Who came up with the idea of the rock cycle?
The rock cycle is attributed to James Hutton (1726-1797), the “father of geology” who explored and documented the landscape of the British Isles.
Hutton proposed the principle of uniformitarianism, the idea that the processes that shape the Earth's crust today also occurred in the past. This brought about the revolutionary idea that given how long it takes for geologic processes to occur, the Earth must be very old for all existing landforms to have been formed, not the 6,000 years as was thought and based on biblical interpretations.
Hutton recognized that the history of Earth could be determined by understanding how processes such as erosion and sedimentation work in the present.
Hutton proposed the principle of uniformitarianism, the idea that the processes that shape the Earth's crust today also occurred in the past. This brought about the revolutionary idea that given how long it takes for geologic processes to occur, the Earth must be very old for all existing landforms to have been formed, not the 6,000 years as was thought and based on biblical interpretations.
Hutton recognized that the history of Earth could be determined by understanding how processes such as erosion and sedimentation work in the present.
Hutton's "unconformity"He wondered at patterns he saw in rocks. For example, in the image to the right, you see a piece of rock that has different striation patterns. The top is horizontal, while the bottom is almost vertical. The drawing was made in 1787, the photo was taken in 2003.
Hutton called these types of non-matching patterns in rocks and mountains "unconformities." To the right is "Hutton's Unconformity" in Scotland. He saw this as evidence of uniformitarianism. Below is another example of unconformity in Missouri. You can see them almost everywhere. |
The Rock Cycle
The rock cycle is the result of energy flowing and matter cycling. The energy is from both the Sun and Earth’s hot interior. The flowing of energy and the cycling of matter create changes in how Earth looks on its surface.
In the water cycle, water undergoes physical changes when it goes from solid to liquid to gas. Rocks also undergo physical changes. For example, lava is an example of liquid rock. It cools down to form a type of solid rock, called "basalt." You can see a basalt formation in the picture to the right. In the picture below, you can see a model of the rock cycle. There are three main types of rock, highlighted in green: sedimentary rock, metamorphic rock, and igneous rock. |
These rocks are formed by several processes: (1) weathering and erosion, (2) transport and deposition, (3) sedimentation, (4) compaction and cementation, (5) temperature and pressure, (6) melting, (7) crystallization, and (8) uplift.
It sounds like a lot of steps, but it'll make sense as we move through the cycle and learn about each one. The rock cycle shows how any rock can undergo changes to become another rock. Rocks are continuously being changed from one form to another. The rock cycle is very slow. Rocks might take tens of thousands of years to change into another type of rock.
It sounds like a lot of steps, but it'll make sense as we move through the cycle and learn about each one. The rock cycle shows how any rock can undergo changes to become another rock. Rocks are continuously being changed from one form to another. The rock cycle is very slow. Rocks might take tens of thousands of years to change into another type of rock.
Sources of Energy: Sun & Earth's Interior
There are two sources of energy for the rock cycle: the sun and Earth's interior.
1. The sun: In the water cycle, the sun is the ultimate source of energy. This energy, in the form of heat, melts and evaporates water to get it moving. As water moves over the Earth's surface, it scrapes, carries, and deposits pieces of rock around. This is essential to the rock cycle. So the sun provides a source of energy for the rock cycle by driving the water cycle.
2. Earth's interior: The inside of our planet is super hot. It is so hot that rock is constantly in liquid form. The heat from inside the earth, and the cooler temperature near the surface move this liquid rock around and around. This is known as convection.
1. The sun: In the water cycle, the sun is the ultimate source of energy. This energy, in the form of heat, melts and evaporates water to get it moving. As water moves over the Earth's surface, it scrapes, carries, and deposits pieces of rock around. This is essential to the rock cycle. So the sun provides a source of energy for the rock cycle by driving the water cycle.
2. Earth's interior: The inside of our planet is super hot. It is so hot that rock is constantly in liquid form. The heat from inside the earth, and the cooler temperature near the surface move this liquid rock around and around. This is known as convection.
