Terms & Phrases
country rock

Rocks are naturally formed, consolidated material composed of grains of one or more minerals. Geologists group rocks into three categories depending on their origin: igneous, sedimentary and metamorphic.

Igneous rocks are formed from solidification of molten material. Sedimentary rocks are formed by the accumulation of fragmental material derived from preexisting rocks of any origin as well as the accumulation of organic material or precipitated material. Metamorphic rocks occur as a result of high pressure, high temperature and the chemical activity of fluids changing the texture and (or) mineralogy of preexisting rocks.


Rock Colors

Perhaps the most apparent feature of rocks to the observer is the coloration. Although most rocks have a rather drab appearance, some have very distinctive and, in some cases, beautiful colors. Shades of red, green, gray and brown may be caused by iron-bearing minerals. Very light-colored rocks are generally lacking in iron-bearing minerals. The coloration of sedimentary rocks reflects the environmental conditions that existed during deposition.


Purple and Red Rocks

Purple, maroon and red rocks are stained by the mineral hematite (iron oxide). Hematite results from the decomposition and oxidation of iron-rich minerals such as magnetite, ilmenite, biotite, hornblende and augite. A rock composed of only several percent hematite may be stained a deep red.


Green Rocks

Green sedimentary rocks are typically formed in a reducing environment where oxygen is not available. For sedimentary rocks, this would normally mean deposition in deeper water than red rocks. In a reducing environment, iron combines with silica compounds to form iron silicate minerals. Then lowgrade metamorphism will convert the iron silicates to the green mineral chlorite. Chlorite in sedimentary rocks indicates a deep-water depositional environment. Where chlorite-rich strata alternate with hematite-rich strata, a change in sea level probably occurred.


Black Rocks

Higher-grade metamorphism (high heat and pressure) will convert the hematite in red rocks and the chlorite in green rocks to the black minerals magnetite and biotite. An abundance of these minerals will yield a gray to dark gray mineral. Traces of black organic matter will also darken a rock to a gray or dark gray.


Weathered Surfaces

Many rocks have a different color on the weathered surface than on a fresh break. Weathering of disseminated pyrite (iron sulfide) in rocks will convert them to brown or yellow iron hydroxide and iron sulfate.



Sedimentary rocks are derived from preexisting igneous, sedimentary and metamorphic rocks. These rocks contain many clues as to their origin and the conditions that existed while they formed. Sedimentary rocks make up 75 percent of the rocks at the earth's surface but only 5 percent of the outer 10 miles of the earth. Sediment, as distinguished from sedimentary rock, is a collective name for loose, solid particles and is generally derived from weathering and erosion of preexisting rock. After formation, sediments are transported by rivers, ocean waves, glaciers, wind or landslides to a basin and deposited. Lithification is the process of converting loose sediment into sedimentary rock and includes the process of cementation, compaction and crystallization.

Sedimentary rock is formed by lithification of sediments, precipitation from solution and consolidation of the remains of plants or animals. Coal is an example of sedimentary rock formed from the compression of plant remains.


Rounding of
Rock Particles

Rounding occurs during the transportation process by one or more of the erosional agents. Current and wave action in water are particularly effective in causing particles to hit and scrape against one another or a rock surface. The larger the particle the less distance it needs to travel to become rounded. For example, the boulders of the melon gravel deposited by the Bonneville flood were rounded after 3 to 6 miles of transportation.


Deposition of Sediment

Sorting of sediment by size is also effectively accomplished by moving water. A river sorts sediment by first depositing cobbles, then pebbles, sand, silt and clay. The larger the size of sediment, the greater the river's energy necessary to transport it. Deposition is the term used to describe the settling of transported sediment.

Look at a diagram that
explains rounding and sorting.


Lithification of
Clastic Rock

Clastic or detrital sedimentary rock is composed of fragments of preexisting rock. The grains are generally rounded and sorted during the transportation process. Clastic sediment is generally lithified by cementation. Cementation occurs when material is chemically precipitated in the open spaces of the sediment so as to bind the grains together into a hard rock. Common cements include calcite, silica and iron oxides. A matrix of finer-grained sediments may also partly fill the pore space.


