Rocks

 Rock: A rock is any solid mass of mineral or mineral-like matter that occurs naturally as part of our planet.  Most rocks contain multiple types of minerals. Each mineral retains its properties within the rock.  There are 3 major types of rocks: Igneous, Sedimentary, and Metamorphic.

Interactions among Earth’s water, air, and land can cause rocks to change from one type to another. This continuous process that causes rocks to change is the Rock Cycle.

Sedimentary Rock: A rock formed by the compaction and cementation of sediments. Weathering: Process where rocks are physically and/or chemically broken down by water, air, and living things. The weathered pieces are called sediments. Weathering and Erosion can change any rock into a sedimentary rock.

Metamorphic Rock: A rock altered by heat and pressure. Heat and Pressure and can change any rock into a metamorphic rock..

Igneous Rock: A rock formed by the solidification of magma. Magma-molten material that forms deep beneath the Earth’s surface. Lava-Magma that reaches the surface In order to become an igneous rock, magma must cool beneath the surface or as a result of a volcanic eruption becomes an igneous rock.

 Processes driven by heat from the Earth’s interior are responsible for forming both igneous and metamorphic rocks.  Weathering and the movement of weathered materials by water, gravity, glaciers, and wind are external processes powered by the energy from the sun. External processes produce sedimentary rocks.

 1. What is a rock?  2. What are the three major types of rocks?  3. What is the rock cycle?  4. What powers the Earth’s rock cycle?  5. How might a sedimentary rock become an igneous rock?

 Igneous Rocks forms from the solidification and crystallization of magma or lava.  There are two types of igneous rocks.  Intrusive  Extrusive

 Intrusive Igneous Rocks form when magma solidifies under the Earth’s surface.  Magma is less dense than the surrounding rocks, so it slowly works its way toward the surface.  As it rises, it cools it allows elements to combine and form minerals. Since the magma cools slowly, the minerals are able to grow in size.

Diorite (Left) and Granite (Right) are both examples of intrusive igneous rocks. The slow cooling of magma beneath the Earth’s surface allows the growth of large mineral crystals.

 Extrusive Igneous Rocks form when lava solidifies above the Earth’s surface.  Since the lava cools much quicker above the Earth’s surface, the minerals in extrusive igneous rocks are not able to grow as large.

Rhyolite (Left) and Basalt (Right) are both examples of extrusive igneous rocks. The rapid cooling of lava resulted in a rock with small, interconnected mineral grains.

 Texture and composition are two characteristics used to classify igneous rocks.  The rate of cooling strongly affects the type texture.  Texture Classifications: Coarse-Grain, Fine- Grain, Glassy, and Porphritic  Composition Classifications: Granitic, Basaltic, Other Compositions

COARSE-GRAIN TEXTURE: THE SLOW COOLING OF MAGMA ALLOWS THE FORMATION OF LARGE CRYSTALS. FINE-GRAIN TEXTURE: THE RAPID COOLING OF MAGMA OR LAVA RESULTS IN SMALL, INTERCONNECTED MINERAL GRAINS.

GLASSY TEXTURE: WHEN LAVA IS SPEWED ONTO THE SURFACE, IONS DON’T HAVE TIME TO ARRANGE THEMSELVES INTO CRYSTALS AND THE RESULT IS A GLASSY TEXTURE. PORPHYRITIC TEXTURE: MINERALS DO NOT FORM AT THE SAME RATE SO IT IS POSSIBLE FOR SOME MINERALS TO BECOME QUITE LARGE BEFORE OTHERS BEGIN TO FORM. THESE LARGE CRYSTALS THAT ARE SURROUNDED BY FINE GRAINED MINERALS ARE CALLED PHENOCYRSTS.

 Granitic Composition: Igneous rocks that are made almost entirely of the light- colored silicate minerals quartz and feldspar.  Granitic rocks are the major rocks of the continental crust. Rhyolite is an extrusive granitic rock. Notice that is light in color’

 Basaltic Composition: Rocks that contain many dark silicate minerals and plagioclase feldspar.  They are rich in iron which makes them darker and denser than granitic rocks. Gabbro is an intrusive igneous rock with basaltic composition.

