The Rock Cycle: _{Uniformitarianism and recycling}

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 the 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.

Hutton argued that in order for uniformitarianism to work over very long periods of time, 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. 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.

Hutton's ideas were not immediately embraced by the scientific community, largely because he was reluctant to publish. He was a far better thinker than 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 important 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, allowing scientists to look back into Earth history and make predictions about the future.

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.

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).

Uplifting, 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 in order for them to be exposed. In either case, as soon as rocks are exposed at 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 every year. The sediment carried by these rivers is deposited and continually buried in floodplains and deltas. In fact, the US Army Corps of Engineers is kept busy dredging the sediments out of the Mississippi in order to keep shipping lanes open (see Figure 4).

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 as cement, gluing individual grains together. The compaction and cementation of sediments creates 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). In order 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.

“Cooked” rocks

If sedimentary rocks or intrusive igneous rocks are not 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 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, while 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.

A North American example

The rock cycle is not just theoretical; we can see all of these processes occurring at many different locations and at 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.

The Cascade Range in the northwestern United States is located near a convergent plate boundary, where the Juan de Fuca plate, which consists mostly of basalt saturated with ocean water is being 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 cools 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. 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 climate 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.