The Rock Cycle

The Rock Cycle: Uniformitarianism and Recycling

BY ANNE E. EGGER, PH.D.

Terms you should know

• Fossil: the preserved impression or remains of an animal or plant whose living tissue has been replaced by minerals.

• Landscape: the natural scenery of a region; a collection of landforms in an area.

• Recycle: to cause to appear again in a new form or function; adapt to new use, form, or function.

Introduction

We all see changes in the landscape around us, but your view of how fast things change is probably determined by where you live. If you live near the coast, you see daily, monthly, and yearly changes in the shape of the coastline. Deep in the interior of continents, change is less evident – rivers may flood and change course only every 100 years or so. If you live near an active fault zone or volcano, you experience infrequent but catastrophic events like earthquakes and eruptions. Throughout human history, different groups of people have held to a wide variety of beliefs to explain these changes. Early Greeks ascribed earthquakes to the god Poseidon expressing his wrath, an explanation that accounted for their unpredictability. The Navajo view processes on the surface as interactions between opposite but complementary entities: the sky and the Earth. Most 17th century European Christians believed that the Earth was essentially unchanged from the time of creation. When naturalists found fossils of marine creatures high in the Alps, many devout believers interpreted the Old Testament literally and suggested that the perched fossils were a result of the biblical Noah's flood.

Uniformitarianism

In the mid-1700s, a Scottish physician named James Hutton began to challenge the literal interpretation of the Bible by making detailed observations of rivers near his home. Every year, these rivers would flood, depositing a thin layer of sediment in the floodplain. It would take many millions of years, reasoned Hutton, to deposit a hundred meters of sediment in this fashion, not just a few weeks allowed by the Biblical flood. Hutton called this the principle of uniformitarianism: Processes that occur today are the same ones that occurred in the past to create the landscape and rocks as we see them now. By comparison, the strict biblical interpretation, common at the time, suggested that the processes that had created the landscape were complete and no longer at work.

Figure 1: This image shows how James Hutton first envisioned the rock cycle.

Uniformitarianism (Continued)

Hutton argued that for uniformitarianism to work over very long periods, Earth materials had to be constantly recycled. If there were no recycling, mountains would erode (or continents would decay, in Hutton's terms), the sediments would be transported to the sea, and eventually, the surface of the Earth would be perfectly flat and covered with a thin layer of water. Instead, those sediments once deposited in the sea must be frequently lifted back up to form new mountain ranges. Recycling was a radical departure from the prevailing notion of a largely unchanging Earth. As shown in Figure 1, Hutton first conceived of the rock cycle as a process driven by Earth's internal heat engine. The heat caused sediments deposited in basins to be converted to rock, heat caused the uplift of mountain ranges, and heat contributed in part to the weathering of rock. While many of Hutton's ideas about the rock cycle were either vague (such as "conversion to rock") or inaccurate (such as heat causing decay), he made the important first step of putting diverse processes together into a simple, coherent theory.

Uniformitarianism (Continued)

Hutton's ideas weren't immediately embraced by the scientific community, largely because he was reluctant to publish. He was a far better thinker than a writer – once he did get into print in 1788, few people were able to make sense of his highly technical and confusing writing (to learn more about Hutton and see a sample of his writing, visit the Resources for this module). His ideas became far more accessible after his death with the publication of John Playfair's "Illustrations of the Huttonian Theory of the Earth" (1802) and Charles Lyell's "Principles of Geology" (1830). By that time, the scientific revolution in Europe had led to widespread acceptance of the once-radical concept that the Earth was constantly changing. A far more complete understanding of the rock cycle developed with the emergence of plate tectonics theory in the 1960s (see our Plate Tectonics I module). Our modern concept of the rock cycle is fundamentally different from Hutton's in a few key aspects: We now largely understand that plate tectonic activity determines how, where, and why uplift occurs, and we know that heat is generated in the interior of the Earth through radioactive decay and moved out to the Earth's surface through convection. Together, uniformitarianism, plate tectonics, and the rock cycle provide a powerful lens for looking at the Earth. This allows scientists to look back into Earth’s history and make predictions.

Multiple Choice

Comprehension Checkpoint

If Earth's materials weren't recycled then

• Earth would be flat and covered with water.

• mountain ranges would continue to grow.

The Rock Cycle

The rock cycle consists of a series of constant processes through which Earth materials change from one form to another over time. As within the water cycle and the carbon cycle, some processes in the rock cycle occur over millions of years and others occur much more rapidly. There is no real beginning or end to the rock cycle, but it is convenient to begin exploring it with magma. You may want to open the rock cycle schematic in Figure 2 and follow along in the sketch; click on the diagram to open it in a new window.

Figure 2: A schematic sketch of the rock cycle. In this sketch, boxes represent Earth materials and arrows represent the processes that transform those materials. The processes are named in bold next to the arrows. The two major sources of energy for the rock cycle are also shown; the sun provides energy for surface processes such as weathering, erosion, and transport, and the Earth's internal heat provides energy for processes like subduction, melting, and metamorphism. The diagram's complexity reflects the complexity of the rock cycle. Notice that there are many possibilities at any step along the way.

The Rock Cycle (Continued)

Magma, or molten rock, forms only at certain locations within the Earth, mostly along plate boundaries. (It is a common misconception that the entire interior of the Earth is molten, but this is not the case. See our Earth Structure module for a more complete explanation.) When magma is allowed to cool, it crystallizes, much the same way that ice crystals develop when water is cooled. We see this process occurring in places like Iceland, where magma erupts out of a volcano and cools on the surface of the Earth, forming a rock called basalt on the flanks of the volcano (Figure 3). But most magma never makes it to the surface and it cools within Earth's crust. Deep in the crust below Iceland's surface, the magma that doesn't erupt cools to form gabbro. Rocks that form from cooled magma are called igneous rocks; intrusive igneous rocks if they cool below the surface (like gabbro), extrusive igneous rocks if they cool above (like basalt).

Figure 3: This picture shows a basaltic eruption of Pu'u O'o, on the flanks of the Kilauea volcano in Hawaii. The red material is molten lava, which turns black as it cools and crystallizes.

Uplift, weathering, and erosion

Rocks like basalt are immediately exposed to the atmosphere and weather. Rocks that form below the Earth's surface, like gabbro, must be uplifted and all of the overlying material must be removed through erosion. This will enable them to be exposed. In either case, as soon as rocks are exposed to the Earth's surface, the weathering process begins. Physical and chemical reactions caused by interaction with air, water, and biological organisms cause the rocks to break down. Once rocks are broken down, wind, moving water, and glaciers carry pieces of the rocks away through a process called erosion. Moving water is the most common agent of erosion – the muddy Mississippi, the Amazon, the Hudson, the Rio Grande. All of these rivers carry tons of sediment weathered and eroded from the mountains of their headwaters to the ocean year. The sediment carried by these rivers is deposited and continually buried in floodplains and deltas. The US Army Corps of Engineers is kept busy dredging the sediments out of the Mississippi to keep shipping lanes open (see Figure 4).

Figure 4: Photograph from space of the Mississippi Delta. The brown color shows the river sediments and where they are being deposited in the Gulf of Mexico.

Multiple Choice

Comprehension Checkpoint

Erosion is caused primarily by

• wind.

• moving water.

Sedimentary rocks

Under natural conditions, the pressure created by the weight of the younger deposits compacts the older, buried sediments. As groundwater moves through these sediments, minerals like calcite and silica precipitate out of the water and coat the sediment grains. These precipitants fill in the pore spaces between grains and act like cement, gluing individual grains together. The compaction and cementation of sediments create sedimentary rocks like sandstone and shale, which are forming right now in places like the very bottom of the Mississippi delta. Because deposition of sediments often happens in seasonal or annual cycles, we often see layers preserved in sedimentary rocks when they are exposed (Figure 5). For us to see sedimentary rocks, however, they need to be uplifted and exposed by erosion. Most uplift happens along plate boundaries where two plates are moving towards each other and causing compression. As a result, we see sedimentary rocks that contain fossils of marine organisms (and therefore must have been deposited on the ocean floor) exposed high up in the Himalaya Mountains – this is where the Indian plate is running into the Eurasian plate.

Figure 5: The Grand Canyon is famous for its exposures to great thicknesses of sedimentary rocks.

