"Landforms and Rock Types In The United States"

Igneous Rocks


1.A pegmatite is a holocrystalline, intrusive igneous rock composed of interlocking phaneritic crystals usually larger than 2.5 cm in size; such rocks are referred to aspegmatitic. Has a brownish and white speckled color.

Andesite: Andesite is an extrusive rock intermediate in composition between rhyolite and basalt. Andesite lava is of moderate viscosity and forms thick lava flows and domes. The word andesite is derived from the Andes Mountains in South America, where andesite is common. Andesite is the diorite.Colour - variable, but typically bluish-grey or grey (lighter coloured than basalt).

Texture - augite) and plagioclase, possibly with minor amounts of amphibole (hornblende) and glass; olivine or amphibole.

Basalt:Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a composition similar to gabbro.Colour - dark grey to black.

Texture - aphanitic (can be porphyritic).

Peridotite  Peridotite is a dark-colored igneous rock consisting mostly of olivine and pyroxene. Peridotite is a hugely important rock type because it is believed that the Earth’s mantle is predominantly composed of this rock type.Colour - generally dark greenish-grey.

Texture - phaneritic (coarse grained).

Diorite: is an intrusive rock intermediate in composition between gabbro and granite. It is produced in volcanic arcs, and in mountain building where it can occur in large volumes as batholiths in the roots of mountains (e.g. Scotland, Norway). Because it is commonly speckled black and white, it is often referred to as "salt and pepper" rock. Diorite is the andesite.Colour - typically speckled black and white.

Texture - phaneritic (medium to coarse grained).

Metamorphic Rocks

Phyllite: Phyllite is an intermediate-grade, foliated metamorphic rock type that resembles its sedimentary parent rock , shale, and its lower-grade metamorphic counterpart, slate . Like slate, phyllite can be distinguished from shale by its foliation, called slaty cleavage, and its brittleness, or fissility. Both slate and phyllite are generally dark-colored; their most common color is dark gray-blue, but dark red and green varieties also exist.is from scientific Latin and means "leaf-stone." It's typically a medium-gray or greenish stone, but here sunlight reflects off its finely wavy face.

Amphibolite: Amphibolite is a coarse-grained metamorphic rock that is composed mainly of green, brown or black amphibole minerals and plagioclasefeldspar. The amphiboles are usually members of the hornblendegroup. It can also contain minor amounts of other metamorphic minerals such as: biotite, epidote, garnet, wollastonite, andalusite,staurolite, kyanite and silimanite. Quartz, magnetite and calcite can also be present in small amounts.Color: Generally dark black, sometimes brown

Gneiss: Colour: Grey or pink but with dark streaks and layers.

Texture: Medium- to coarse grained. Characterized by discontinuous, altering light and dark layers, the former usually having a coarsely granular texture while the latter, which often contains mica, may be foliated.

Structure: In addition to the gneissose texture described above, gneisses tend to be banded on a large scale with layers and streaks of darker and lighter coloured gneiss. Granite and quartz veins and pegmatites are common. May be folded.

Feldspar: Feldspar is abundant and, together with quartz, forms the granular, lighter coloured layers. Muscovite, biotite and hornblende are commonly present, while any of the minerals characteristic of higher grades of regional metamorphism may occur.

Quartzite: Quartzite is a nonfoliated metamorphic rock that formed by the metamorphism of pure quartz sandstone. The intense heat and pressure of metamorphism causes the quartz grains to compact and become tightly intergrown with each other, resulting in very hard and dense quartzite. Quartzite is usually white or gray, but can be other light colors depending on the impurities in the parent sandstone. It has a glassy luster, as would be expected considering the quartz in sandstone has a vitreous or glassy luster.

Hornfels:Hornfels is a metamorphic rock formed by the contact between mudstone / igneous body, and represents a heat-altered equivalent of the original rock. This process is termed equigranular, mineral grains fitting closely together like the fragments of a mosaic or a rough pavement.Hornfels can often be identified without microscopic observation as it is typically non-descript (lacking distinctive features). Under the microscope, the hornfels structure maintains small, equigranular mineral grains that fit together like mosaic fragments. Hornfels maintain a relatively high hardness, and a colour that varies primarily from grey to black. It is also worth noting that hornfels are generally smooth to the touch.

Sedimentary Rocks

Breccia:Breccia is a term most often used for clastic sedimentary rocks that are composed of large angular fragments (over two millimeters in diameter). The spaces between the large angular fragments can be filled with a matrix of smaller particles or a mineral cement that binds the rock together.Breccia is a rock formed from angular gravel and boulder-sized calcite, silica or iron oxide. Texture is clastic. The color variable, dependent on clast and matrix composition.

minerals dominate.
Other features - rough to touch due to angular clasts.

Limestone: Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of the mineral calcite. It most commonly forms in clear, warm, shallow marine waters. It is usually an organic sedimentary rock that forms from the accumulation of shell, coral, algal and fecal debris. It can also be a chemical sedimentary rock formed by the precipitation of calcium carbonate from lake or ocean water.Texture - non-clastic.Grain size - variable, can consist of clasts of all sizes.Colour - variable, but generally light coloured, grey through yellow.

Chert:Chert is a microcrystalline or cryptocrystalline sedimentary rockmaterial composed of silicon dioxide (SiO2). It occurs as nodules, concretionary masses and as layered deposits. Chert breaks with a conchoidal fracture, often producing very sharp edges. Early people took advantage of how chert breaks and used it to fashion cutting tools and weapons. The name "flint" is also used for this material. Texture - non-clastic.

Grain size - cryptocrystalline, cannot be seen except under very high magnification.
Hardness - hard.
Colour - all colours, dependent on impurities present when precipitated.

Oil Shale: The term oil shale generally refers to any sedimentary rock that contains solid bituminous materials (called kerogen) that are released as petroleum-like liquids when the rock is heated in the chemical process of pyrolysis. Oil shale was formed millions of years ago by deposition of silt and organic debris on lake beds and sea bottoms. Over long periods of time, heat and pressure transformed the materials into oil shale in a process similar to the process that forms oil; however, the heat and pressure were not as great. Oil shale generally contains enough oil that it will burn without any additional processing, and it is known as "the rock that burns". It is dark black and sometime mistaken for coal.

Coal: Coal is a sedimentary rock of biochemical origin. It forms from accumulations of organic matter, likely along the edges of shallow seas and lakes or rivers. Flat swampy areas that are episodically flooded are the best candidates for coal formation. During non-flooding periods of time, thick accumulations of dead plant material pile up. As the water levels rise, the organic debris is covered by water, sand and soils. The water (often salty), sand and soils can prevent the decay and transport of the organic debris. If left alone, the buried organic debris begins to go through the coal series as more and more sand and silt accumulates above it. The compressed and/or heated organic debris begins driving off volatiles, leaving primarily carbon behind. The sand and soils form the rocks sandstone,shale, and if the soils are limey, limestone. Black and has a Rough texture. It's streak is black.