Earth Space II - Intro to Petrology - Unit 7

What is a Rock?

  • Rock - a mixture of minerals

  • Rocks can be made of only 1 mineral (example: Rock salt/Halite)

  • 15 mineral make up 95% of the crustal rocks of Earth

  • Examples of Common Rocks:

    • Granite - has a mixture of 4 silicate minerals: Quartz, Mica, Hornblende, and Feldspar

      • Quartz - usually clear

      • Mica - tan to black

      • Hornblende - dull black

      • Feldspar - can be white, pink, or red (the color in granite)

  • Most solid rock exists as a buried layer of Bedrock (covered by regolith and soil)

  • An outcrop is a solid rock exposed at the surface

Three Basic Types of Rocks

  • Igneous rocks - formed by the cooling and solidifying of hot, molten material

    • Make up 95% of all rocks of Earth

    • Found deep in Earth, mountains, and volcanic areas

    • Example: Granite

  • Sedimentary rocks - formed by the deposit, hardening, and cementing of sediments (sand, clay, silt, and lime)

    • Make up 4% of all rocks, but cover 75% of the surface of the Earth

    • Example: Sandstone

  • Metamorphic rocks - formed from mainly igneous and sedimentary rocks that have been changed by heat and pressure

    • Make up only 1% of all rocks

    • Found deep in mountains and volcanic areas

    • Example: Marble

The Rock Cycle

  • Rock Cycle - the process where one rock type changes to another rock type over a long period of time

Igneous Rocks

  • Formed by the cooling and hardening of molten material in the Earth

  • Igneous is Latin for fire

What is the Origin of Igneous Rocks?

  • Magma - molten rock deep inside the Earth. Hotter and chemically different than lava.

  • Lava - molten rock on or near the Earth’s surface. Temperature of 1,800 to 2,200*F

  • Pyroclastics - tiny pieces of solid volcanic rock that have been explosively ejected out onto the surface by a volcano.

  • Tuff or Tephra - welded pyroclastics

How are Igneous Rocks Classified?

  • Mineral Composition - the chemicals in the molten material

    • Felsic rocks are lighter in color and have a higher silica content

    • Mafic rocks are darker in color and contain more iron and magnesium

  • Texture - size of the mineral gains or crystals in the rock

    • The texture is based on the cooling rate of the molten material

    • Igneous rocks are interlocking and the crystals are at random (no pattern)

    • Contain no fossils

Basic Types of Igneous Rocks

  • Plutonic (Intrusive)

    • Slow cooling of magma due to the surrounding rocks that insulate it

    • Form deep in the Earth

    • Coarse-grained texture (large and viable crystals)

    • Example: Granite

  • Volcanic (Extrusive)

    • Fast cooling of lava on or near the surface of the Earth

    • Fine-grained texture (very small to microscopic grains)

    • Example: Basalt

Special Types of Igneous Rocks

  • Porphyry - large crystals in a fine-grained rock

    • Formed by irregular cooling of magma

  • Vesicular - composed of many bubble shaped vesicles

    • Formed when gas escapes from lava

    • Example: Scoria

  • Glassy - rock has no crystals with smooth, glass-like texture

    • Example: Obsidian

    • Formed when lava is cooled superfast, such as in the ocean

Intrusive Igneous Rock Types

  • Granite - most common coarse grained igneous rock

    • Composed mainly of light silicate minerals (felsic)

    • Color varies from white, gray, pink, or red

  • Diorite - coarse grained having a dark gray, black, to greenish-black color

    • Composed mainly of dark silicate minerals (mafic)

Extrusive Igneous Rock Types

  • Rhyolite - fine grained igneous rock with the same mineral composition as granite. Can show gas bubbles

  • Andesite - fine grained with same minerals as Diorite. Name comes from the Andes Mountains

  • Basalt - most common fine grained igneous rock. Has the same minerals as Gabbro.

Formation of Plutons

  • Batholith - large, irregular rock mass under the surface (greater than 40 mi2).

    • Usually made up of granite (continental bedrock)

    • Called a stock if less than 40 mi2

    • Example: El Capitan in Yosemite, CA

  • Sill - igneous rock intrusion (sheet-like) that cuts parallel with other rock layers

  • Dike - igneous rock intrusion (wall) that cuts across other rock layers

  • Laccolith - dome or mushroom-shaped igneous intrusion forming deep in the Earth.

    • Dike is “feeder stem” of laccolith

    • Example: Stone Mt. in Georgia

  • Lava flow - extrusive igneous rock from a volcano

Sedimentary Rocks

  • Formed by the deposit, hardening, and sometimes cementing of sediments

  • Sediments come from the weathering and erosion of rocks/minerals on the surface. This allows the rock cycle to continue on.

