What are Rocks?
To understand what rocks are, let’s get down to basics. Atoms make up elements, elements make up minerals and minerals make up rocks.
Start with Individual Atoms
Atoms are the smallest part of any element that can exist alone and still behave like that element at any point in time.
Next Up: Elements
Elements are pure substances that cannot be broken down into simpler forms by any form of chemical reaction. Although it is possible to break down objects made of a single element into a number of smaller size pieces by a physical means, the element retains its original properties no matter how small it is broken. For example, a gold bar contains the element gold (Au), and no matter how small the gold bar is broken down, each new size formed will still retain all the properties of gold. So although elemental objects can be broken down into smaller size objects, nothing can be done to break them down chemically. In both cases, the element remains pure and retains its original properties.
Minerals Consist of Two or More Elements
Let’s consider two elements common in nature, silicon and oxygen. When these two different elements combine, we have something called silicon dioxide (SiO2). This SiO2 is no longer an element but a mineral. In other words, when we combine elements together we get minerals. The SiO2 mineral is what we commonly call sand.
Minerals Can Combine to Form a Rock
One grain of sand contains the SiO2 mineral. Now, imagine a plastic container containing some water. If we fill this container to the brim with sand grains and leave it undisturbed long enough, we’ll discover that the sand grains have joined together and become one compact solid mass. This compact solid mass of SiO2 minerals is sandstone.
Figure 1. A sandstone rock made of the mineral sand that is made of the elements silicon and oxygen.
Sandstone is a rock. Armed with this understanding, we can go ahead to define rocks. A rock is formed when mineral grains become compacted over time into a solid mass. The minerals in a rock could be the same throughout the rock, or the rock could be formed from a combination of different minerals. What is important here is that they are consolidated or compacted into one solid mass; else it is not a rock. If we broke down this sandstone into tiny bits using a hammer, it returns back into sand (the mineral SiO2) and cannot be called a rock anymore.
How Rocks are Formed - The Rock Cycle
The natural process to form rocks on earth requires millions of years. (To learn more about the earth's natural processes, read Earth Matters in Oil and Gas.) Let’s start with igneous rocks. Inside volcanoes are liquid minerals called magma. The heat within the earth is high enough to melt minerals into liquid (magma) form. On the surface the temperature is much colder compared to that found several miles beneath the earth's surface. When volcanoes erupt, the magma flows upwards to the surface and empties itself on the sides of the volcano.
Figure 2. Diagram of how rocks are formed naturally.
Remember that the temperature at the surface is much cooler. Magma that has found its way to the surface is called lava. Due to the much colder surface temperature, the lava cools from the liquid minerals that existed inside the earth to solid minerals on the surface. We've already established that when minerals become compacted they form a rock. A volcanic igneous rock forms from this volcanic eruption when the lava cools. Sometimes the magma cools to a sufficiently low temperature before reaching the surface and solidifies. This type of igneous rock called intrusive igneous rock.
It is possible for a rock to break down into minerals, like the sandstone example that was broken into tiny bits using a hammer. For the volcanic igneous rock, instead of a hammer, natural elements such as wind, water and ice can break the rock into tiny bits and carry these bits away to other places on the earth.
Although we could finish grinding a piece of sandstone within minutes, nature requires millions of years. Weathering is the gradual process of rocks naturally breaking down over time.
Let’s consider how water can break down a rock. When it rains, a raindrop that strikes the rock can wear away small rock minerals located on the surface. As more raindrops continually strike the same point over time, they wear away enough minerals from the rock to create a groove or a small hole. This groove will now cause even more damage to the rock because it serves as a container to collect water.
Pockets of water will collect in the space now created within the solid rock mass. This water will gradually weaken the bond between minerals and cause them to easily break down and be carried away more readily. The process by which these broken rock bits are carried away is called erosion.
The rock bits will be carried as far as possible until the force of the carrying agent (e.g., running water) weakens, at which point the rock bits will be said to have been deposited.
These rock bits form part of what is known as sediments. When another set of sediment is carried downstream during the next rainstorm, they will be deposited on top of the former set, and thus layers of sediments begin to form. This can continue until we have several layers of deposited sediments, one lying on top of the other. Over time, the pressure from the upper layers becomes high enough to compact the lower layers, becoming one compact solid rock.
The process by which downward pressure compacts sediments into rocks is called lithification, and rocks formed this way are called sedimentary rocks. Sedimentary rocks are formed from the weathering, erosion, deposition and lithification of fragments of other rocks. During lithification, water present in the depositional environment may also react with the rock minerals to help form cement that binds the rock grains together even more tightly.
We have seen an example of how igneous rock can become sedimentary rock due to weathering and erosion of rock fragments over time. One thing to note though is that the deeper we go down into the earth, the hotter it becomes.
If sedimentary rock is buried deep enough, the temperature and pressure at that depth could become so high that the minerals present in the sedimentary rock become altered or change due to the great heat and pressure. The rock formed when the minerals of other rocks are altered this way is called metamorphic rock; in other words, the rock undergoes a form of metamorphosis and is now different from what the way it was previously. If the metamorphic rock is pushed deep enough, it will warm to the temperature at which the minerals in the rock become liquid once more, waiting to be pushed to the surface to form igneous rock.
Figure 3. Diagram of the rock cycle from igneous to sedimentary to metamorphic.
Why Hydrocarbons are found in Sedimentary Rocks
The oil and gas industry is most concerned with sedimentary rocks because hydrocarbons are typically found in sedimentary rocks. Looking at the way these rocks are formed, we can see that of all rock types, it is easier for organic matter to be found in sedimentary rocks than in any other rock type.
There are several mechanisms by which this may happen:
- The organic matter can be carried with the sediments during rainstorms and deposited.
- In oceanic environments, when organic matter such as phytoplankton die and fall to the sea floor, sediments are deposited on top of them and they become trapped in the sedimentary rock.
- One kind of sedimentary rock, limestone, is formed from the precipitation of minerals in the sea instead of weathering and erosion of sediments. These minerals precipitate out of the water and trap dead organic matter that falls to the seabed.
Under the right conditions, sedimentary rocks may contain hydrocarbon reservoirs of oil and gas.