What are Igneous, Sedimentary, & Metamorphic Rocks?

By Rebecca Hylland

What are igneous, sedimentary, and metamorphic rocks and their associated rock types? A rock is a rock, right? Not to geologists. To aid in their study of the earth, geologists group rocks into three categories based on their origin: igneous, sedimentary, and metamorphic. Each category is then further subdivided.

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 Rocks

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: (pronounced “nice”) 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 Rocks

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.

Glad You Asked article, Survey Notes, v. 28 no. 2, April 1996