Rocks and the Rock Cycle


Rocks, rocks, rocks.  Weathering is all about rocks.  Rocks, rocks, rocks.  To understand weathering, we must first learn about rocks.  

Rocks are the most common of all materials on earth.  They are familiar to everyone.  You may recognize rocks in the form of a mountain near your hometown, the gravel in a driveway, the cliffs lining your favorite fishing hole, or the granite or sandstone or limestone your fingers and toes cling to at your favorite climbing area.

Rocks should be considered products of their environment - when their environment changes, so will the ways the rock weathers and erodes, or is otherwise shaped.

Rocks form in one of two distinct environments:  either below the surface of the earth or at its surface.  Rocks are composed of minerals.  Minerals are naturally occurring, inorganic substances, which have an ordered internal structure giving them a specific appearance.  Interestingly, ice is a mineral.  Some of the more common rock-forming minerals are quartz, feldspar, pyroxene, hornblende, and olivine.  Minerals should not be confused with rocks.  For example, granite is a rock.  it is composed of several minerals such as quartz, mica, and feldspar.

The triad (don't you love that word, triad?) of rocks that compose the planet are igneous, sedimentary, and metamorphic.  

IMPORTANT!  Don't PANIC because of the MASSIVE amount of text on this Web page...  You can learn about rock types and other things in great detail below.  But here's the real important stuff about rocks and rock types that are integral to understanding this module:


In this Module, you will encounter a LOT of sandstone.  Sandstone is a sedimentary rock that is highly resistant to weathering.  Other sedimentary rocks you will encounter are mudstone and siltstone.  These are soft and easily weathered (lots in the Painted Desert).  Yet another sedimentary rock you will see is limestone.  Limestone is composed of calcium carbonate, which is water soluble (it dissolves in water).  Therefore, the limestone you find in a dry area will remain a rock a lot longer than limestone you find in a MOIST area.

Limestone in Missouri.  Jacks Fork River.

Soft mudstone/siltstone in the Painted Desert, Arizona.

Sandstone on the Colorado Plateau.  Brain Morrato, Pete Ogden, and Brandon Vogt for scale.

Regarding metamorphic rocks, there really aren't many of these on the Colorado Plateau.. only in the bottom of the Grand Canyon!  So, good thing for you, we are really not going to cover this rock type in this Module.

You will see a few igneous rocks in this Module...  You see granite (intrusive igneous) and basalt (extrusive igneous).  The granite you will see is high up in the La Sal Mountains and the basalt you will see sits as cap rocks on top of softer, underlying sediments.  A cap rock is a comparatively more resistant rock that protects the rocks beneath it from weathering and eroding.

Basalt "sculpted" by water.

Granite broken apart by frost.

The key to understanding rock weathering is to realize that rocks weather at different rates and into different forms based on their chemical composition and based on what weathering processes dominate the area.

For WAY more information on rocks than you are required to know for this Module, visit the following links:

Jump to igneous  |  Jump to metamorphic  |  Jump to sedimentary

THE ROCK CYCLE:  (some content and the diagram below borrowed from NASA)

The rock cycle best explained as the relationship between earth’s internal and external processes.  Weathering (breaking down rock) and erosion (transporting rock material) at or near the earth's surface breaks down rocks into small and smaller pieces.  These smaller pieces of rock (such as sand, silt, or mud) can be deposited as sediments that, after hardening, or lithifying, become sedimentary rocks. Extreme pressure from burial, increasing temperature at depth, and a lot of time, can alter any rock type to form a metamorphic rock.   If the newly formed metamorphic rock continues to heat, it can eventually melt and become molten (magma). When the molten rock cools it forms an igneous rock. Metamorphic rocks can form from either sedimentary or igneous rocks. The sedimentary particles from which a sedimentary rock is formed can be derived from a metamorphic, an igneous, or another sedimentary rock. All three rock types can be melted to form a magma. Thus, the cycle has continued over the ages, constantly forming new rocks, breaking those down in various ways, and forming still younger rocks.  Rocks at the surface of the earth range in age from over three billion years old to a few hundred years old. 

