Rocks and its Types

In geology, rock is a naturally occurring solid aggregate of one or more minerals or mineraloids. For example, the common rock granite is a combination of the quartz, feldspar and biotite minerals. The Earth's outer solid layer, the lithosphere, is made of rock.

Igneous Rocks

Igneous rocks are those that solidify from a molten or partially molten state. These rocks are characterized as either extrusive or intrusive.

Extrusive Rocks
Extrusive igneous rocks solidify from molten material that flows over the earth’s surface (lava). Extrusive igneous rocks typically have a fine-grained texture (individual minerals are not visible unless magnified) because the lava cools rapidly when exposed to the atmosphere, preventing crystal growth. Common extrusive rocks are basalt, andesite, and rhyolite.

Basalt: Basalt is characteristically a dense, black, massive rock, high in calcium and iron-magnesium- bearing minerals and low in quartz content. Great examples of basaltic lava flows can be found in the Black Rock Desert, Millard County.

Andesite: Andesite has a higher quartz content than basalt and is usually lighter in color. Crystals of the minerals amphibole, biotite, and feldspar are sometimes visible without magnification. In Utah andesite can be seen at Signal Peak in the Tushar Mountains, Piute County.

Rhyolite: Rhyolite is typically a fine-grained, white, pink, or gray rock, high in quartz and feldspar content with some amphibole and biotite. A well-known example is the Topaz Mountain rhyolite in the Thomas Range, Juab County.

Intrusive Rocks
Intrusive rocks form from molten material (magma) that flows and solidifies underground. These rocks usually have a coarse texture (individual minerals are visible without magnification), because the magma cools slowly underground, allowing crystal growth. Common rock types within the intrusive category are granite and diorite.

Granite: Granite is the intrusive equivalent of rhyolite but has a coarser texture. A 12-square-mile outcrop of granite is visible on the southwestern flank of the Sheeprock Mountains, Tooele and Juab Counties.

Diorite: Diorite has the same texture as granite but has the mineral composition of an andesite, which is diorite’s extrusive equivalent. Diorite forms the summits of Haystack Mountain, Mt. Tomasaki, Mt. Mellenthin, and Mt. Tuckuhnikivatz in the La Sal Mountains, Grand and San Juan Counties.

Sedimentary Rocks

Erosion and deposition play a key part in the formation of sedimentary rocks. Wind, water, ice, and chemicals break down existing rock into sediment that is then transported and deposited by wind, water, and glaciers.

As sediment accumulates with time (thousands of years) it becomes compacted and cemented (lithified), eventually forming rock. Over a period spanning hundreds of millions of years, oceans, rivers, and great deserts covered Utah and deposited the sediment that has lithified into the sedimentary rocks we see today. Some common sedimentary rocks are shale, sandstone, limestone, and conglomerate.

Shale: Shale is lithified clay and consists of layers that typically break into thin sheets. A well-known shale formation is the Wheeler Shale of the House Range, Millard County, which contains numerous Cambrian Period (500 to 570 million years ago) trilobite fossils that are found by splitting the shale along its layers. Another shale formation is the widespread Mancos Shale, visible along Utah State Highway 6 between Price and Interstate 70, Carbon and Emery Counties.

Sandstone: Sandstone is composed of cemented sand grains and is the cliff-forming rock commonly seen in southern Utah. Two famous formations are the Entrada Sandstone visible in Arches National Park, and the Navajo Sandstone which forms Checkerboard Mesa in Zion National Park.

Limestone: Limestone is composed of more than 50% calcium carbonate (calcite). The remainder of the rock may contain fine rock fragments, clay, quartz, and seashells. A limestone that is readily visible is the Bridal Veil Member of the Oquirrh Formation, at Bridal Veil Falls, Utah County. The Twin Creek Limestone can be viewed at the cement quarry near the mouth of Parley’s Canyon, Salt Lake County.

Conglomerate: Conglomerate is well-rounded gravel in a matrix of sand, clay, and natural cementing agents. Two of the many conglomerates in Utah are the Price River Formation visible along Highway 6 between Thistle and Soldier Summit, Utah County, and the Shinarump Conglomerate Member of the Chinle Formation exposed along the central part of the Burr Trail, east of Boulder, Garfield County.

Metamorphic Rocks

Metamorphic rocks are any rock type that has been altered by heat, pressure, and/or the chemical action of fluids and gases. Metamorphic rocks are classified by their structure and their dominant minerals. Metamorphic rock structure is either foliated (has a definite planar structure) or nonfoliated (massive, without structure).

Foliated Metamorphic RocksCommon foliated metamorphic rocks are slate, phyllite, schist, and gneiss (pronounced “nice”).

Slate: Slate is fine grained, dense, and brittle and is a metamorphosed form of shale. Slate can be seen west of Patters spring in the Pilot Range, Box Elder County.

