Just as any person can be put into one of two main categories of human being, all rocks can be put into one of three fundamentally different types of rocks. They are igneous, sedimentary and metamorphic rocks:
Figure 11 – Types of rocks Igneous Rocks
Igneous rocks are crystalline solids, which form directly from the cooling of magma.
This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state.
Each mineral forms a characteristic type of crystal. For example, the well known igneous rock, Granite, is composed of three main minerals, Quartz, Mica and Feldspar, all of which look different and can be clearly seen in a sample.
Figure 12 - The three main minerals in granite Black=Mica, White=Feldspar, Grey =Quartz
The size of the crystals is usually determined by the speed at which the molten rock material cools. Quick cooling produces small crystals, slow cooling produces larger crystals.
The earth is made of igneous rock - at least at the surface where our planet is exposed to the coldness of space. Igneous rocks are given names based upon two things:
composition (what they are made of) and texture (how big the crystals are).
Magmas occur at depth in the crust, and are said to exist in "magma chambers," a rather loose term indicating an area where the temperature is great enough to melt the rock, and the pressure is low enough to allow the material to expand and exist in the liquid state. Many different types of igneous rocks can be produced. The key factors to use in determining which rock you have are the rock's texture and composition.
Texture
Texture relates to how large the individual mineral grains are in the final, solid rock. In most cases, the resulting grain size depends on how quickly the magma cooled. In general, the slower the cooling, the larger the crystals in the final rock. Because of this, we assume that coarse grained igneous rocks are "intrusive," in that they cooled at depth in the crust where they were insulated by layers of rock and sediment. Fine grained rocks are called "extrusive" and are generally produced through volcanic eruptions.
Grain size can vary greatly, from extremely coarse grained rocks with crystals the size of your fist, down to glassy material which cooled so quickly that there are no mineral grains at all. Coarse grain varieties (with mineral grains large enough to see without a magnifying glass) are called phaneritic. Granite and gabbro are examples of phaneritic igneous rocks. Fine grained rocks, where the individual grains are too small to see, are called aphanitic. Basalt is an example. The most common glassy rock is obsidian.
Obviously, there are innumerable intermediate stages to confuse the issue.
Composition
The other factor is composition: the elements in the magma directly affect which minerals are formed when the magma cools. Again, we will describe the extremes, but there are countless intermediate compositions.
The composition of igneous magmas is directly related to where the magma is formed. Magmas associated with crustal spreading are generally mafic, and produce basalt if the magma erupts at the surface, or gabbro if the magma never makes it out of the magma chamber. It is important to remember that basalt and gabbro are two different rocks based purely on textural differences - they are compositionally the same. Intermediate and felsic magmas are associated with crustal compression and subduction. In these areas, rock and sediment from the surface is subducted back into the crust, where it re-melts. This allows the differentiation process to continue, and
the resulting magma is enriched in the lighter elements. Intermediate magmas
produce diorite (intrusive) and andesite (extrusive). Felsic magmas, the final purified result of the differentiation process, lead to the formation of granite (intrusive) or rhyolite (extrusive).
Sedimentary Rocks
Figure 13 – Sedimentary Rock
Sedimentary rocks are formed at the surface of the Earth, either in water or on land. They are layered accumulations of sediments-fragments of rocks, minerals, or animal or plant material. Temperatures and pressures are low at the Earth's surface, and sedimentary rocks show this fact by their appearance and the minerals they contain.
Most sedimentary rocks become cemented together by minerals and chemicals or are held together by electrical attraction; some, however, remain loose and unconsolidated. The layers are normally parallel or nearly parallel to the Earth's surface; if they are at high angles to the surface or are twisted or broken, some kind of Earth movement has occurred since the rock was formed. Sedimentary rocks are forming around us all the time.
Sand and gravel on beaches or in river bars, look like the sandstone and conglomerate they will become. Compacted and dried mud flats harden into shale. Scuba divers who have seen mud and shells settling on the floors of lagoons find it easy to understand how sedimentary rocks form. Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks.
There are three main types of sedimentary rocks:
Clastic sedimentary rocks:
Clastic sedimentary rocks are accumulations of clasts: little pieces of broken up rock which have piled up and been "lithified" by compaction and cementation.
Sandstone
Figure 14 – Sandstone rocks
Sandstone is composed of mineral grains (commonly quartz) cemented together by silica, iron oxide, or calcium carbonate. Sandstones are typically white, gray, brown, or red. The red and brown sandstone is colored by iron oxide impurities. Most sandstones feel gritty, and some are easily crushed (friable) and break up to form sand. Sandstones have pore spaces between each grain of sand; this property, called porosity, makes them good reservoirs for oil and natural gas. Sandstones are very resistant to erosion and form bluffs, cliffs, ridges, rapids, arches, and waterfalls.