Igneous rock can change into sedimentary rock or into metamorphic rock. Sedimentary rock can change into metamorphic rock or into igneous rock. Metamorphic rock can change into igneous or sedimentary rock.

Igneous rock forms when magma cools and makes crystals. Magma is a hot liquid made of melted minerals. The minerals can form crystals when they cool. Igneous rock can form underground, where the magma cools slowly. Or, igneous rock can form above ground, where the magma cools quickly.

Notice how many time "weathering" and "erosion" (green arrows) appear in the above diagram?

When it pours out on Earth's surface, magma is called lava. Yes, the same liquid rock matter that you see coming out of volcanoes.  Igneous rock that pours out onto the Earth's surface is called igneous extrusive, whereas igneous rock that cools beneath the surface is called igneous intrusive.

On Earth's surface, wind and water can break rock into pieces (weathering!). They can also carry rock pieces to another place (erosion)!. Usually, the rock pieces, called sediments, drop from the wind or water to make a layer. The layer can be buried under other layers of sediments. After a long time the sediments can be cemented together to make sedimentary rock. In this way, igneous rock can become sedimentary rock.

All rock can be heated. But where does the heat come from? Inside Earth there is heat from pressure (push your hands together very hard and feel the heat). There is heat from friction (rub your hands together and feel the heat). There is also heat from radioactive decay (the process that gives us nuclear power plants that make electricity).

So, what does the heat do to the rock? It bakes the rock.

Remember, all rocks are made up of mineral crystals, or pieces of other rocks made up of crystals.  Baked rock does not melt, but it does change. It forms crystals. If it has crystals already, it forms larger crystals. Because this rock changes, it is called metamorphic. Remember that a caterpillar changes to become a butterfly. That change is called metamorphosis. Metamorphosis can occur in rock when they are heated to 300 to 700 degrees Celsius.

When Earth's tectonic plates move around, they produce heat. When they collide, they build mountains and metamorphose the rock.

The rock cycle continues. Mountains made of metamorphic rocks can be broken up and washed away by streams. New sediments from these mountains can make new sedimentary rock.

The rock cycle never stops.


Igneous Rocks:  Igneous rocks are a type of rock formed from extremely hot (2,200 degrees F) molten masses known as magma. Magma is present everywhere below the surface of the earth’s crust. Generally, magma lies about 90 miles below the surface. In certain places, such as Yellowstone National Park, the magma is as close as 40 miles below the surface. On average, every 100 feet you dig down into the earth, the temperature will increase about 1.4 degrees F (this is a bit scary - keep this in mind the next time you visit a coal mine in Pennsylvania or the next time you descend into a lava tube near Flagstaff!. 

Sometimes magma forces its way up to the surface through a vent (such as a volcano) and spills onto the surface. This happened near Flagstaff, Arizona at Sunset Crater less than 1,000 years ago. Once magma comes out onto the surface of the earth it is called lava, and it cools rapidly at the surface. The resulting rock is called extrusive igneous rock (it is extrusive because the rock “extruded” out of the ground onto the surface). Extrusive igneous rocks can be distinguished by their small crystal sizes. In all extrusive igneous rock, it is nearly impossible to detect crystals without the aid of a microscope. The faster the rock cools, the smaller the crystals. Some extrusive igneous rocks cool so quickly that they have a glassy texture. Common extrusive igneous rocks are: andesite, basalt, dacite, pumice, rhyolite, and obsidian. The crystals are so fine in obsidian, that paleo-people and modern surgeons alike use/used the rock to manufacture razor-sharp tools.

However, a much greater volume of this fluid magma never reaches the earth’s surface – it only intrudes into the upper part of the earth’s crust and solidifies. The rocks that form by this process are known as intrusive igneous rocks (it is intrusive because the rock did not “extrude” out of the ground onto the surface). Intrusive igneous rocks solidify over a period of thousands of years. This slow cooling rate allows better development of mineral crystals. Intrusive igneous rocks will have eye visible crystals and will appear coarse-grained. Igneous intrusive rocks eventually will become exposed at the surface of the earth by erosion of the overlying material. Common intrusive igneous rocks are: granite, diorite, gabbro, and peridotite.