Common Types of Sedimentary Rock

Conglomerate is the coarsest-grained sedimentary rock formed by the cementation of gravel-sized sediments. The gravel is generally rounded; however, it probably did not travel very far. Conglomerates are generally deposited by a river.

Sandstone is a medium-grained sedimentary rock formed by the cementation of sand-sized sediments, with silt and clay forming the matrix. Sandstones may be deposited by rivers, wind, waves or ocean currents.

Shale is a fine-grained sedimentary rock composed of clay- and silt sized fragments. Shale's noted for its thin laminations parallel to the bedding. Compaction is very important in the lithification of shales. Before compaction, shale may consist of up to 80 percent water in the pore spaces.

Chemical Sedimentary Rocks are formed by material precipitated from solution. Examples include rock salt, gypsum and limestone.

Organic Sedimentary Rocks consist mostly of the remains of plants and animals. Coal is an organic rock formed from compressed plant remains.

Limestone is a sedimentary rock composed of mostly calcite. Some limestones are chemical precipitates, whereas others consist mostly of elastic grains of calcite or shells of marine invertebrates. The calcite grains in limestone recrystallize readily so as to form new and larger crystals.


Sedimentary Structures

Sedimentary Structures in sedimentary rock are formed either during the deposition process or shortly after deposition. One of the most important structures is bedding. An important principle of geology holds that sedimentary rocks are deposited in horizontal layers. The bedding plane is the nearly flat surface separating two beds of rock. Bedding planes originate by a change in grain size, a change in grain composition or a pause in deposition during the depositional process.

Mud Cracks are sedimentary structures that are abundant in many of the formations of the Belt Supergroup as well as in many Paleozoic marine sedimentary formations in Idaho. Mud cracks are polygonal cracks formed in clay- and silt-sized sediments. They are caused by the exposure of lake bottoms, river bottoms and tidal flats to the sun after being beneath water. The cracks are caused by the sun drying and shrinking the upper several inches of the exposed mud flat.

Ripple marks are small ridges, generally less than one inch high and 2 to 8 inches wide. The ridges are developed by moving water and form perpendicular to the direction of water movement. If the profiles of the ripple marks are symmetrical, they are caused by waves; if the profiles are asymmetrical, they are caused by currents. The steep sides occur in the down-current direction.


Sedimentary Rocks
of Idaho

Sedimentary rocks of Idaho were generally deposited in marine environments; however, a significant part are of continental origin. Most Precambrian and Paleozoic strata are marine; Mesozoic strata include both marine and continental deposits; all Cenozoic formations are continental. Marine deposits are noted for being thick and distributed over a large area. Most of the marine rocks in Idaho were deposited on the continental shelf and slope that slowly subsided over hundreds of millions of years. Even though a pile of sediments more than 50,000 feet thick may form, most of the material is deposited in shallow water such as in the intertidal zone.




Igneous rocks are those rocks that have solidified from an original molten state. Temperatures within the earth are so hot that many rocks and minerals are able to exist in a molten condition called magma. This molten rock exists deep below the earth's surface in large pools called magma chambers. Many magmas or portions of magmas are lighter than the surrounding rock and tend to rise toward the surface of the crust; also, the high pressure at depth facilitates the upward movement of magma, Molten materials that extrude through the surface of the earth are called eruptive, extrusive or volcanic rocks. Those magmas that crystallize and solidify at depth, never reaching the earth's surface before consolidation, are called intrusives or plutonic rocks. Of course after consolidation, plutonic rocks may be exposed at the earth's surface by the process of erosion.

Look at Bowen's Reaction Series.

The crystal size of igneous rocks is very diagnostic of their origin. Volcanic or extrusive rocks have a very small average grain size which is generally too small to discern with the naked eye. Extrusive rock has a very high component of glass because it was quickly frozen from the molten stage before crystals had time to grow. The more deeply-buried plutons cool more slowly and develop a coarse texture composed of large crystals. Therefore, large mineral crystals of more than one inch in diameter indicate formation at a depth of 6 to 12 miles.

Look at chart of Mineral Proportions.

