An Introduction to Rocks and Minerals

 Rocks and minerals are often referred to as the same thing. In fact, they are not the same.  A mineral is a chemical compound or a mixture of compounds.  A rock is an aggregate of minerals. An aggregate is a mixture that is cemented or mechanically joined together.  In other words, rocks are composed of minerals.

Examples of Minerals Quartz Potassium Feldspar Biotite Mica

Rock: An Aggregate of Minerals Granite A rock composed of minerals: Quartz Potassium Feldspar Biotite Mica

 Rocks are classified by their method of formation.  There are three classes of rocks: 1. Igneous 2. Sedimentary 3. Metamorphic

 Igneous means “rock from fire”.  Igneous rocks form from a melt, a complex mixture of liquids and gases, also known as lava or magma.  Melts form deep within the Earth and can have temperatures over 2000 o C.  Magma is melt material that never reaches the Earth’s surface before it cools.  Lava is melt material which erupts to the surface and cools.

 Igneous rock has two types: 1. Intrusive 2. Extrusive  These types of igneous rock are determined by where the rock forms and the time it takes to cool.

 Intrusive igneous rock forms from magma which never reaches the surface.  The type is determined by looking at the size of mineral crystals in the rock.  When magma does not reach the surface, it cools slowly. This allows minerals in the rock to form large crystals.  A common intrusive igneous rock is granite.

 Extrusive igneous rock forms from lava cooling on the surface.  Cooling can be due to exposure to air (on land) or water (on the seafloor).  Rapid cooling keeps the minerals in the rock from growing so they remain small and dense.  Examples of extrusive igneous rock are basalt and rhyolite.

Extrusive Igneous Rock Basalt Rhyolite

 Water is the common factor in the formation of all sedimentary rock.  Seventy-five percent (75%) of all the visible rock on the Earth’s surface are sedimentary.  Sedimentary rock often has a layered pattern when exposed. These layers, called strata, results from gradual deposition of sediments over a long period of time.

 The most common method of formation of sedimentary rock is weathering and erosion of existing rock.  Rain or moving water carries eroded sediments until the flow of water slows and the sediments are deposited.  If the water dries up, the sediment dries out and becomes cemented, a process called lithification.  This type of formation is called clastic sedimentation.

Sandstone arch in Utah Shale strata in New Mexico

 Chemical sedimentary rocks are formed when compounds dissolved in water precipitate out or the water evaporates.  Organic sedimentary rocks are formed from the remains of plants and animals, including shells, that lithify to form solid rock.

Other types of Sedimentary Rock Chemical Sedimentary Halite Gypsum Organic Sedimentary Limestone Coquina

 Metamorphism means change.  Metamorphic rock is formed by changing existing rock with heat and pressure.  All rocks, igneous, sedimentary or metamorphic, under the right conditions, can become metamorphic.  Metamorphism can change the shape, texture and mineral composition of an existing rock.

 Metamorphic rock can be identified as being one of two types: 1. Foliated Texture 2. Nonfoliated Texture

 A foliated rock forms when heat melts the minerals then pressure rearranges them in bands on cooling.  An example would be granite becoming gneiss.

 Nonfoliated metamorphic rock forms when the minerals are melted and dissolved then reform as a single mass with no layering.  An example would be limestone morphing into marble.

 Marble (limestone → marble)  Slate (shale → slate)

 What should be clear by now is that no rock lasts forever and that existing rocks can be changed.  This cycle of change, or recycling, is called the Rock Cycle.  Even though this is a continuing cycle, the time required is longer than the human lifetime. Keep in mind that when looking at geologic time, nothing is permanent!

 Although the Rock Cycle can start with any type of rock, it is easiest to start with igneous rock. 1. Magma cools over time and begins to be weathered and eroded to form sediment. 2. Sediments compact and lithify to form sedimentary rock. 3. The sedimentary rock gets buried deeper and is exposed to heat and pressure becoming metamorphic rock. 4. The metamorphic rock can be carried deeper toward the mantle where it melts again to magma.