Unit 9-2: The Rock Cycle: Sedimentary Rocks

Well, I’ve broken out of wrestling, and now have to get the the set of another Scorpion King movie. I’ve asked a buddy to help out with the next section.

Well, lets take a look at what makes sedimentary rock different from igneous rock. Lets start with sediments. Well, there are some nice sediments. There are three types of sediments: -Clastic -Chemical -Organic

Chemical Sediments: -Mineral grains that formed due to a chemical process. -Examples include acid rain, oxidation (rust), ocean salt wearing away the material. Clastic Sediments: -Grains of rock and mineral -These broke off of other rocks due to erosion or weathering. -Can be a variety of sizes. Organic Sediments: -Made from pieces of once living material. -Examples: plant materials such as wood.

Sediments have a range of sizes, from largest to smallest: -Pebbles, -Gravels, -Sand, -Silt, -Clay. However, there are different sizes of sediments. I should probably explain that part! The key to learning how clastic rocks form is by looking at what makes them. These rocks are formed by sediments that are cemented together by time and pressure.

We sort the sediments by size using a sieve. -Sediments are placed into the top sieve, -The lower sieves have progressively smaller screens to screen out the sediments. -Once the system is turned on, the sieve vibrates. -After several minutes, we can see the different sediments, sorted by size.

Nature also sorts sediments. -In rivers, fast moving waters carry all sediments. -As the rivers slow down, or approach the edge of the riverbed, sediments are dropped off. -The largest and heaviest (pebbles) get dropped off first. -The smallest (clay) stay in the river the longest.

-Dissolved minerals in the ocean or river water act as cement that holds the sediments together. -Silica, lime, and iron can act as cement. -The cement works its way into the pores and irregularities in the sediments. -The cement binds to the rock sediments and holds them together. -Clays and silts can bind together when pressure is high enough, but heavier materials need cement to hold them together. -Pebbles and gravels form a conglomerate. -Sand forms sandstone. -Silt and clay forms shale.

-Conglomerate is the coarsest of the clastic sedimentary rocks. -The pebbles and gravels that make up the conglomerate are easily visible to the naked eye. -Conglomerates can be made out of any type of pebble or gravel. -Quartz is very commonly found in conglomerates because of its durability. -The cement that holds the pebbles and gravels together is usually very fine grained.

-Sandstone is made of sand sized sediments. -Generally, all the sand sediments are quartz grains. -Cement holds the sand grains together. -Because the cement never fills all the spaces between the sand grains, there can be up to 30% of air space between the grains. -Sandstone is rough, gritty and durable. -In some areas of the world, sandstone was commonly used as a building material and as a sculpting material.

-Shale is made of clay and silts. -Clay is composed mostly of kaolin, so shale is mostly kaolin. -The grains in shale are very tightly packed together. -Shale is a very soft and smooth stone. -It breaks very easily. -Fossils are found in shales, since they form in rivers.

Some sedimentary rocks form through chemical processes. -When minerals are dissolved in water, they can react with other minerals or other chemicals. -This produces a new material. -The most common chemical sedimentary rocks are: -Limestone -Rock salt -Rock gypsum

-Limestone forms from tiny grains of of calcite. -Calcium can also come from shells or bones. -The tiny grains of calcite are usually deposited from sea or lake water. -Limestones are usually light gray or white in color. -Due to the heavy nature of the atoms that make the minerals, limestone is very dense. -Limestone will feel smooth to the touch.

-Rock salt is the natural form of table salt. -Rock salt is made almost entirely out of halite. -Rock gypsum is very much like rock salt, but made out of gypsum. -Both form through the evaporation of salt lakes or ocean bays cut off by sandbars.

Now, there is one last type of sedimentary rock we should look at, and that is sedimentary rocks that form through organic means. -Limestone can also form through organic means. -Shells of sea creatures, plankton, bones, etc. pile up on the ocean floor. -As time progresses, these calcium deposits compact into limestone. -On land, a common sedimentary rock is formed from plant remains: coal.

-Sedimentary rocks show special features that help to identify them. -One method we use is stratification. -Stratification is the arrangement in visible layers. -How does stratification develop?

-When a change of sediments occurs, a new layer develops. -Different size grains, or new sediments appear, a new layer of sedimentary rock forms. -This is how sedimentary rocks become stratified.

-Aha! New sediments result in new layers. -That’s why we see sandstone on top of shale, on top of sandstone. -Now, there are a couple other features we can look at: bedding planes and cross bedding.

-Bedding plane: horizontal layers where stratification begins. -Cross bedding: A angled section or layer in stratification. -It can be due to wind, erosion, or tectonic forces.

-Many sandstones show ripple marks on the surface of the bedding plane. -These formed by the winds and water that formed and deposited the sediments. -Mud cracks develop when deposits of wet clay dry out and contract. -In a bedding plane, these cracks are filled with other materials.

-Limestones can sometimes contain hollow spheres of silica. -These hollow spheres contain crystals. -This is a geode. -It appears that geodes formed by water wearing away a hallow in the stone. -Over time, this hallow gets quartz deposited in it, which grows into crystals. -Also in limestones, there can be lumps of chert or flint.

How do you know so much about these rocks, Mr. Holmes? It’s sedimentary, my dear Watson!