Mafic and Felsic Magmas

Magmas are thought to be generated in the outer 60 to 180 miles of the earth where temperatures are hot enough to cause melting. Magmas rich in magnesium, iron and calcium are called mafic. Those rich in sodium, potassium and silicon are called felsic. Those that are transitional between mafic and felsic are called intermediate. Felsic magmas are generated mostly within the continental crustal regions where the source of parent rocks are abundant- whereas, mafic magmas may be derived from parent materials rich in magnesium, iron and calcium which occur beneath the crust. Mafic magmas, coming from a deep hot source, are about 1,200 degrees centigrade when they reach the earth's surface; whereas, felsic magmas are much cooler - about 700 degrees centigrade upon reaching the earth's surface.


Origin of Basalt

Most basalt originates at spreading centers such as the mid-oceanic ridge system. Basalt magma originates from partial melting of mantle material. The fluid magma rises through fissures formed by tensional forces of two diverging plates.


Origin of Andesite
and Granite

Intermediate and felsic magma in Idaho are believed to have originated where a cool slab of oceanic lithosphere of basalt and overlying sedimentary rock descended beneath the continental crust of the western United States. The descending plate of lithosphere becomes hotter with increasing depth. Water trapped in the descending plate also lowers the melting temperature so that partial melting of basalt takes place. While the basaltic magma rises through the overriding continental crust, the magma absorbs some of the more silica rich rocks to become intermediate in composition. Also, the very hot basaltic magma chambers in the continental crust could melt the surrounding felsic rocks and create granitic magmas.


Emplacement of Magma

Bodies of intrusive rocks exist in almost every shape and size. Regardless of shape or size, they all come under the general term pluton. Most of them appear to be emplaced in the surrounding country rocks (host rocks) by the process of forceful injection. By forceful injection, the body is intruded along zones of weakness, such as fractures, by pushing apart the surrounding rock. A pluton is also emplaced by melting rock around it and prying out blocks of the country rock. The surface between the pluton and the country rock is the intrusive contact. Magma is also aided in its upward movement because it is generally less dense than the surrounding rock. When the magma stops moving it begins to crystallize. Those plutons that reach shallow to intermediate depths tend to be porphyritic, that is, large crystals are contained in a finer crystalline groundmass.


Types of Plutons

Dikes are small tabular plutons which cut across layering in the host rocks. Dikes may range from one inch to tens of feet thick. They are much longer than wide and can commonly be traced a mile or more. Dikes are generally intruded along fractures and tend to have the composition of pegmatite, aplite (white, sugar-textured dikes) and basalt. In almost every roadcut through the Idaho Batholith of central Idaho, aplitic and pegmatitic dikes can be seen.

Sills are also tabular bodies of the same approximate size and shape range as dikes. However, sills are concordant or parallel to the layers of the surrounding host or country rock. The Purcell sills are examples of such plutons in northern Idaho.

The largest plutons consist of granite and diorite and are found in the cores of mountain ranges. The Idaho Batholith is a good example. A batholith is defined as a pluton with a surface exposure in excess of 40 square miles. If the exposure is less than that, the pluton is called a stock. It is commonly believed that buried batholiths underlie large areas of widespread silicic volcanics in Idaho. Many of the large batholiths such as the Idaho Batholith are known to be a composite of many granitic plutons.



Pegmatite bodies have a relatively larger grain size than the surrounding igneous rocks. Individual crystals are known to reach more than 30 feet in length. A pegmatite may have the composition of a granite, diorite or gabbro. All three types are exposed in the large granitic plutons of Idaho. However the granitic pegmatites are by far the most common. In practically every exposure of granitic rock in the state, there are one or more granitic pegmatite dikes exposed. Although most of these pegmatites do not exceed 10 feet in thickness; an uncommonly large pegmatite more than 300 feet along its smallest dimension is exposed in the City of Rocks near the town of Oakley.

The extremely large crystal size (generally 2 to 8 inches), is attributed to both slow cooling and low liquid viscosity. Pegmatites are the last portion of a pluton to crystallize. These residual fluids are much richer in certain elements than the original magma. High amounts of silica and ions of elements that are necessary to crystallize sodium plagioclase and potassium feldspar must be abundant in the fluids. The fluids are also rich in certain elements that could not be used in the crystal structure of the previously crystallized minerals. Water is also very abundant which promotes slower cooling and a lower temperature of crystallization. Many pegmatites were intruded along existing fractures.

Most Idaho pegmatites are composed of orthoclase feldspar, quartz and muscovite. Careful inspection will also reveal small red garnets, black tourmaline and bluish-green aquamarine. Aquamarine is generally only found in the tertiary plutons.


