Sedimentary Rocks

Sedimentary rocks are the most abundant rocks on the earth's surface, so if you want to impress your friends with your geologic wisdom, you've got to know these! For the entire earth, igneous rocks are more common, but most of them are covered by a thin veneer of sediments.

1. Formation of sedimentary rocks.

Sedimentary rocks are created in three distinctive steps. 1.) The weathering of rock to produce sediments (small fragments of rock or dissolved ions). 2.) The transportation of sediments to a new area. 3.) The lithification (turning to a rock) of sediments.

A. Weathering.

Early in earth history all rocks were igneous, formed directly from the solidification of magma. However, as soon as these rocks crystallized on the surface, they began to change. This process of change is called "weathering". As the name implies, weathering is in part a result of exposure to the atmosphere, primarily water and oxygen. Weathering may also be produced by biological activity, temperature change and a variety of other natural phenomena. For ease of classification, all weathering processes are considered to be either "physical" or "chemical".

a. Physical weathering.

Physical weathering is simply the breaking of big pieces of rock into little pieces. There is no change in the types of minerals present, they are just reduced to fragments. This is important for two reasons. First, these smaller pieces are more easily transported. Many natural processes transport rock fragments, the most important of these is running water. The brown color possessed during most rivers, particularly during periods of high discharge, is produced by weathered rock carried in the flow. Wind, gravity, glaciers and ocean waves are also capable of moving large quantities of sediment.

Physical weathering is also important because it increases the surface area of rocks. Chemical weathering (discussed in the next section) only occurs on exposed outer surfaces. The minerals on the inside of a rock are protected by those on the outside. Consider a round chunk of rock. Only the relatively few minerals exposed on the outside of this rock are vulnerable to chemical attack. The majority on the inside are protected. Now imagine that you take a sledge hammer and smash this rock into tiny fragments. You still have the same mass of rock, but there is much more surface area. All of the minerals once hidden inside the rock are now exposed.

You can experience the accelerated rates of chemical weathering produced by increases in surface area. Obtain a piece of LifeSaver candy. Pop it in your mouth. As the water in your mouth dissolves (an important chemical weathering process) the LifeSaver, flavor is released. Initially, the flavor is strong because the piece of candy is relatively large and so has a high surface area. As the LifeSaver dissolves, surface area decreases, and so the flavor decreases. As your mouth becomes increasingly bored by the taste, you can't resist crunching up the candy with your teeth. This physical weathering process (just breaking it up, with no change in composition), greatly increases the surface area exposed to solution, and so there is a sudden burst of flavor as the candy rapidly dissolves.

b. Chemical weathering.

Chemical weathering destroys old minerals and creates new ones. There are many different chemical weathering processes, and we do not have the time to study them all in detail. Do remember that most of these destructive processes are caused by water or oxygen. Water and oxygen are chemically very reactive. Because these substances are so common, we often forget how powerful they are. Consider the blade on a high-quality pocket knife. It is made of extremely hard and durable steel, and with proper care will last many lifetimes. What happens if you lose that knife out in the woods? In a matter of weeks, due to exposure to water and oxygen, a formerly expense knife becomes a rusty piece of junk. The minerals in the knife have chemically weathered, changing from iron (Fe) to iron oxides (Fe2O3, hematite or FeOOH, goethite) better known as rust. This is an example of the chemical weathering process called oxidation.

The new minerals created during chemical weathering are formed from the remnants of the old, with the possible addition of new atoms from oxygen, water, and substances already dissolved in that water. At the same time, as the original mineral's crystalline structure breaks down, some atoms may be carried away by water. These dissolved ions are transported away in solution, but may later recrystallize and form new minerals to create new sedimentary rocks.

All minerals are affected by chemical weathering, but not in the same way and at the same rates. As a general rule: minerals are the most stable under conditions similar to those in which they were formed. Minerals are the least stable under conditions different from those in which they were formed. Consider intrusive igneous rocks. These form under conditions of high temperatures and pressures, and in the absence of water and oxygen. The minerals in intrusive igneous rocks are stable under these extreme conditions. Conditions on the surface of the earth are relatively cold, low-pressure, and both oxygen and water are abundant. As a result, most of the minerals contained in igneous rocks begin to chemically weather as soon as they are exposed on the surface.

As unstable minerals weather, they change into new minerals. These new minerals, having formed on the surface, are stable under these conditions. Because they are relatively unaffected by weathering they are called "stable end-products of weathering". The three most important of these are: quartz, clay and the iron oxide. Examine a piece of granite (#1). In this igneous intrusive rock, the gray, glassy crystals are quartz, the whitish crystals are feldspar, and the black crystals are biotite. As granite weathers, the feldspar turns to clay, the biotite to rust and the quartz remains behind as grains of sand.

B. Transportation.

a. Of detrital fragments of weathered rock.

The small fragments of rock produced by physical weathering are easily transported by running water. The size of particle that a river can carry is proportional to the velocity of the water. Fast-moving, high-energy, water can carry relatively large particles. Slow-moving, low-energy, water only transports small particles. There are four basic particle-sizes: gravel, sand, silt and clay.

Gravels are particles greater than 2 mm. in size (about the size of the head of a match). Look at specimens #31 and #32. Both are composed primarily of large pieces of gravel. This indicates that the water transporting these sediments had a high velocity. The water carrying this sediment had enough energy to remove all the smaller particles, but not the large pieces of gravel.

