Rocks and Minerals
In studying environmental geology the use of rocks and minerals and their affect on human populations are more important then the study of the rock or mineral itself.
Mineral: a naturally occurring inorganic substance with a range of chemical composition arranged in a definite crystalline configuration of regular repeating pattern of atoms. Properties of minerals are the result of the crystalline structure.
Hardness
Crystal form
Cleavage
Color impurities
Streak
Other
Classes of minerals:
Silicates: have silica in their formula, they make up the majority of the rock forming minerals. Arranged in different patterns silicates contain silica and other elements:
Quartz and quartz varieties: composed of mostly pure silica. Include many stones used in jewelry such as amethyst and opal.
Feldspars: contain sodium, aluminum and potassium as well as silica.
Ferromagnesium (ferromags) minerals: rich in iron and magnesium as well as silica.
Nonsilicates: do not contain silica in their formula.
Carbonates: composed predominantly of calcium carbonate, dissolve easily.
Sulfates: contain SO4 in their formula, often associated with mineral ores. These minerals often cause problems for the environment.
Native elements: include gold, silver, copper, lead. Important as mineral resources, discussed later in chapter 12.
Gems: rarity and beauty important.
Diamonds: found associated with volcanic areas. Most common in South Africa, Australia, South America and India. Some have been found in Arkansas but these are not commercially important.
Emeralds: most valuable gem carat for carat.
Case Study: Asbestos and other toxic Earth materials
Rocks:
Rocks form from the combination of two or more minerals. Three rock classes exist: Igneous, Metamorphic and Sedimentary.
1) Igneous Rocks: form from the solidification of a melt (lava/magma). Cooling rate dictates rock that forms. In general igneous rocks have little to no pore space and are physically strong. Used for building stone and decorative facings. Most common igneous rocks are basalt and granite.
2) Metamorphic Rocks: form from alteration of any type of rock from heat or pressure or both. During metamorphism mineral components do not melt but can recombine to form larger mineral crystals, interlocking crystals or new mineral crystals. Also minerals within a rock can align parallel to each other as foliation.
Contact metamorphism: forms from the presence of a magma nearby. Often associated with important resource minerals.
Regional metamorphism: forms from pressure due to large scale continental movements. Often these rocks are folded and can contain rock cleavage planes.
Metamorphic rocks can be physically strong but can also contain rock cleavage planes that act as planes of weakness. The typically do not have pore space. Often used for decorative purposes.
3) Sedimentary Rocks: any rock will undergo weathering when exposed at the surface. The result of weathering is sediment.
Sediment: the small bit and pieces of rocks and minerals that result from weathering. Varies in size from clay to boulders.
Sedimentary rocks form at or near the surface from the weathering products of other rocks.
Lithification: the formation of sedimentary rocks through compaction/dessication, cementation or chemical precipitation.
Sedimentary rocks contain fossils and other important information about the Earth's history.
Sedimentary rocks often contain important economic resources such as oil, gas, water and coal.
Rock Cycle: the representation of rock formation through time and Earth processes.
In environmental geology there is not as much emphasis on the rocks and minerals themselves as in the properties that make them useful as a resource or hazardous and problematic to the environment.
Porosity: the total amount of "empty" space in a rock
Permeability: the ability for a fluid to flow through the pore spaces of a rock.
Porosity and permeability are important in the study of groundwater (use and pollution), oil and gas.
Strength of rocks is a factor of stress and strain.
Stress: the force being applied to a rock. Much rock movement and stress is very slow and continuous.
Compressive: squeezing
Tensile: pulling apart
Shearing: movement in directions opposite to one another.
Rock stress is often transferred to the layers within the rock.
Strain: deformation that results from stress. Can be either permanent or temporary.
Elastic Strain: material returns to its original shape after stress is removed. Elastic deformation in rocks and rock units not readily apparent and often on the order of thousands to millions of years.
Plastic Strain: material deforms permanently without cracking or breaking. Especially common with metamorphic rocks, the rocks appear to have been folded or look like they flowed.
Fracturing: brittle material that breaks under stress.
Most rocks can be either ductile or brittle and will deform differently under different conditions. At depth (under higher temperatures and pressures) rocks tend to deform plastically. At the surface rocks tend to be brittle and fracture.
Rock Hall of Fame:
Rocks with religious or mythical importance:
Shiprock: is believed by Navajo Indians to be the remains of a sailing ship that brought them to Earth. It is in reality a volcanic neck.
Ayers Rock: is a sacred aboriginal site. It is an erosional remnant of sandstone hills.
Famous Gemstones:
Hope Diamond: this diamond has an interesting, unlucky history.
Largest diamond in the world: Cullinan I or Great Star of Africa. Part of the Crown Jewels of England. 530 carats
Star of India
Rocks in movies:
Rocks and rocky locations are often used as backdrops for movies. Here are some that you may recognize:
The landscape most often used for movie backdrops is Monument Valley . The majority of western films were here. Next time you watch an old western you will probably see Monument Valley. It is actually a good example of differential weathering.
"North by Northwest" , the film by Alfred Hitchcock and starring Cary Grant used Mount Rushmore in its finale.
And who can forget the final scene in "Close Encounters of the Third Kind"? Devil's Tower became the landing site of the space craft. Devil's Tower is an example of a volcanic neck.
National Monuments and other miscellaneous rocks
Mount Rushmore
Crazyhorse Monument
Blarney Stone
Rosetta Stone