How are Sedimentary Rocks Identified?

  • Texture - the size of the sediments or materials making up the rock

  • Mineral composition

    • Example: Limestone is made out of Calcite

General Characteristics of Sedimentary Rocks

  • Texture is determined by the degree of weathering and erosion of the original material

  • Non-interlocking texture

  • Usually found in layers or strata

  • Can contain fossils of plants/animals

  • Slow formation (usually indicates an aquatic environment)

Classification of Sedimentary Rocks

  • Clastic or Detrital - Composed of fragments of preexisiting rocks in the form of cobbles, gravel, sand, silt, or clay.

    • Examples of Clastic Rocks:

      • Conglomerate - composed of rounded, pebble-sized fragments

        • Minerals can have quartz sand and pebbles, gravel, clay, chert, flint, etc.

        • Nicknamed “Nature’s Concrete

      • Breccia - composed of angular fragments

      • Sandstone - fine to medium grained rock composed of compacted and cemented sand (Quartz)

        • Has a variety of colors (white, brown, red, yellow) due to impurities in the quartz and/or cement

      • Shale - fine grained rock composed of hardened mud, clay, silt (weathered silicateS)

        • Nicknamed “Creek Rock” (used for skipping across water)

        • The most abundant sedimentary rock on Earth

  • Chemical and or Organic Rocks - formed by the precipitation of soluble materials or more complicated chemical reactions

    • Examples of Chemical/Organic Sedimentary Rocks:

      • Limestone - fine to medium-grained rock composed of the mineral calcite (lime).

        • Bubbles in HCL acid

        • #1 rock in Indiana

        • Types of Limestone:

          • Lithographic - crystalline form, commonly called gravel

          • Coquina - over 90% shell fragments cemented together. Common on Florida beaches

          • Fossiliferous - contains many fossils and organics in a darker, more compact rock

          • Travertine - composed of light/dark bands of precipitated calcite

            • Found in caves and hot springs like Yellowstone National Park

          • Chalk - soft, white, porous rock (Dover, England)

          • Oolitic - made up of small, spherical grains formed by back and forth waves in a shallow sea

      • Dolostone - fine to medium grained rock composed of the mineral dolomite

        • Nicknamed is “Pink Limestone”

        • Can bubble in HCL acid if rock is powdered

      • Rock Salt - formed from the mineral Halite

      • Rock Gypsum - formed from the mineral Selenite Gypsum

        • Both are usually found in massive pieces, and formed as an evaporite in shallow seas

      • Coal - organic fossil fuel classified as a rock

Stages of Coal’s Formation

  • Started as a swampy plany environemnt around 300 million years ago.

  • Peat - decayed plant material (usually brown in color and woody texture)

  • Lignite - formed from increased pressure from peat. Soft, brown to black color with many impurities.

  • Bituminous - sedimentary coal with dull, black color

    • Composed of mainly Carbon with some Sulfur

    • Most common coal in the United States

  • Anthracite - hard metamorhpic coal formed by heat and/or pressure from bituminous coal

    • Shiny, black color composed of almost pure Carbon with minor impurities

    • Best coal for energy use, but not widespread and expensive to extract

  • Graphite - formed by heat and pressure on anthracite coal deep in mountainous areas

    • Silver when pure; layers of carbon atoms

  • Diamond - formed by heat and pressure on Graphite deep in the Earth. Carbon atoms have a complex pattern.

Metamorphic Rocks

  • Formed from other rocks that were changed by heat pressure or chemicals

  • Can be recrystallized and/or minerals can be rearranged in order

How are Metamorphic Rocks Classified?

  • Mineral composition

  • Texture - size of the mineral grains in the rock and how the grains are arranged (foliated vs. non-foliated)

What are general characteristics of Metamorphic Rocks?

  • Interlocking texture

  • Mineral grains can be at random like igneous or in layers like sedimentary

    • Foliation - when metamorphic rocks have layers like sedimentary rocks

  • Usually no fossils due to extreme heat and pressure

Foliated Metamorphic Rocks

  • Gneiss - coarse grained minerals in parallel bands or layers. Usually comes from granite (igneous), but can form from certain conglomerates (sedimentary)

  • Schist - fine to medium grained texture in thin, flaky layers. Usually comes from Slate. Varities include biotite, muscovite, garnet, talc, staurolite, graphite, etc.

  • Slate - fine grained texture in thin, dense layers. Shiny with a variety of colors. Comes from shale (sedimentary)

Non-foliated Metamorphic Rocks

  • Quartzite - medium to fine grained hard rock usually white or pink. Non porous. Comes from Sandstone (sedimentary)

  • Marble - medium to fine grained texture. Very shiny with many colors (white, pink) Will bubble in HCL acid. Comes from Limestone (sedimentary)