The La Sal Mountains were formed by widespread igneous activity that began about 40 million years ago. Caldera explosions erupted thousands of cubic miles of volcanic rocks from several locations. Volcanoes spewed ash and lava. For 20 million years these extrusive volcanic rocks smoothed the landscape, filling depressions with accumulations of ash, flows, and debris literally miles thick. These mostly pastel-colored extrusive rocks still blanket much of the high areas of central and southwestern Utah. Not all of the molten rising igneous material erupted as volcanic rocks; some material, along with its mineral-bearing fluids, congealed in the earth's crust. Several of these intruded masses having been exposed by erosion or encountered out by exploration drilling became great mining districts, such as at Alta, Brighton, Bingham, Park City, and Cedar City. In the Colorado Plateau, bodies of intrusive rocks domed the overlying sedimentary rocks to form the Abajo and Henry Mountains (as well as the La Sal Mountains). NOTE:  This paragraph on laccoliths borrowed from here

Metamorphic Rocks: morphos is the Greek word for ``form'' or ``shape'', and metamorphism means ``change of form''.

Inside the Earth, the temperature and pressure are much higher than on the surface, and these effects, along with fluids percolating through rock, can change the chemistry, mineralogy, and/or structure of the rocks, without melting them. Rocks which have undergone these sorts of changes are called metamorphic rocks.

Agents of Matamorphosis

The three primary agents which metamorphose rock are temperature, pressure, and fluids.

1) Temperature

High temperatures can change rock by changing the structure of the minerals which make up the rocks; changing the structure of the minerals changes them into new minerals (remember the definition of a mineral). Two primary sources for high temperatures inside the Earth are:

2) Pressure (aka Stress)

Pressure (or more properly, stress) can also change rock. There are two main kinds I want you to know about:

3) Fluids

Fluids which metamorphose rock are not pore fluids remaining from when sedimentary rocks were deposited. Instead, they come from two main sources: hydrothermal fluids from magmatic intrusions and dehydration of minerals, like clay, which contain water in their structures (hydrous minerals).

Whatever the source, fluids contain ions dissolved from other rock or from their original source. As fluids percolate through rocks, they can exchange ions with the existing minerals and thus change the chemical makeup of those minerals. The other way fluids change minerals is by hydrating minerals which previously did not contain water.

Either way, fluids change the chemical makeup of minerals, turning them into new minerals, which changes the rocks which were made of the previous minerals. This process of change by fluids is called metasomatism.

Types of Rock Metamorphism

Some kinds of metamorphism:

Metamorphic Rocks and Rock Textures

Three major texture and rock types for metamorphic rocks that you need to know:

Metamorphic Grade

Geologists who study metamorphic rocks have come up with the concept of metamorphic grade to describe how 'metamorphosed' a rock is. It runs from low grade, where the rocks are hardly changed from their original form due to low pressures and temperatures, to high grade, where the rocks are heavily altered due to high pressures and/or temperatures.

More specific distinctions can be made through lab experiments in which various kinds of rocks are squeezed and heated up and the changes observed. Through this kind of work, geologists have found a set of index minerals, which are common minerals which form under particular combinations of pressure and temperature. Armed with knowledge from these experiments, field geologists can go out and make maps of mineral location to determine how metamorphism is distributed over large regions of rocks.

Metamorphic Facies

Two fundamental points about metamorphism are:

Keep that in mind: you get different minerals from different rocks under the same conditions and you get different minerals from the same rocks under different conditions.

Geologists have formalized these statements into a system of classifications for rocks by pressure and temperature conditions, so that a given combination of pressure and temperature will give a specific class of rocks. These classifications are called metamorphic facies.

The most important thing about metamorphic facies is that if you know the kind of metamorphic rock you have, you can work backward to find out the pressure/temperature conditions under which it formed. This is vital information for figuring out past tectonic conditions in the region, since certain facies form in certain plate tectonic environments.