Common Igneous Rocks

Igneous rocks are classified on the basis of their texture and composition. Although more than several hundred names have been given to igneous rocks, only a few major divisions are discussed below.

Granite is the most common coarse-textured rock. It is formed at great depths within the earth and has crystals ranging from microscopic to more than one inch in size. Granite typically contains quartz, feldspar, mica and hornblende. Granites are generally light in color and may have a salt and pepper appearance. The feldspar may cause it to be white, gray, pink or yellowish brown. Most of the large bodies of plutonic rocks in Idaho have typical granitic texture and composition. Potassium feldspar and plagioclase feldspar make up most of the rock, though quartz may represent up to 25 percent of the bulk composition. The black minerals are commonly hornblende and biotite mica. Muscovite is also common in some granites.

Gabbro is a dark, coarse-grained igneous rock. It is generally composed of plagioclase feldspar and augite. Gabbro is generally dark green or dark gray in color. Idaho has relatively little gabbro compared to granite.

Pumice is lava that solidified while gases were released from it. It is essentially a frozen volcanic froth. Because of the abundance of gas cavities, pumice is so light in weight that it can float in water. Pumice is generally light gray or tan and has the same chemical composition as obsidian, rhyolite and granite.

Diorite is a coarse- to fine-grained plutonic rock and has a mineral composition that places it midway between granite and gabbro. It has little quartz or potassium feldspar. Diorite tends to be a gray rock due to the high amounts of plagioclase feldspar and iron-rich minerals.

Andesite is much finer grained than diorite but has the same mineral composition. Andesites are more common than rhyolites, but less common than basalts.

Rhyolite is a volcanic rock with the same composition as granite. The major difference is its fine-grain size or glassy texture. Rhyolite is generally light colored and may be gray, white, tan or various shades of red. It has a characteristic streaked texture called flow banding. Flow banding is caused by slow flowage of highly viscous lava.

Obsidian forms when magma of a rhyolitic composition cools so fast that crystallization of the minerals is not possible. Thus volcanic glass is essentially a frozen liquid. It is a lustrous, glassy black or reddish black rock. Obsidian has a conchoidal fracture giving it very sharp edges. Because of this property, it was commonly used to make tools and weapons by early man. One of the best-known obsidian flows occurs at Obsidian Cliffs in Yellowstone National Park.

Basalt is the fine-grained compositional equivalent of gabbro. It is by far the most abundant volcanic rock. For example, the volume of basalt in the Columbia Plateau is estimated to be 74,000 cubic miles. Basalt is normally coal black to dark gray when not weathered. Common constituent minerals include pyroxene, calcic plagioclase and olivine. Basalt commonly has small cavities called vesicles. Basalt flows are characterized by columnar Jointing which causes polygonal vertical columns that look like giant fence posts stacked on end. Most of the large basalt flows are extruded from large fissures in the earth's crust. Basalts are very common throughout Idaho, especially western and southern Idaho.

Look at an example of
Basalt of Portneuf Valley.

Look at an example of granodiorite.


Porphyritic Texture

Some fine-grained rocks such as basalt, rhyolite and, most commonly, andesite have a mixed texture of large and small grains. This texture is called porphyritic and is characterized by large crystals called phenocrysts surrounded by a groundmass (background) of smaller crystals.


Pyroclastic Rocks

In addition to the fluid lava extruded from a volcano, a great amount of lava is blown out the vent by violent gas explosions. All material driven out explosively is called pyroclastic. Large fragments such as spindle-shaped volcanic bombs fall near the vent. However, the dust-size fragments called ash can be carried hundreds of miles by prevailing winds. Volcanic ash is composed of fragments of volcanic glass and small crystals. When air-fall ash deposits consolidate, they are called ash-fall tuffs. Excellent examples of most of these volcanic products can be observed at Craters of the Moon National Monument.

One type of pyroclastic rock very common in southern and east-central Idaho is the welded ashflow tuff. This material consists of a very hot mixture of fragments of pumice, cinders, crystals and glass shards, many of which are more than one inch in size. They flow out of the vent and downslope somewhat like a lava flow, but riding on a cushion of hot gases. When the deposit settles and comes together, the angular fragments are so hot they weld together. Unlike rhyolite flows, a single ash flow tuff unit may extend up to 100 miles. These tuffs make distinctive rim formers above the lake-bed deposits in the Snake River Plain.