Multiple Choice

Comprehension Checkpoint

Most uplift happens

• along plate boundaries.

• in fast-moving rivers.

“Cooked” rocks

If sedimentary rocks or intrusive igneous rocks aren't brought to the Earth's surface by uplift and erosion, they may experience even deeper burial and be exposed to high temperatures and pressures. As a result, the rocks begin to change. Rocks that have changed below the Earth's surface due to exposure to heat, pressure, and hot fluids are called metamorphic rocks. Geologists often refer to metamorphic rocks as "cooked" because they change in much the same way that cake batter changes into a cake when heat is added. Cake batter and cake contain the same ingredients, but they have very different textures, just like sandstone, a sedimentary rock, and quartzite, its metamorphic equivalent. In sandstone, individual sand grains are easily visible and often can even be rubbed off; in quartzite, the edges of the sand grains are no longer visible, and it is a difficult rock to break with a hammer, much less rubbing pieces off with your hands. Some of the processes within the rock cycle, like volcanic eruptions, happen very rapidly. However, others happen very slowly, like the uplift of mountain ranges and weathering of igneous rocks. Importantly, there are multiple pathways through the rock cycle. Any kind of rock can be uplifted and exposed to weathering and erosion; any kind of rock can be buried and metamorphosed. As Hutton correctly theorized, these processes have been occurring for millions and billions of years to create the Earth as we see it: a dynamic planet.

Multiple Choice

Comprehension Checkpoint

All processes in the rock cycle take millions of years.

• true

• false

A North American example

The rock cycle isn’t just theoretical; we can see all of these processes occurring at many different locations and many different scales all over the world. As an example, the Cascade Range in North America illustrates many aspects of the rock cycle within a relatively small area, as shown in Figure 6.

Figure 6: Cross-section through the Cascade Range in Washington state. Image modified from the Cascade Volcano Observatory, USGS.

A North American example (Continued)

The Cascade Range in the northwestern United States is located near a convergent plate boundary. This is where the Juan de Fuca Plate, which consists mostly of basalt saturated with ocean water, is subducted or pulled underneath the North American plate. As the plate descends deeper into the Earth, heat and pressure increase, and the basalt is metamorphosed into a very dense rock called eclogite. All of the ocean water that had been contained within the basalt is released into the overlying rocks, but it is no longer cold ocean water. It too has been heated and contains high concentrations of dissolved minerals, making it highly reactive, or volatile. These volatile fluids lower the melting temperature of the rocks, causing magma to form below the surface of the North American plate near the plate boundary. Some of that magma erupts out of volcanoes like Mt. St. Helens, cooling to form a rock called andesite, and some cool beneath the surface, forming a similar rock called diorite. Storms coming off of the Pacific Ocean cause heavy rainfall in the Cascades, weathering and eroding the andesite. Small streams carry the weathered pieces of the andesite to large rivers like the Columbia and eventually to the Pacific Ocean, where the sediments are deposited. Continual deposition of sediments near the deep oceanic trench results in the formation of sedimentary rocks like sandstone.

A North American example (Continued)

Eventually, some sandstone is carried down into the subduction zone, and the cycle begins again (see the Experiment! section in the Resources for this module). The rock cycle is inextricably linked not only to plate tectonics but to other Earth cycles as well. Weathering, erosion, deposition, and cementation of sediments all require the presence of water, which moves in and out of contact with rocks through the hydrologic cycle; thus weathering happens much more slowly in a dry climate like the desert southwest than in the rainforest (see our module The Hydrologic Cycle for more information). Burial of organic sediments takes carbon out of the atmosphere, part of the long-term geological component of the carbon cycle (see our module The Carbon Cycle module); many scientists today are exploring ways we might be able to take advantage of this process and bury additional carbon dioxide produced by the burning of fossil fuels (see News & Events in Resources). The uplift of mountain ranges dramatically affects global and local climates by blocking prevailing winds and inducing precipitation. The interactions between all of these cycles produce the wide variety of dynamic landscapes we see around the globe.

Did you know?

Did you know that interactions between natural cycles produce the dynamic landscapes we see across the globe – and can even change the global climate? One significant cycle on Earth is the rock cycle, which has neither a beginning nor an end. Rather, through the rock cycle, Earth’s materials just change from one form to another.