For example, blueschists form under low temperatures and moderate-to-high pressures, which indicates that the material which metamorphosed was shoved down into the Earth so quickly it didn't have much time to warm up. What kind of plate tectonic environment displays these features? Subduction zones!

Sedimentary Rocks:

Once material is weathered from rocks, it is transported away and later deposited somewhere else, and eventually is turned into new rocks. Such rocks are called sedimentary rocks, and they're the subject of this lecture.

What are sediments?
What is a sedimentary rock?

Sediments are loose particles of former rocks. They can either be bits of rock (ranging from mineral grains all the way to boulders) or material which was dissolved and then later precipitated from water to form solid crystals.

After these sediments are deposited, they may be buried and undergo a set of physical and chemical changes which turn them into solid rock. Rocks formed from sediments are called sedimentary rocks.

Types of Sedimentary Rocks

There are three main types of sedimentary rocks I want you to know about:

Sedimentary Cycle

All sedimentary rocks go through a cycle similar to this one:

  1. Weathering from previous rocks
  2. Erosion of weathering products
  3. Transport of particles from weathering region to someplace else
  4. Deposition of sediments
  5. Burial of sediments
  6. Diagenesis
We will illustrate these processes with clastic rocks, then discuss chemical and biochemical rocks.

Clastic Rocks: Transport

There are a number of different ``transportation agents'', or ways in which sediments are moved from one place to another; practically all of them move things downhill or downstream. Here's a brief list of some:

All these modes can transport clastic particles of various sizes, and the size each can transport depends on the force with which it flows. For example, a river can transport clastic particles of all sizes when it is flowing rapidly, high in the mountains or during a flash flood. As the current slows, the river drops big particles first, then the next size, then the next, and so on. When the current is very gentle, only the smallest particles are carried. This phenomenon is called sorting.

Another phenomenon resulting from transport of clastic particles is called rounding. Basically, as particles are moved downstream, they bounce around and chips are taken off, especially around the corners. In this way, the sharp edges are worn away and a smoother, rounder particle is left. The farther a particle travels, the rounder and smoother it will be.

Clastic Rocks: Deposition

Rivers (and all other modes) transport clastic material downstream and then drop it someplace; this process is called deposition or sedimentation. Places where this occurs are called sedimentary environments.

Clastic Rocks: Burial/Diagenesis

As sediments are deposited, they build layers of material. These layers are buried by later sediments, which are buried by later sediments, and so on and so on.

As more and more sediments are deposited, the sediments near the bottom get heated up and squeezed more and more. The sediments begin to undergo diagenesis, which is an umbrella term for physical and chemical changes which turn sediment into sedimentary rocks.

Physically, the primary change is compacting sediments and squeezing out fluid in the pores between sediment grains; imagine squeezing water out of a sponge. One of the major chemical changes is that new minerals can form between the clastic grains and cement the grains together; calcite is a common cement. On the other hand, some of the more soluble clastic grains may dissolve over time or be replaced by other materials.

Classification of Clastic Rocks

There are two major kinds of clastic sedimentary rocks to discuss: sandstones and fine-grained rocks.

When sand lithifies, it turns into a sandstone. Sandstones can be classified by a number of different criteria, but the most common are grain size and shape and mineralogy. There are three major kinds of fine-grained clastic sedimentary rocks:

Chemical/Biochemical Rocks: A Brief Overview

Having dealt with clastic rocks, we now look at chemical and biochemical rocks. The raw materials to make these rocks are chemical solutions of water containing various ions dissolved from pre-existing rocks. These ions are dissolved in the water and flow with it downstream into lakes and the ocean. Once the material arrives in the ocean, it can be precipitated out of the water through either biochemical or inorganic chemical means.

Chemical and biochemical materials, once precipitated as solids, go through diagenesis just as clastic rocks do, though chemical changes are more important for these rocks than are physical changes. One common chemical change is changing aragonite, which is fine-grained calcium carbonate, to less fine-grained calcite.

Chemical and biochemical rocks can be classified by chemistry, and here are three major kinds: