What are Rocks

Rocks and classifications

Making up the majority of the Earth's crust, rock is usually defined as a mixture of common minerals. Rocks can be hard or soft, as small as a grain or as large as a building. They have been an integral part of the history of mankind, first being used as tools for hunting and defense, and as a building materials to construct shelters and monuments. Combined with the effects of tectonics, weathering and vegetation, rocks define the natural landscapes we see around us. The minerals and metals we find in rocks are essential to the prosperity and cultural splendor of human civilization. There are many kinds of rock, and they can be classified in a number of ways. However, geologists classify rocks based on how the rocks were formed. The three classes are igneous rocks (formed directly from liquid rock), metamorphic rocks (formed by direct alteration of existing rocks), and sedimentary rocks (formed by eroded materials from other rocks). What Are Rocks? Rocks are combinations of one or more minerals that we find in nature. For example: Limestone is composed of only one mineral – calcite Basalt is commonly composed of three minerals – feldspar, pyroxene, and olivine Granite is commonly composed of five minerals – two kinds of feldspar, mica, amphibole, and quartz Just like minerals, there are many different kinds of rocks. Geologists group rocks into three different categories; igneous, sedimentary, and metamorphic. Igneous rocks form from magma or lava Sedimentary rocks form from pieces of other rocks or are formed by minerals being precipitated from water Metamorphic rocks form by recrystallizing the minerals in a rock under high pressures or temperatures Igneous Rocks Igneous rocks are formed from lava or magma. Magma is molten rock that is underground and lava is molten rock that erupts out on the surface. The two main types of igneous rocks are plutonic rocks and volcanic rocks. Plutonic rocks are formed when magma cools and solidifies underground. Volcanic rocks are formed from lava that flows on the surface of the Earth and other planets and then cools and solidifies. The texture of an igneous rock depends on the size of the crystals in the rock. This tells us if the rock is plutonic or volcanic. When magma cools underground, it cools very slowly and when lava cools above ground, it cools quickly. When magma and lava cool, mineral crystals start to form in the molten rock. Plutonic rocks, which cool slowly underground, have large crystals because the crystals had enough time to grow to a large size. Volcanic rocks, which cool quickly above ground, have small crystals because the crystals did not have enough time to grow very large. The type of igneous rock is also dependent on its composition (the elements that are present). There are many different compositions of magma and lava. Fortunately, most igneous rocks are one of three basic compositions: Felsic igneous rocks contain relatively high amounts of silicon, sodium, aluminum, potassium (Si, Na, Al, and K) and relatively low amounts of iron, magnesium, and calcium (Fe, Mg, Ca). Mafic rocks contain relatively low amounts of silicon, sodium, aluminum, potassium (Si, Na, Al, and K) and relatively high amounts of iron, magnesium, and calcium (Fe, Mg, Ca). Intermediate rocks are what their name sounds like. Their composition is in-between mafic and felsic rocks. The different elements present in the different igneous compositions will form different minerals. Rocks with high amounts of iron (Fe) tend to form minerals that are dark in color (such as olivine and pyroxene). As result, mafic rocks tend to be dark in color and felsic rocks tend to be lighter in color. An example of a mafic rock is basalt, the black rock that forms from lava flows in places like Hawaii. An example of a felsic rock is granite, the light colored rocks that we find in places like the Sierra Nevada mountains in California. Sedimentary Rocks Most sedimentary rocks fall into one of two categories: Clastic sedimentary rocks are formed from pieces of other rocks that are glued together. For example, a sandstone is made up of pieces of sand. Chemical sedimentary rocks are formed by precipitation of different elements dissolved in water. For example, halite (rock salt) forms when water evaporates away and leaves the dissolved salt behind. There are also organic sedimentary rocks, which are made up of plant material. Coal is considered an organic sedimentary rock. Metamorphic Rocks Graphite Diamond Marble Metamorphic When rocks are heated up or put under a lot of pressure, they can change drastically. This is because the minerals that make up the rocks form only at certain temperatures and pressures. At high temperatures or pressures, the