Convection
The model on the right shows convection currents in Earth's mantle. Super hot, liquid rock rises to the surface. The rock molecules spread apart as they are heated (just like in evaporation!). As the liquid rock nears the surface where it is not as hot, the liquid rock cools down a bit. The cooling causes the liquid rock to condense (just like water vapor condenses into clouds). This makes the liquid rock heavy, and it sinks down. This is an on-going cycle of getting hot and rising, and cooling down and sinking.
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The circular motion of convection inside Earth's mantle
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Plate Tectonics
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The video shows a computer simulation, or computer model of convection inside the planet. Continents are on huge solid rock "plates" that move around because of this convection. This is called plate tectonics, which causes earthquakes to happen and volcanos to erupt. The important thing to understand is that the continual movement of liquid and solid rock is an ongoing cycle that started billions of years ago. This movement is caused by the heat inside the planet. |
Three Types of Rock
1. Igneous Rock
Igneous rock: Hot, liquid rock inside the planet is called magma. Magma can cool to solid form inside the earth's crust, as well as on the surface. Rocks that are formed from the cooling of magma or lava are called igneous rocks.
There are two kinds of igneous rock:
There are two kinds of igneous rock:
- Intrusive igneous rock: If magma cools inside the crust, it is called "intrusive igneous rock." This cooling process is quite slow, so the rock has time to form crystals. This is crystallization of magma, and it looks coarse (grainy). Remember crystallization means to take on a permanent form with crystal structure. An example would be granite (see examples below). Through the movement of rock and plate tectonics, these rocks have been pushed up to the surface in the form of mountains.
- Extrusive igneous rock: If magma rises up through a volcano and then cools on the surface, it is called "extrusive igneous rock." This cooling is quicker, and the rocks look more fine-grained. An example here would be basalt or pumice. If lava explodes and cools rapidly in the air, there is no time for crystals to form. Obsidian is an example of this rapid cooling (see examples below).
igneous rock: rocks formed from the cooling and crystallization of magma and lava.
Examples of intrusive igneous rock
Examples of extrusive igneous rock
Weathering
Weathering: Mountains and rocks on the surface of Earth are broken down by wind, water, sand, and chemicals. This is called weathering.
The broken bits rock are called sediments. Weathering also involves erosion and deposition.
The broken bits rock are called sediments. Weathering also involves erosion and deposition.
- Erosion: These tiny bits, or sediments, can be moved around and transported by wind, water, and gravity. This is called erosion.
- Deposition: When the sediments are dropped in a new location, they are said to be deposited. This is called deposition.
Sedimentary Rock
Sedimentary rock: Sediments pile on top of each other. Over a very long period of time, they are pushed close together tightly by the pressure of the rocks above. This is called compaction and cementation. It is like they are being glued together, or cemented together, by pressure and time.
Sedimentary rocks can look grayish or reddish (due to iron oxides). They are fine-grained, and fossils are most commonly found in sedimentary rocks (see examples below).
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- sedimentary rock: formed when sediments are compacted and cemented together.
- weathering: the breaking down of rocks into sediments by wind, water, sand, and chemicals.
- erosion: sediments moved around and transported by wind, water, ice, and gravity.
Examples of sedimentary rock
Metamorphic Rock
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Metamorphic rock: You may have heard of the term "morph" before. It means to change or transform. Metamorphic rocks are when rocks are changed by heat and pressure. Any type of rock (igneous, sedimentary, and metamorphic) can form a new metamorphic rock if given enough time, heat, and pressure.
Pressure and heat change the molecules of a rock's minerals so it becomes an entirely different rock. They have wavy patterns, or they may have unusual minerals (see images below). The change in the appearance of these rocks takes an extremely long time, and these rocks make up a large part of Earth's crust. |
Rock Cycle Recap |
Bill Nye on Metamorphic Rock |
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NGSS MS-ESS2-1: Develop a model to describe the cycling of Earth’s materials and the flow of energy that drives this process.
Photo used under Creative Commons from john.purvis