Aim: What are the three types of rocks? I. Rocks – made up of more than one mineral. Rocks are classified according to how they were formed. Igneous, sedimentary, and metamorphic are the three main groups of rocks. A. Igneous Rocks– rocks that form from hot magma (molten rock) when it cools and solidifies. 1. Lava – magma that has reached earth’s surface.
2. Rate of cooling can be determined by crystal size. a. Intrusive igneous rocks - have large crystals due to the slow cooling of magma inside the earth. b. Extrusive Igneous Rocks – have small crystals due to the fast cooling of lava on the earth’s surface. 3. Igneous rocks are classified according to crystal size and color. GO TO PAGE 6 OF YOUR ESRT’S
a. Felsic igneous rocks – light in color, low in density, and rich in feldspar and silica. b. Mafic igneous rocks – dark in color, high in density, and rich in magnesium and iron.
B. Sedimentary Rocks – rocks that are formed by the hardening and cementing of layers of sediments. 1. Three types of Sedimentary Rocks a. Clastic – rocks formed from fragments of other rocks. Ex: shale, sandstone, conglomerate. b. Chemical (Chrystaline) – rocks formed from mineral grains that fall out of a solution (precipitate) by evaporation or chemical reaction. Ex: rock salt and limestone.
c. Organic – rocks formed from the remains of plants and animals. Ex: coal and limestone made of shell fragments 2. Characteristics of Sedimentary Rocks a.Have layers (stratification) b. Contain fossils (remains, impressions, or any other evidence of plants and animals preserved in rock.
Conglomerate rocks are sedimentary rocks. They are made up of large sediments like sand and pebbles. The sediment is so large that pressure alone cannot hold the rock together; it is also cemented together with dissolved minerals.
Sandstone rocks are sedimentary rocks made from small grains of the minerals quartz and feldspar. They often form in layers as seen in this picture. They are often used as building stones.
Shale rock is a type of sedimentary rock formed from clay that is compacted together by pressure. They are used to make bricks and other material that is fired in a kiln.
Limestone rocks are sedimentary rocks that are made from the mineral calcite which came from the beds of evaporated seas and lakes and from sea animal shells. This rock is used in concrete and is an excellent building stone for humid regions.
Gypsum rocks are sedimentary rocks made up of sulfate mineral and formed as the result of evaporating sea water in massive prehistoric basins. It is very soft and is used to make Plaster of Paris, casts, molds, and wallboards.
3. Metamorphic Rocks – rocks that have been exposed to great heat and pressure. a. Dynamic (Regional) Metamorphism – large areas of rock under great heat and pressure. (mountain building) b. How heat and pressure causes rocks to change: Pressure squeezes grains closer together causing the rocks to become more dense and less porous. Rearrange particles (atoms). Minerals are reformed or new ones are created.
c. Thermal (Contact) Metamorphism – igneous intrusions bake overlying rock. The changes in the rock are less drastic than regional metamorphism and foliation is not produced. d. Characteristics of Metamorphic Rocks: Foliation – grains are in parallel layers. Ex: slate Banding – bands of light colored minerals alternate with bands of dark colored minerals.
Gneiss is a high grade metamorphic rock. This means that gneiss has been subjected to more heat and pressure than schist. Gneiss is coarser than schist and has distinct banding. This banding has alternating layers that are composed of different minerals. The minerals that compose gneiss are the same as granite. Feldspar is the most important mineral that makes up gneiss along with mica and quartz. Gneiss can be formed from a sedimentary rock such as sandstone or shale, or it can be formed from the metamorphism of the igneouse rock grantite. Gneiss can be used by man as paving and building stone.
Quartzite is composed of sandstone that has been metamorphosed. Quartzite is much harder than the parent rock, sandstone. It forms from sandstone that has come into contact with deeply buried magmas. Quartzite looks similar to its parent rock. The best way to tell quartzite from sandstone is to break the rocks. Sandstone will shatter into many individual grains of sand while quartzite will break across the grains.
Slate is a fine-grained metamorphic rock with perfect cleavage that allows it to split into thin sheets. Slate usually has a light to dark brown streak. Slate is produced by low grade metamorphism, which is caused by relatively low temperatures and pressures. Slate has been used by man in a variety of ways over the years. One use for slate was in the making of headstones or grave markers. Slate is not very hard and can be engraved easily. The problem with slate though is its perfect cleavage. The slate headstones would crack and split along these cleavage planes. This is not a desirable attribute for a head stone. Slate was also used for chalk boards. The black color was good as a background and the rock cleaned easily with water. Today it is not very advantageous to use this rock because of its weight and the splitting and cracking over time.
Schist is a medium grade metamorphic rock. This means that it has been subjected to more heat and pressure than slate, which is a low grade metamorphic rock. As you can see in the photo above schist is a more coarse grained rock. The individual grains of minerals can be seen by the naked eye. Many of the original minerals have been altered into flakes. Because it has been squeezed harder than slate it is often found folded and crumpled. Schists are usually named by the main mineral from which they are formed. Bitotite mica schist, hornblende schist, garnet mica schist, and talc schist are some examples of this.
Marble is metamorphosed limestone or dolomite. Both limestone and dolomite have a large concentration of calcium carbonate (CaCO3). Marble has many different sizes of crystals. Marble has many color variances due to the impurities present at formation. Some of the different colors of marble are white, red, black, mottled and banded, gray, pink, and green. Marble is much harder than its parent rock. This allows it to take a polish which makes it a good material for use as a building material, making sink tops, bathtubs, and a carving stone for artists. Today, headstones are made from marble and granite because both of these rocks weather very slowly and carve well with sharp edges. Marble is quarried in Vermont, Tennessee, Missouri, Georgia, and Alabama.