 Other compositional groups: Rocks with a composition between granitic and basaltic have an andesitic composition. They contain at least 25% dark silicate minerals.  Ultramafic: Peridotite is an important igneous rock that is composed mostly of olivine and some pyroxene. It is composed almost entirely of dark silicate minerals so its composition is referred to as ultramafic.

 All sedimentary rocks form when existing rocks are broken down into sediments.  These sediments are compacted and cemented together.

 Weathering: Chemical or physical breakdown of rock into smaller pieces called sediments.  Erosion: The carrying away of the sediments by agents such as water, wind, ice, or gravity.  Deposition: When the agent loses energy and drops the sediments. Sediments are deposited according to size.  Large sediments are deposited first.

 Compaction and cementation turns sediments into sedimentary rock.  Compaction: is a process that squeezes sediments. It is caused by the weight of the sediments.  Cementation: takes place when dissolved minerals are deposited in the tiny spaces among the sediments.

 Sedimentary rocks can be classified into two main groups according to the way they form.  Clastic Sedimentary Rocks: Made of weathered bits of rocks and minerals.  Chemical/Biochemical Sedimentary Rocks: Forms when dissolved minerals precipitate from water solutions.

 Grouped according to the size of the sediments in the rocks.

Conglomerate is a clastic sedimentary rock that contains large (greater then two millimeters in diameter) rounded particles. Sandstone is a clastic sedimentary rock made up mainly of sand- size (1/16 to 2 millimeter diameter) weathering debris Siltstone is a clastic sedimentary rock that forms from silt-size (between 1/256 and 1/16 millimeter diameter) weathering debris. Shale is a clastic sedimentary rock that is made up of clay-size (less then 1/256 millimeter in diameter) weathering debris

 The precipitation that forms chemical sedimentary rocks occurs when the water evaporates or boils off leaving a solid product. Rock Salt is a chemical sedimentary rock that forms from the evaporation of ocean or saline lake waters

 The many unique features of sedimentary rocks are clues to how, when, and where the rocks formed.  In undisturbed sedimentary rocks, the oldest layer of deposition is at the bottom and the youngest is at the top.  Ripple marks and mud cracks can give clues as to the environment that the rock formed in

 Fossils: traces or remains of ancient life are unique to some sedimentary rocks such as limestone.  Fossils play a key role in matching up rocks from different places that are the same age.  Fossils can be used to help answer questions about the rocks that contain them such as did the rock form in land or water? Was the climate hot or cold? When did the rock form?

 Form when existing rocks are changed by heat and pressure, usually a few kilometers below the Earth’s surface and extend to the upper mantle.  Often the changed rock looks different and has a different chemical composition than the parent rock (parent rock=original rock).  There are two types of metamorphism: Contact and Regional

 Contact Metamorphism: Hot magma moves into rock causing contact metamorphism to take place.  Produces low-grade metamorphism which are minor changes in the rock. Example: Marble

 Regional Metamorphism: During mountain building, large areas of rocks are subjected to extreme pressure and temperatures.  Produces high-grade metamorphism and large-scale deformation.

 The agents of metamorphism are heat, pressure, and hydrothermal solutions.  Heat: The most important agent because it provides energy to drive chemical reactions.  Two sources of heat: Magma and depth  Pressure: Just like temperature, pressure increases with depth. Pressure causes the spaces between mineral grains to close.  Hydrothermal Solutions: Hot, water based solutions that escape from magma. Promote recrystallization by dissolving minerals and depositing new ones.

 Like igneous rocks, metamorphic rocks are classified based on texture and composition.  There are two groups: foliated and nonfoliated

 When rocks undergo contact metamorphism, they become more compact and dense. The pressure causes minerals to align in a similar direction. This gives the rock a layered or banded appearance.  Under extreme conditions, minerals recrystallize with a preferred orientation-at a right angle to the direction of force.

Gneiss is foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. It typically contains abundant quartz or feldspar minerals

 A metamorphic rock that does not have a banded texture.  Most nonfoliated rocks contain only one mineral.

Quartzite is a non-foliated metamorphic rock that is produced by the metamorphism of sandstone. It is composed primarily of quartz Marble is a non-foliated metamorphic rock that is produced from the metamorphism of limestone. It is composed primarily of calcium carbonate