Phyllite: Phyllite is similar to slate but has a satin-like sheen on its foliation planes. Phyllite can be seen on the north and south flanks of Grizzly Peak east of Willard, Box Elder County.

Schist: Schist has the same satin-like sheen as phyllite but has a coarse texture due to its high mica (muscovite or biotite) content. The Little Willow Formation is a schist that is visible on the north side of the mouth of Little Cottonwood Canyon, Salt Lake County.

Gneiss: Gneiss is a high-grade (high heat and pressure) metamorphic rock in which the foliation results from a layering of different mineral groups, which give this rock a banded look of dark (mica, amphibole, and other iron-magnesium minerals) and light (quartz and feldspar) minerals. Good examples of gneiss are visible in the Farmington Canyon Complex in Farmington Canyon, and at Fray Peak on Antelope Island, both in Davis County.

Nonfoliated Metamorphic RocksCommon nonfoliated metamorphic rocks are quartzite and marble.

Quartzite: Quartzite is typically a metamorphosed form of sandstone. Unweathered quartzite has a “sugary” looking surface. Individual quartz grains are deformed, interlocked, and fused together. When the rock breaks, it typically breaks through the grains. Some quartzite formations retain their original bedded (layered) structure such that when broken they form flagstones that are commonly used in landscaping or as veneer for buildings. Quartzite is quarried from the Raft River and Grouse Mountains for use as building stone. It can also be seen at Storm Mountain in Big Cottonwood Canyon, Salt Lake County.

Marble: Marble is metamorphosed limestone. The calcite crystals in marble are large and interlocking, forming a dense crystalline rock. Marble of the Deseret Limestone and Gardison Formation can be seen in Big Cottonwood Canyon, Salt Lake County.

The Rock cycle

When rocks are pushed deep under the Earth's surface, they may melt into magma. If the conditions no longer exist for the magma to stay in its liquid state, it will cool and solidify into an igneous rock. A rock that cools within the Earth is calledintrusive or plutonic and will cool very slowly, producing a coarse-grained texture. As a result of volcanic activity, magma (which is called lava when it reaches Earth's surface) may cool very rapidly while being on the Earth's surface exposed to theatmosphere and are called extrusive or volcanic rocks. These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass, such as obsidian. Any of the three main types of rocks (igneous, sedimentary, and metamorphic rocks) can melt into magma and cool into igneous rocks.

Transition to igneous

Secondary changes

Epigenetic change (secondary processes) may be arranged under a number of headings, each of which is typical of a group of rocks or rock-forming minerals, though usually more than one of these alterations will be found in progress in the same rock. Silicification, the replacement of the minerals by crystalline or crypto-crystalline silica, is most common in felsic rocks, such as rhyolite, but is also found in serpentine, etc. Kaolinization is the decomposition of the feldspars, which are the most common minerals in igneous rocks, into kaolin (along with quartz and other clay minerals); it is best shown by granites and syenites. Serpentinization is the alteration of olivine toserpentine (with magnetite); it is typical of peridotites, but occurs in most of the mafic rocks. In uralitization secondary hornblende replaces augite; this occurs very generally indiabases; chloritization is the alteration of augite (biotite or hornblende) to chlorite, and is seen in many diabases, diorites and greenstones. Epidotization occurs also in rocks of this group, and consists in the development of epidote from biotite, hornblende, augite or plagioclase feldspar.

Transition to metamorphic

Rocks exposed to high temperatures and pressures can be changed physically or chemically to form a different rock, called metamorphic. Regional metamorphism refers to the effects on large masses of rocks over a wide area, typically associated with mountain building events within orogenic belts. These rocks commonly exhibit distinct bands of differing mineralogy and colors, called foliation. Another main type of metamorphism is caused when a body of rock comes into contact with an igneous intrusion that heats up this surrounding country rock. Thiscontact metamorphism results in a rock that is altered and re-crystallized by the extreme heat of the magma and/or by the addition of fluids from the magma that add chemicals to the surrounding rock (metasomatism). Any pre-existing type of rock can be modified by the processes of metamorphism.

Transition to sedimentary

Rocks exposed to the atmosphere are variably unstable and subject to the processes of weathering and erosion. Weathering and erosion break the original rock down into smaller fragments and carry away dissolved material. This fragmented material accumulates and is buried by additional material. While an individual grain of sand is still a member of the class of rock it was formed from, a rock made up of such grains fused together is sedimentary. Sedimentary rocks can be formed from the lithification of these buried smaller fragments (clastic sedimentary rock), the accumulation and lithification of material generated by living organisms (biogenic sedimentary rock - fossils), or lithification of chemically precipitated material from a mineral bearing solution due toevaporation (precipitate sedimentary rock). Clastic rocks can be formed from fragments broken apart from larger rocks of any type, due to processes such as erosion or from organic material, like plant remains. Biogenic and precipitate rocks form from the deposition of minerals from chemicals dissolved from all other rock types.