Sands vary in size from 2 mm. to 0.05 mm. A 0.05 mm. sand grain is the smallest individual particle that a normal person can see or feel. For this reason, sands feel gritty. Examine specimens #27, #28 and #29. All of these rocks are composed largely of sand grains. You can both see and feel these grains. Sand grains are usually composed of quartz, a stable end-product of chemical weathering. Where chemical weathering has been incomplete, other less-stable minerals, such as feldspar, will be present.

Silts vary in size from 0.05 mm. to 0.002 mm. You can not see or feel individual particles of silt. Your fingerprints are about the same width as one of these particles, so the silt grains just drop into your prints, where they cannot be individually felt. If you pick up a handful of silt, the plugging of your fingerprints produces a smooth, slippery feel. A silt-sized particle that you may have some experience with is ordinary flour. If you rub flour between your fingers, you cannot feel individual particles, it just feels kind of slick. Examine specimen #26. This rock is composed predominantly of silts. Specimen #25 also is formed from silt. Note that you cannot see or feel individual particles. In composition, silts are often quartz or feldspars in the process of weathering into clays.

Clays vary in size from 0.002 mm. down to a single molecule. This incredibly small size gives clays unusual properties. The most noticeable is that they are sticky. At some time in your life, you've taken a handful of wet dirt and squeezed it together to make a mudball. The clay is what holds the mud together. Sand and silt are not sticky. Think about laying out on a nice sandy beach. You stand up, quickly brush your legs, and you are clean. Try laying out in some mud. These sticky particles can't just be brushed off, they must be vigorously scrubbed with soap and water. Clay puts the dirty in dirt. The term clay can be confusing because it refers to both a particle-size and a specific mineral. This double usage has come about because most clay-size particles are clay minerals, and most clay minerals are only found as clay-size particles. Examine specimen #24. This rock is composed primarily of clay. Specimen #25 also contains considerable amounts of clay. Note that you cannot see or feel individual particles.

As fragments of rock are transported, they change in shape. This is most noticeable with gravels. Freshly broken gravels are very angular, just like freshly broken glass. Consider how a river would transport gravel. These large fragments are far too heavy to be carried aloft in the water (while swimming, I doubt you've ever encountered a piece of gravel floating along). Instead, they are dragged, rolled and bounced along the bottom of the channel, where they constantly grind against other sediments in the stream bed. The resulting abrasion wears down the corners, producing rounded shapes. Compare specimen #32 to #31. Rock #32 contains very angular gravels. This indicates that these fragments were not transported far from their source. Rock #31 contains very rounded gravels, indicating a long journey.

b. Transportation of ions in solution.

As chemical weathering destroys mineral structures, some of the ions released will be taken into solution by water. The dissolved ions contained in a river are called its solution load. Because this load consists of individual ions, typically the water will still appear to be clear. Although not visible, the solution load can be tasted. Pure water is almost tasteless. Dissolved ions give water its flavor.

C. Lithification.

a. Of detrital fragments.

Once particles have been transported to a new area, they must be transformed from a collection of loose sediment into new, solid rock. This process is called lithification ("lith" means stone). Lithification is a combination of two processes: compaction and cementation.

Compaction occurs when sediments are squeezed together. Freshly depositied sediments, like that tiny particles of silt and clay settling on the bottom of the ocean, typically contain a lot of air-space. As more and more material accumulates, the increased pressure on the underlying sediments drives out the air, creating a much denser deposit. As sediments are squeezed together, their rough edges and irregular shapes tend to catch on each other and lock the mass together. Additionally, sticky clays will hold these sediments together. Think about making a mudball. You take a handful of loose, moist soil, squeeze it tightly, and by compaction create a nice, hard, rock-like dirt clod. Examine specimen #25. This rock was formed simply by the compaction of mud.

Cementation occurs when particles are glued together by other minerals. These mineral cements are formed by the recrystallization of ions in solution. Common mineral cements are quartz (SiO2), calcite (CaCO3) and iron oxide (hematite or goethite). Examine specimen #32. The gravel-sized fragments in this specimen were cemented together with quartz. Examine specimen #27. The reddish/brown colors in this rock have been produced by the iron oxide cement holding the sand grains together.

b. Lithification of dissolved ions.

It is possible for ions dissolved in water to be "undissolved" and turned into new minerals. This process is called precipitation and may occur either inorganically or organically.

Inorganic precipitation is usually produced by evaporation. Imagine a lake in the desert. Because water is such a good solvent, the lake will have a solution load. In nature, there is no such thing as pure water. On a hot day, water will evaporate and return to the atmosphere, but the minerals dissolved in the lake have to stay behind. As a result, the concentration of dissolved ions in the remaining water increases. Eventually, the water cannot keep all of the dissolved material in solution. The ions will join together to form new mineral crystals which then drop to the bottom of the lake the form new sedimentary rocks.

Organic precipitation results from biological activities. Many organisms have the ability to remove dissolved ions from solution. They use these ions to build hard mineral structures, such as shells, teeth and bones. You do the same thing when you drink milk to obtain the calcium needed to build strong bones and teeth. When marine organisms die, their hard parts drop to the bottom and form new sedimentary rocks.

2. Classification of Sedimentary Rocks.

Sedimentary rock are classified as either "detrital" or "chemical". Detrital rocks are formed from fragments of other rocks. These rocks are classified primarily on the size of these fragments. Secondarily, the shapes or mineral compositions of these fragments are considered.

Chemical sedimentary rocks are those formed by the precipitation of dissolved ions out of solution. This can occur either organically or inorganically. Chemical sedimentary rocks are usually classified based on the types of minerals they contain.