Summary

Earth’s materials are in constant flux. Some processes that shape the Earth happen quickly; others take millions of years. This module describes the rock cycle, including the historical development of the concept. The relationship between uniformitarianism, the rock cycle, and plate tectonics is explored in general and through the specific example of the Cascade Range in the Pacific Northwest.

Key Concepts

• The rock cycle is the set of processes by which Earth materials change from one form to another over time.

• The concept of uniformitarianism, which says that the same Earth processes at work today have occurred throughout geologic time, helped develop the idea of the rock cycle in the 1700s.

• Processes in the rock cycle occur at many different rates.

• The rock cycle is driven by interactions between plate tectonics and the hydrologic cycle.

Explanation

Rocks that form below Earth’s surface must be uplifted. Then the overlying material must be removed through erosion in order for the rocks to be exposed. This wearing-away process can happen by the action of water, wind, or glacial ice.

Multiple Choice

If a sedimentary rock ISN'T immediately uplifted, which of the following is most likely to happen?

• weathering

• melting

• metamorphism

Explanation

If sedimentary rocks aren't brought to the Earth’s surface by uplift, they become buried deeper. This results in metamorphic rocks, rocks that have changed below the Earth’s surface due to exposure to heat, pressure, and hot fluids. Geologists often refer to metamorphic rocks as “cooked” because they change in much the same way that cake batter changes into a cake when heat is added.

Multiple Choice

Which of the following locations describes one example of a place where igneous rocks are forming?

• in a streambed near a volcano

• on the surface of the sea floor

• everywhere beneath the earth's surface

• within the crust below an active volcano

Explanation

Magma, or molten rock, forms only at certain locations within the Earth. Rocks that form from cooled magma are called igneous rocks. Some magma erupts out of a volcano and cools on the surface of the Earth, but most magma never makes it to the surface. Rather, it cools within the Earth’s crust.

Multiple Choice

Any kind of rock can melt to form magma.

• true

• false

Explanation

Through the rock cycle, Earth’s materials change from one form to another. Earth's internal heat provides energy to melt rock. Magma is rock in a melted form.

Multiple Select

Which of the following processes in the rock cycle involve water?

• cementation

• uplift

• erosion

• metamorphism

• weathering

Explanation

Weathering, erosion, deposition, cementation, and melting all require the presence of water, which moves in and out of contact with rocks through the hydrologic cycle.

Multiple Choice

What kind of rock is forming in the picture?

• extrusive igneous

• intrusive igneous

• metamorphic

• sedimentary

Explanation

Rock that forms from cooled magma is called igneous rock. There are 2 types of igneous rock: (1) intrusive igneous rock forms if magma cools below Earth’s surface; (2) extrusive igneous rock forms when magma cools above Earth’s surface, such as on the flanks of a volcano. The photo shows rock that has formed from magma that has cooled above Earth’s surface.

Multiple Select

James Hutton's ideas about uniformitarianism and the rock cycle initially met resistance among scientists of the time because

• He didn't explain his ideas very well.

• most people believe that Earth was billions of years old.

• everyone understood how heat was generated in Earth's core.

• most people didn't believe that Earth was millions of years old.

• no one understood how heat was generated in Earth's core.

Explanation

Hutton's ideas weren't broadly accepted by his contemporaries for a variety of reasons: a strict biblical interpretation of Earth's age, a lack of knowledge about Earth's interior heat engine, and Hutton's own technical and confusing writing.

Multiple Choice

The principle of uniformitarianism is best described by which of the following statements?

• Earth has remained uniform since its beginning.

• All changes on Earth happen at a uniform rate.

• Any process that happens now also could have happened in the past.

• Earth's internal heat supplies all of the energy for change on the surface.

• Earth has remained uniform since its ending.

Explanation

According to the principle of uniformitarianism, the physical processes that are shaping the world today – weathering, volcanic eruptions, tectonic plate movement, etc. – have been working throughout the geological past as well.

Multiple Choice

All of the processes involved in the rock cycle occur too slowly for humans to see.

• true

• false

Explanation

While some of the processes in the rock cycle happen very slowly, like the uplift of mountain ranges, others, like volcanic eruptions, occur very rapidly.

Poll

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