The Rock Cycle

There are three types of rocks; igneous, sedimentary, and metamorphic There are three types of rocks; igneous, sedimentary, and metamorphic. The interrelationship between the rock types is referred to as the rock cycle.

Igneous Rocks

Igneous Rocks Igneous rocks form from molten rocks called magma while still in the earth, or from lava once the rock has reached the surface. The characteristics of this type of rock are determined by where it forms and how fast it cools. Rocks that form underground from magma that cools slowly are called intrusive and have large crystals. Rocks that form on the earth’s surface from lava erupted from a volcano cool quickly and have small crystals. These rocks can have a glassy texture because of the speedy cooling or even have gas bubbles that leaves holes in the rock as it cooled and solidified. This type of rock is called extrusive rock. Basalt, obsidian, and granite are types of igneous rocks.

Metamorphic Rocks

Metamorphic Rocks Any rock (igneous, sedimentary, or metamorphic) can become a metamorphic rock. Rock can be buried deep beneath the earth’s surface. The weight of the thick layers pushes the rock down toward the heat of the mantle. Heat and great pressure deep in the Earth can change rock into metamorphic rock. If rocks are buried deep in the Earth at high temperatures and pressures, they form new minerals and textures all without melting. Marble and gneiss are examples of metamorphic rock.

Sedimentary Rocks

Sedimentary Rocks The forces of wind, rain, snow, and ice combine to break down or dissolve (weather), and carry away (transport) rocks exposed at the earth’s surface. These particles eventually come to rest (deposited) and become hard rock through cementation and compaction (lithification). Clastic sedimentary rocks form by weathering processes which break down rocks into pebble, sand, or clay particles by exposure to wind, ice, and water. Non-clastic sedimentary rocks form from chemical reactions, chiefly in the ocean. Some sedimentary rocks may contain fossils which are the hardened remains of once living organisms that where buried. Limestone and sandstone are types of sedimentary rocks.

All three rock types get weathered to create sedimentary rock All three rock types get weathered to create sedimentary rock. All three rock types melt to form magma. Magma forms igneous rock. All three types of rock form metamorphic rock when buried deep inside the earth and undergo heat and/or pressure.

Rocks and Minerals: What’s the Difference? A mineral is a homogeneous, naturally occurring, inorganic solid. (Homogeneous means made of parts or elements that are the same kind.) Each mineral has a definite chemical composition and a characteristic crystalline structure. A mineral may be a single element such as copper (Cu) or gold (Au), or it may be a compound made up of a number of elements.

Rocks are made up of one or more minerals.

Minerals are divided into two kinds, the rock-forming minerals and the ore minerals. An ore is a metal bearing rock or mineral that can be mined. Each mineral has its own characteristics and can be identified by its uniqueness. One characteristic of minerals is their crystal shape. When minerals are allowed to grow unhindered, they develop a three dimensional shape. Color is another useful identification feature of minerals, but should be used with other identifying characteristics since some minerals occur in a variety of colors.

Another way minerals can be identified is by the hardness of the rock Another way minerals can be identified is by the hardness of the rock. Scientists have developed a scale to compare the hardness of minerals to. This scale is called Moh’s Hardness Scale and is ordered from the softest mineral at 1 to the hardest mineral at 10. Hardness Mineral Description 1 Talc Soft, feels greasy 2 Gypsum Scratched by fingernail 3 Calcite Scratched by a penny 4 Fluorite Scratched by a knife 5 Apatite Hard to scratch with a knife 6 Orthoclase Scratches glass 7 Quartz Scratches glass 8 Topaz Scratches quartz 9 Corundum Scratches topaz 10 Diamond Hardest known substance on Earth

Minerals can also be tested for reactions to magnets and hydrochloric acid (__HCl__). Mass, taste, and smell can also identify a mineral.

The color of a mineral’s powder is called streak The color of a mineral’s powder is called streak. You can observe streak by rubbing a mineral across the surface of an unglazed porcelain tile. Transparency describes the way light passes through a mineral. A mineral that does not allow light to pass through it is opaque. If light can pass through the mineral but an object can not be seen through the mineral, then the mineral is translucent. A mineral that allows objects to be seen through it is transparent.

Luster describes the way light reflects off the surface of a mineral. Adamantine - very gemmy crystals Dull - just a non-reflective surface of any kind Earthy - the look of dirt or dried mud Fibrous - the look of fibers Greasy - the look of grease Gumdrop - the look of a sucked on hard candy Metallic - the look of metals Pearly - the look of a pearl Pitchy - the look of tar Resinous - the look of resins such as dried glue or chewing gum Silky - the look of silk, similar to fibrous but more compact Sub-metallic - a poor metallic luster, opaque but reflecting a little light Vitreous - the most common luster, it simply means the look of glass Waxy - the look of wax