Different rocks hold different types of information and only specific types of rocks can contain fossils. As you probably learned in middle or high school, there are three main classes of rocks: Igneous, Metamorphic, and Sedimentary. We will go through some basic information on each below:
Igneous rocks are rocks that are formed either inside of the Earth’s crust (intrusive), or on the outside of the Earth by volcanic processes (extrusive). These rocks are very common, and can have very large mineral grains that make up the rock or very small grains. The size of the grains tells geologists something about how fast the rock cooled: Igneous rocks with very big mineral grains cooled very slowly within the Earth, and those with very small grains, or that have no visible grains at all, were cooled very quickly, likely at the surface of the Earth. Common igneous rocks include: pumice, you may have a pumice rock that you use to exfoliate; granite, commonly used in driveways, also called gravel, and for kitchen counter tops; and obsidian, a black to dark grey rock that looks like a chunk of glass.
Metamorphic rocks are igneous, sedimentary, or other metamorphic rocks that have been subjected to heat and/or pressure. Rocks are heated by being buried with additional sediments or through the movement of tectonic plates. This movement creates high temperatures and pressures. Rocks that are just subjected to heat usually grow larger mineral grains. A common mineral that appears when rocks are subjected to intense heat is garnet. When rocks are subjected to both heat and pressure, they begin to exhibit foliation. This is when the flaky minerals contained within the rock align perpendicular to the stress direction, leading to a banding appearance in the rock. Common metamorphic rocks include: gneiss pronounced ‘nice’, and yes, it is usually the subject of many cheesy geology puns; schist, a rock that has lots of mica, and appears very shiny, and also the centerpiece of many geology puns; and slate, less shiny than schist, commonly used for walkways, fireplaces, and sometimes counter tops.
Thus, the best type of rocks have been saved for last (at least in the opinion of most paleontologists and paleoclimatologists). Sedimentary rocks are rocks that are made from pieces of other rocks. They are the most common type of rock that covers the Earth’s surface, with approximately 75% of the rocks at the Earth’s surface being sedimentary. They are of vast importance to geologists because they contain fossils as well as sedimentary structures that tell us about ancient animals and plants and even environments. Sedimentary structures are visible features within a sedimentary rocks that record parts of the environment in which they were formed. Ripples and mud cracks are both common features found in modern environments, surely at the beach and perhaps around a drying puddle or near a lake. Sometimes, these features become preserved in the sediment and make it into the rock record.
Fossils and sedimentary structures are preserved in several different types of sedimentary rocks. Although there are several, we will only focus on the most common sedimentary rocks in more detail below.
Limestone is a type of carbonate rock (chemical formula CaCO3) that will dissolve if any acidic liquid comes into contact with it. Notice that limestone is made mostly of calcite.
Limestones are formed from carbonate sediments, such as carbonate sand that makes up the beaches of the Bahamas, and other tropical areas. Corals and seashells are also made of carbonate, as are several fossil groups (such as brachiopods). Learn more about what minerals make up animal shells and skeletons on the ‘Common Minerals in Biomineralization‘ page.
Carbonate sand, and thus limestone, mostly forms in tropical areas due to certain environmental factors. Another characteristic of limestones is that they typically contain several fossils of marine animals. Limestones, like sandstones, can also contain sedimentary structures, such as burrows, ripples, and cross bedding, that tell us something about the environment in which they were deposited.
Shale is a very important sedimentary rock because it is the rock from which oil and natural gas are produced. Shale is a very fine-grained rock (the individual grains are too small to see with your naked eye) that contains a lot of organic material (decayed plant and animal remains, poop, algae, bacteria) that gives the rock its characteristic dark grey/black color. When enough shale accumulates and is ‘cooked’ at higher temperatures within the Earth, it will eventually produce oil and gas through a complex process. Shale is formed in very low energy environments, such as lakes and in the deep parts of the ocean. Because of this, shale does not typically preserve larger organisms, but rather preserves small planktic organisms (those that float in the upper part of the oceans) and microfossils.
Siltstone & Mudstone
Siltstone and mudstone are two different sedimentary rock types, but are often hard to tell apart. Both rocks contain grains that are very small, and the common way to test if a rock is a siltstone or mudstone is to bite a piece off the rock. This sounds gross (and it kind of is, we’re willing to admit), but is very informative: siltstone has slightly larger grains than mudstone, so will feel gritty between your teeth. Mudstone has very small particles, so small that the rock will not feel gritty.
Both rocks are created in very low energy environments, such as in the deep ocean, or in a lake, or perhaps even in the floodplain of a river. Although somewhat similar to shale, these rocks do not contain as much organic matter.
Sandstone is a very common type of sedimentary rock that is formed from mostly quartz (also called silica) minerals. Silica is very abundant in the Earth, and is also very hardy; thus, it lasts a very long time and does not erode away easily. Most beaches are made of quartz grains because of this property of the mineral.
Sandstones commonly contain sedimentary structures such as ripples, cross bedding, and burrows. Sometimes, they do contain fossils, but because the rocks represent deposition in a high energy environment (such as a beach that is constantly subjected to energy from waves and wind), the fossils are fragmented and not very well preserved.
Sedimentary rocks that contain large (2mm) pieces of other rocks stuck together are called conglomerates. Often, the larger pieces of rock are cemented together by smaller grains of sand, or even silt or mud. Thus, this rock type is unique in that the grain sizes can cover a huge range. Conglomerates are often formed in alluvial fans, landslides, and even by glaciers.