muscovite white/brassy plates
Quartz clear/glassy

Mafic Dark
olivine, pyroxene, amphibole, calcic or intermediate plagioclase
Intermediate Intermediate
amphibole, intermediate plagioclase
Felsic Light
quartz, orthoclase, sodic plagioclase, biotite

Sedimentary Rocks



Sedimentary rocks which cover over two-thirds of the Earth's surface, are produced by the transformation of pre-existing rocks by gravity, atmospheric agencies, and living organisms. They are the result of the consolidation of sediments, loose material derived from the mechanical accumulation of fine and coarse fragments of rock (clastic sediments) or from precipitation from solution, with or without intervention by living organisms (organogenetic and chemical deposits).

Of the eight rock-forming minerals in Bowen's Reaction Series, only quartz does not weather. It is released from the rock by weathering but remains in the environment as sand-sized grains. In a simple model the remaining seven minerals weather into two new end products: clay, made of very small crystals stable at earth's surface, and calcite in solution. If weathering is complete, then only quartz, clay, and calcite remain. When solidified, the three kinds of sediment form three kinds of sedimentary rock: quartz sandstone, shale, and limestone. However, during the weathering process there are many transition processes and sediments.

Quartz Kaolinite Calcite
Magnetite Limonite Dolomite
Garnet Hematite Halite
Zircon Bauxite Gypsum
Rutile Chert

Mechanical Weathering - rocks are broken into smaller sizes, but the original mineral content does not change. Lithic fragments are initially angular but these become rounded during transportation. Eventually, most lithic fragments are chemically weathered too.

Chemical Weathering - all minerals, except quartz, decompose to new minerals stable at the earth's surface. These include clay, iron oxides, and a variety of minerals in solution. On Bowen's Reaction Series, the minerals at the top weather most quickly, those at the bottom, most slowly.

Large, heavy particles require more effort (greater water velocity) to transport than do small, light particles. In a stream,

  • minerals in solution leave first, even if the water is only a trickle and nothing else moves.

  • clay and iron oxides, the smallest sedimentary particles, move next, they are kept in suspension by gentle flow and travel almost as fast as the stream.

  • sand grains move in water of moderate energy by bouncing and rolling along the bottom.

  • larger mineral and lithic fragments finally begin to move only when water velocity is very high.

In a swift stream, if the water velocity slows even a little, the larger particles stop moving first and get left behind.

Rounding and Size Reduction
Angular sedimentary particles have not been transported very far from their source. Particles which are transported by running water have their sharp edges knocked off by abrasion with other particles and become ever more rounded.

Depositional Environment
Swift-flowing rivers with great turbulence transport larger particles easily and carry away the finer particles, producing a coarse grained sedimentary rock. Deeper, gentler-flowing water transports only the finer particles producing a shale. Depositional environments evolve in a systematic and predictable way downstream:

alluvial fan —> braided river —> meandering river —> delta/beach —> shelf —> submarine fan —> basin

Classification of Sedimentary Rocks

Sedimentary rocks are grouped together based on their method of origin: clastic, chemical, and biochemical.

Clastic Rocks are made of the undissolved weathering products of preexisting rocks. The weathering products are called clasts. The Wentworth Scale divides the clast sizes into clay, silt, sand, and gravel categories.

Chemical Rocks form when the weathering products of preexisting rocks go into solution as dissolved minerals. Certain kinds of limestone and dolomite are the most abundant chemical rocks. Halite and gypsum precipitate from highly concentrated seawater in arid climates with high evaporation. Chert usually forms from the recrystallization of some other form of silica, either skeletons of sponges or microorganisms, or volcanic ash.

Biochemical Rocks are formed form the skeletons of organisms. Most limestones are biochemical, although they intergrade with chemically formed limestone and dolomite. Examples are micrite (lime mud - clay sized calcite crystals) and chalk.


Metamorphic Rocks


Metamorphic rocks are derived from pre-existing rocks by mineralogical, chemical, and/or structural changes, essentially in the solid state, in response to marked changes in temperature, pressure, shearing stress, and chemical environment, generally at depth in the earth's crust. Metamorphic processes cause predictable changes in the preexisting (parent) rocks. A parent rock can be any preexisting rock, including igneous, sedimentary, and other metamorphic rocks. As a rock is buried deeper and deeper, both the pressure and temperature affecting it increase causing metamorphism. Temperature goes up because the core of the earth is hot enough to be molten and the closer a rock gets to it, the hotter it gets. Pressure goes up with burial because the deeper a rock is buried, the more overlying rock weighs down on it.

Geothermal Gradient is the rate of increase of temperature in the earth with depth. The gradient differs from place to place depending on the heat flow in the region and the thermal conductivity of the rocks. The average geothermal gradient approximates 25°C/km of depth. Not only does the temperature rise, but the pressure also increases and is measured in bars. A bar is about 1 atmosphere of pressure, which is near 14.5 psi at the earth's surface.

Metamorphic Grade is the intensity of metamorphism, measured by the degree of difference between the parent rock and the metamorphic rock. It indicates in a general way the P-T (Pressure-Temperature) environment or facies in which the metamorphism took place. For example, conversion of shale to slate or phyllite would be low-grade metamorphism, whereas its continues alteration to a garnet-sillimanite schist would be high-grade metamorphism.

Metamorphic Facies is a set of metamorphic rocks characterized by particular mineral associations, indicating origin under restricted temperature-pressure conditions.

  • Regional Metamorphism - steadily increasing temperature and pressure (greenschist, amphibolite, granulite facies)

  • Blueschist Facies Metamorphism - low-temperature, high-pressure

  • Eclogite Facies Metamorphism - high-temperature, very-high-pressure

  • Contact/Hydrothermal Facies Metamorphism - high-temperature, low-pressure, usually with hot, chemical-rich fluids

Rock Fabric is the spatial and geometrical configuration of all those components that make up a deformed rock, including texture, structure, and preferred orientation.

  • Granular - a term applied to a crystalline rock made up of grains of nearly the same size.

  • Foliation - as crystals grow, pressure arranges them so that all the flat surfaces line up forming foliation. This foliation causes the rock to break to break into perfectly flat, parallel layers. The foliation always forms at right angles to the stress, os the rock may have two sets of layers running through it, one a trace of the former bedding, the other the foliation.

  • Mineral Banding - a result of alternation of layers, stripes, flat lenses, or streaks differing in mineral composition.