Conglomerate
Conglomerate is a sedimentary rock usually composed of rounded quartz pebbles, cobbles, and boulders surrounded by a matrix of sand and finer material, and cemented with silica, iron oxide, or calcium carbonate. The rock fragments are rounded from being rolled along a stream bed or a beach during transportation. If the fragments embedded in the matrix are angular instead of rounded, the rock is called a breccia (pronounced BRECH-i-a).
Figure 15 -Conglomerate Rock Shale
Figure 16 - Shales
Shale is the most abundant of all sedimentary rocks. It is composed primarily of soft clay minerals, but may include variable amounts of organic matter, calcareous material, and quartz grains. Shale may be any color, but is generally greenish gray to grayish black. It is relatively soft and has a smooth, greasy feel when freshly exposed, but is hard and brittle when dry. Most shales split into thin plates or sheets and are termed fissile, but others are massive (nonfissile) and break into irregular blocks. Shales weather very easily to form mud and clay.
Clays
The term "clay" is applied to various earthy materials composed dominantly of hydrous aluminum magnesium silicate minerals. The most familiar characteristic of clay is plasticity or the ability of moist clay to be fashioned into a desired shape. The physical properties of a clay are plasticity, strength, and refractoriness. Plasticity enables the clay
to be molded; strength permits it to be handled during the forming, drying, and burning processes; and refractoriness permits it to be burned into a hard body of permanent form Bentonite
Bentonite is a soft, low-specific-gravity, expandable clay. It is altered volcanic ash and is found in central Kentucky in beds up to 3 feet thick near the top of the Tyrone Limestone.
Drillers have labeled these bentonite beds the Mud Cave and Pencil Cave. Because of its peculiar property of expanding when wet, bentonite is effective as a water sealer,
especially to prevent pond leakage, and is also used in rotary drilling muds to prevent contaminating formations with drilling fluid.
Chemical sedimentary rocks:
Mny of these form when standing water evaporates, leaving dissolved minerals behind.
These are very common in arid lands, where seasonal "playa lakes" occur in closed depressions. Thick deposits of salt and gypsum can form due to repeated flooding and evaporation over long periods of time. Other chemical sedimentary rocks include sedimentary iron ores, evaporites such as rock salt (Halite), and to some extent flint, limestone and chert.
Organic sedimentary rocks
Any accumulation of sedimentary debris caused by organic processes. Many animals use calcium for shells, bones, and teeth. These bits of calcium can pile up on the seafloor and accumulate into a thick enough layer to form an "organic" sedimentary rock. These include Limestone, Chalk and Coal.
Clues that may help you recognize a sedimentary rock are...
• It looks like bits of other rocks stuck together.
• It has a gritty feel and bits can be rubbed off it.
• It contains fossils, bits of shell or pebbles.
• There are no, or very few crystals in it.
• All the grains look rounded and worn.
Metamorphic Rocks
The metamorphics get their name from "meta" (change) and "morph" (form). Any rock can become a metamorphic rock. All that is required is for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions.
The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks. New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks. Some kinds of metamorphic rocks--granite gneiss and biotite schist are two examples--are strongly banded or foliated. (Foliated means the parallel arrangement of certain mineral grains that gives the rock a striped appearance.) Pressure or temperature can even change previously metamorphosed rocks into new types.
In most cases, this involves burial which leads to a rise in temperature and pressure.
The metamorphic changes in the minerals always move in a direction designed to restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble.
The Geological Time Scale
A sequence of divisions of geological time comprising in order from oldest to youngest: Precambrian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quaternary.
Each of the geological periods is characterised by groups, or suites, of fossils. The picture below shows a typical fossil embedded in a rock.
Figure 17 – Fossil embedded in a rock
The geological periods are grouped into three major divisions of Phanerozoic time. The block of "ancient life" is dated from some 540 million years before present (the
Cambrian) to about 245 million years before present (the Permian). Fossils such as trilobites, graptolites, early fish and ancestral plants belong to this "Era", known as the Paleozoic. The Paleozoic Era is replaced by the time of "middle life" (the Mesozoic Era), charcterised by dinosaurs and marine organisms such as the great marine reptiles and the ammonites. The Mesozoic Era commenced with the Triassic Period (starting about 245 million years ago) and concluded with the Cretaceous Period (66.4 million years ago).
The last block of geological time is the Cenozoic Era with two geological periods, the Tertiary and the Quaternary. This era is characterised by widespead evolution of the mammals, and concludes with the appearance of modern Homo sapiens (our own species), in late Quaternary time. We are living in the Quaternary Period.
EON ERA PERIOD