Volcanic Cones

Volcanoes are vents in the earth's crust through which molten rock and other volcanic products are extruded. There are three types of volcanic cones: cinder cones, shield volcanoes (lava domes), and composite cones (stratovolcanoes). All three types are common in southern Idaho.

Cinder cones are formed entirely of pyroclastic material, mostly of cinders. These cones consist of a succession of steeply-inclined layers of reddened scoriaceous cinders around a central crater. They are generally less than 1,000 feet in height and are susceptible to erosion because there is generally nothing holding the mass together. This type of cone has the steepest flanks of the three types of volcanic cones. Hundreds of cinder cones are distributed throughout the Snake River Plain, generally aligned along fractures in the crust. These cones disrupt the otherwise flat, featureless plain.

Shield volcanoes are built almost entirely of basaltic lava flows. They have gently-rounded profiles with a circular outline. This type of cone is the most stable and least susceptible to erosion.

Composite or stratovolcanoes are composed of alternating sheets of lava and pyroclastic material. these volcanic mountains are cone shaped and may be as much as 12,000 feet high. The alternating pyroclastic layers and lava layers indicate that the pyroclastic material was produced during periods of explosive activity, whereas the lava eruptions occurred at times of quiescence.

Caldera, are nearly circular basin-shaped depressions in the upper part of volcanoes. They are much larger than craters and are generally more than 6 miles in diameter. There are two types: explosive calderas and collapse or subsidence calderas. Most of those in Idaho are thought to have formed by collapse caused by the sudden withdrawal of supporting lava. Such calderas are common in southern and east-central Idaho.


Metamorphic rocks are those that have transformed from preexisting rock into texturally or mineralogically-distinct new rocks by high temperature, high pressure or chemically-active fluids. One or more of these agents may induce the textural or mineralogical changes. For example, minute clay minerals may change into coarse mica. Heat is probably the most important single agent of metamorphism. Metamorphism occurs within a temperature range of 100 to 800 degrees centigrade. Heat weakens bonds and accelerates the rate of chemical reactions. Two common sources of heat include friction from movement and intrusion of plutons. Pressure changes are caused primarily by the weight of overlying rock. Where there are more than 30,000 feet of overlying rock, pressures of more than 40,000 psi will cause rocks to flow as a plastic. Pressure may also be caused by plate collision and the forceful intrusion of plutons.

Chemically-active fluids (hot water solutions) associated with magma may react with surrounding rocks to cause chemical change. Directed pressure is pressure applied unequally on the surface of a body and may be applied by compression or shearing. Directed pressure changes the texture of a metamorphic rock by forcing the elongate and platy minerals to become parallel to each other. Foliation is the parallel alignment of textural and structural features of a rock. Mica is the most common mineral to be aligned by directed pressure.


Types of Metamorphism

There are two types of metamorphism: contact metamorphism and regional metamorphism. Contact metamorphism is the name given when country rock is intruded by a pluton (body of magma). Changes to the surrounding rocks occur as a result of penetration by the magmatic fluids and heat from the intrusion. Contact metamorphism may greatly alter the texture of the rock by forming new and larger crystals. In contact metamorphism, directed pressure is not involved so the metamorphosed rocks are not foliated.


Regional Metamorphism

Most metamorphic rocks are caused by regional metamorphism. This type of metamorphism is caused by high temperature and directed pressure. These rocks are typically formed in the cores of mountain ranges, but may be later exposed at the surface by erosion. Typical rock types include foliated rocks such as slates, phyllites, schists and gneisses.


Metamorphic Rocks

Marble is a coarse-grained rock consisting of interlocking calcite crystals. Limestone recrystallizes during metamorphism into marble.

Quartzite forms by recrystallization of quartz-rich sandstone in response to heat and pressure. As the grains of quartz grow, the boundaries become tight and interlocking. All pore space is squeezed out; and when the rock is broken, it breaks across the grains. Quartzite is the most durable construction mineral. Although both marble and quartzite may be white to light gray, they may be readily distinguished because marble fizzes on contact with dilute hydrochloric acid, whereas quartzite does not. Also, marble can be scratched with a knife, whereas quartzite cannot.