elements will rearrange their crystal structure to form a different mineral. Graphite and diamond are two minerals that are both made entirely out of carbon. If we put graphite under a huge amount of pressure, the carbon atoms will be squeezed together and will rearrange themselves into the more compact crystal structure of diamonds. This change happens without any melting of the rock. Everything remains solid while the metamorphism occurs. So why are these rocks interesting? We know from experiments that certain minerals form only at very specific temperatures and pressures. If we find a rock that has minerals (such as a diamond) that only form at high pressures, we know that it must have formed deep in the Earth. Something must have happened to bring the rock up to the surface where we could find it. If we find a rock that formed at high temperatures (such as marble), we know that the rock must have been heated up. This often occurs deep in the Earth or near magma underground. We often find metamorphic rocks in mountain ranges where high pressures squeezed the rocks together and they piled up to form ranges such as the Himalayas, Alps, and the Rocky Mountains. Metamorphic rocks are forming deep in the core of these mountain ranges. As the mountain ranges are eroded away over millions of years, the metamorphic rocks are eventually uncovered. A good example of where this has happened is the Appalachian Mountains in the Eastern United States. The Appalachian Mountains used to be a very large mountain range. Over time the mountains have eroded away and we now can see many different types of metamorphic rocks in the region. Clastic Sedimentary Rocks Clastic sedimentary rocks are made up of pieces of other rocks. For example, sand on a beach or in a dune can get buried. Under the pressure of burial, the sand is pressed together and compacted. The individual sand grains are glued together by minerals such as calcite (calcium carbonate) and hematite (iron oxide) that are precipitated (see chemical sedimentary rocks) out of ground water that travels between the sand grains. The name of a sedimentary rock depends on the size (and sometimes the shape) of the rock pieces that it is made of: Conglomerate is composed of rounded gravel, pebbles, cobbles, or boulders along with smaller rock pieces. Breccia is similar to a conglomerate except its gravel, pebbles, cobbles, or boulders have a sharper, angular shape. Sandstone is composed of sand. Siltstone is composed of silt. Shale is composed of clay sized particles. The word "clay" can mean several different things to a geologist. In this case, clay means very small (smaller than 0.000004 meters in diameter) rock particles. The difference in particle size tells a lot about what kind of place that the rock formed in. We will find mud, not large cobbles, in the middle of a quiet lake. The rock that can eventually form that mud is a shale. We do find gravel, pebbles, cobbles, and boulders in the middle of a mountain stream. A conglomerate can form from these materials. Chemical Sedimentary Rocks Chemical sedimentary rocks form by precipitation of minerals from water. Precipitation is when dissolved materials come out of water. For example: Take a glass of water and pour some salt (halite) into it. The salt will dissolve into the water. If you set the water in a hot and dry place (like Arizona) the water, but not the salt, will evaporate away. As the water evaporates, the concentration of salt gets higher and the water will eventually become saturated and will not be able to dissolve any more salt. At this point, as the water continues to evaporate, the salt will come out of solution and will be precipitated in the glass. This is a common way for chemical sedimentary rocks to form and the rocks are commonly called evaporites. They are typically made up of the minerals halite (calcium chloride, or rock salt) and gypsum (calcium sulfate). Another way to precipitate minerals out of water is to change the temperature (or the acidity) of the water rather than evaporate the water. This is how rocks such as limestones form. Limestones commonly form in oceans, which do not evaporate away. For example, when cold sea water heats up, it can no longer hold as much calcium carbonate (calcite, the mineral that limestones are made out of) in solution. When this happens, the calcium carbonate is precipitated out of the water as calcite, forming limestone. Limestones can also form from the shells of different sea critters that settle out on the bottom of the ocean. So, when we find a limestone, we know that there was a large lake or ocean present at that location in the past. When we find evaporites, we can tell that we were probably in a hot and dry environment such as a desert playa.