Slate is a low-grade metamorphic equivalent of shale. It is a fine-grained rock that splits easily along flat, parallel planes. Shale, the parent rock, is composed of submicroscopic, platy clay minerals. These clay minerals are realigned by metamorphism so as to create a slaty cleavage. In slate, the individual minerals are too small to be visible with the naked eye.

Phyllite is formed by further increase in temperature and pressure on a slate. The mica grains increase slightly in size but are still microscopic. The planes of parting have surfaces lined with fine-grained mica that give the rock a silky sheen.

A schist is characterized by coarse-grained minerals with parallel alignment. These platy minerals, generally micas, are visible to the naked eve. A schist is a high-grade, metamorphic rock and may consist entirely of coarse, platy minerals.

A gneiss is a rock consisting of alternating bands of light and dark minerals. Generally the dark layers are composed of platy or elongate minerals such as biotite mica or amphibolite. The light layers typically consist of quartz and feldspar. A gneiss is formed under the highest temperatures and pressures which cause the minerals to segregate into layers. In fact, slightly higher temperatures than necessary to convert the rock into a gneiss would cause the rock to melt. If temperatures become sufficiently high, the rock begins to melt and magma is squeezed out into layers within the foliating planes of the solid rock. The resulting rock is called a migmatite - a mixed, igneous and metamorphic rock.



Orbicular rocks are ellipsoidal-shaped masses of rock consisting of successive shells of dark minerals (biotite) and light minerals (feldspar). The occurrence of orbicular rocks is a rare phenomenon. There are fewer than 200 known localities throughout the earth. The State of Idaho happens to have at least three of these localities:

(1) one in the Buffalo Hump area in central Idaho
(look at explanation),

(2) one in southwest Idaho near Banks and

(3) one near Shoup in east-central Idaho. The orbicular rocks near Shoup crop out for about 2 km along the south side of the Salmon River.


Orbicular Rocks
Near Shoup, Idaho

The shape of the intrusion containing the orbicular rocks is very irregular. Along its periphery, numerous dikes of quartz diorite interfinger and discordantly penetrate the augen gneiss country rock.

Evidence in the field is persuasive for a dynamic emplacement of the intrusion. More than 50 percent of the total volume of the intrusion consists of angular xenoliths, xenocrysts and autoliths in a medium-grained, quartz diorite matrix. In other parts of the intrusion, the quartz diorite matrix represents up to 90 percent of the rock volume. Primary-flow foliation and schlieren tend to give the intrusion a gneissic appearance.

Breccia fragments are primarily xenoliths of augen gneiss, quartzite, biotite gneiss and biotite schist. Some of the xenoliths may have been transported a long distance because they are dissimilar to the enclosing rock types. The size range of the xenoliths is variable, with some blocks of augen gneiss almost 100 m in diameter.

The orbicules, which occur in clusters, were formed by the crystallization of alternate layers of plagioclase and biotite around the nucleus. However, in some cases nucleation occurs around xenocrysts and autoliths. Typically, the orbicules have a nucleus of coarse-grained biotite schist. These biotite schist xenoliths probably were brought up from deep in the crust because they are different from any rocks in the area. Although most xenoliths are mantled by at least one layer of plagioclase, many of the large angular xenoliths have several shells of biotite and plagioclase. The single plagioclase mantles are found on xenoliths of all rock types. Orbicules have up to 10 shells of plagioclase with each shell 3 mm to 1 cm thick (look at explanation).

The individual orbicules generally have a sharp contact with the surrounding matrix. For the most part, the external shape of the orbicules depends on the shape of the xenolithic nucleus. Shapes vary from spherical to ellipsoidal masses. Fragments of orbicule shells indicate that some orbicules may have been brittle at the time of emplacement; however, other orbicules were apparently deformed in a ductile condition as they were blasted against the host rock.

 Source Information 


Supplemental Material

The field guides below are a part of the Guidebook to the Geology of Eastern Idaho :  

 The Idaho Overview modules were created by Digital Atlas staff members 
 Jacqueline Harvey, Vita Taube and Diana Boyack. Please E-MAIL the Digital Atlas 
 with any comments, concerns, suggestions or corrections.