Sedimentary Rock Help (page 2)
Introduction to Sedimentary Rock
At home, sediment is the leftover stuff that tends to build up, if it isn’t cleaned up. It is the bits of string, the sand tracked in from the beach, the dog/cat hair and dandruff, and the playground gravel. It is the stray grape, the lost earring, the twig, and the broken pieces of cookie. It is made up of lots of stuff that were once whole or a part of something larger.
Sediment is made up of loose particulate matter like clay, sand, gravel, and other bits and pieces of things.
I like to think of the formation of sedimentary rock as a lot like the gathering of dust bunnies in a house. Or how about the clothes, books, pens, papers, hair clips, shoes, CDs, and other stuff that collects on the floor of a teenager’s room?
Sedimentary rock is formed when all the bits and pieces of different rocks, soils, and organic things are crunched together under pressure. They join into one tight mass that hardens into rock. It’s a lot like when you leave oatmeal in a bowl too long. That stuff becomes concrete in a hurry!
Sedimentary rocks are formed from rocks and soils that came from other locations and have become cemented together with the remains of dead organisms.
A collection of many different soil and rock types is known as regolith . Regolith is the loose rock material found scattered around the crust’s solid, lower bedrock. It is made of volcanic ash, glacial drift, wind-driven deposits, plant accumulations, soils, and various eroded rock waste of every sort. Sedimentary rock that is carried by a glacier is usually deposited along underneath the ice or out to the sides.
Sedimentary rocks, originally from the buildup of material that gathers on the Earth’s surface, have been compressed and cemented into solid rock over time. Most people can say they know something about sedimentary rock, since it is the most common rock type. Beautiful multicolored layers of sedimentary rock can be seen along rock walls when highways or railroads (cut across hillsides and mountains) expose the different types of sediment layered there.
Sedimentary rock forms a broad blanket over the igneous and metamorphic rocks lying beneath its surface. In fact, Leonardo da Vinci wrote about the sedimentary rock of northern Italy in his journals. He compared sedimentary rock high on the mountainside with the sand and mud that he saw when visiting the Mediterranean coast. He noted that the color and textures were often the same.
The word, lithification , comes from the Greek word lithos , meaning stone. Lithified soil is made up of sand, silt, and organic material. Lithification can take place soon after being deposited or much later. The rate of compaction and cementation also plays a big part in eventual lithification. Additionally, the heat needed for lithification is less intense than that found deeper in the mantle, so it’s possible for lithification to happen in the top few kilometers of the crust.
When sediment hardens into sedimentary rock, it is called lithification.
Grains squeezed together by the weight of overlying sediments during compaction are formed into rock denser than the original sediments. These dense layers are then sealed together by the precipitation of minerals in and among the layers.
This is how sandstone, formed from sand and limestone hardens after lithification with the hard skeletons and shells of marine organisms. When sandstone and limestone from different time periods are layered, the different texture and color of the layers can be easily seen.
Diagenesis causes lithification of sediment by physical and chemical processes like compaction, cementation, recrystallization, and dolomitization.
Sediments become rock ( lithified ) through a combined process called diagenesis . Diagenesis is controlled a lot by temperature. But instead of the hot temperatures of igneous or metamorphic rock, diagenesis takes place at lower temperatures of around 200°C. Diagenesis can take place without complete hardening into rock, but you can’t have lithification without diagenesis.
Diagenesis, which takes place in sedimentary rock after its first deposit, is specific to sedimentary rock during and after its slow hardening into rock. The word diagenesis is not used for weathering changes, soil formation, or metamorphism of rock into other rock types.
The four main parts of diagenesis are: (1) compaction; (2) cementation; (3) recrystallization; (4) chemical changes (oxidation/reduction).
These usually take place in a stepwise way, but not always. They are often compressed and compacted first, then slowly cemented together by pressure that squeezes out water and air. Then, after being covered by more layers, unstable minerals recrystallize into a more stable matrix form or are chemically changed (like organic matter) into coal or hydrocarbons.
Lithification through compaction is simple. As you pile more and more sediments on top of early laid-down sediments, the weight and pressure builds. The heavier the weight, the more the lower layers get smashed together or compacted. The sediment’s total volume is reduced since it is squeezed into a smaller space. Drying adds to the sediment’s reduced volume.
Have you ever tried to guess the number of beans in a big jar? It isn’t easy, but one thing you have to consider is how tightly the beans are packed within the jar. If there is a lot of air space between the beans, then the total number in the jar will be less. As more and more beans are added, they add weight at the top and pack the lower beans tighter. Some of the beans may even line up in the same direction. As even more pressure is added, the beans begin to crush and stick together. Eventually, with seasoning and some lemon juice, you get bean dip!
The same thing happens with compacted sediments. When shale grains are compacted and align in the same direction, they form rock that splits along a flat plane in the same direction as the flattened, parallel grains. Figure 7-1 shows how sedimentary particles are compacted when under pressure.
Cementation of sediments happens when compacted grains stick together. Since most sediments are deposited in water, they have water molecules in the spaces between particles. The surrounding water contains different dissolved minerals that eventually fall out of solution and stick to the sediment grains. Minerals like calcite, silica, iron oxide, and magnesium cement the grains together into a solid mass that dries, is compressed further, and becomes rock.
Compaction and cementation can happen at the same time. The squashed sediments can be so tightly packed that they shut out the flow of mineralcontaining water. Figure 7-2 shows how sedimentary particles are cemented and lithified as calcite and silica precipitate out of the surrounding water.
Fig. 7-2. Minerals in the water surrounding sediments can cement them together.
Additionally, minerals within the sediments can be dissolved away when water flows through. This creates pockets and places for other minerals or oils to gather. Petroleum geologists look for oil in these types of pockets. When sedimentary minerals dissolve and react with minerals in water to form other compounds, it is called dolomitization .
Dolomitization happens when limestone turns into dolomite by a mineral substitution of magnesium carbonate for calcium carbonate.
Crystallization And Chemical Changes
Chemical and biochemical sediments and sedimentary rocks can be classified by their chemical makeup and properties. The ions of the most common elements dissolved into seawater are shown in Fig. 7-3. Although silica (SiO 2 ) and phosphorus play a big part in the makeup of sedimentary rock, they are only found in small amounts in seawater. When the water evaporates, the ions crystallize to form rock.
Fig. 7-3. There are a variety of ions dissolved in seawater.
Carbonate sediments come from the biochemical precipitation of the decayed shells of microorganisms. Other chemical sediments that are high in calcium (Ca 2+ ) and bicarbonate are precipitated out of seawater as calcium carbonate (CaCO 3 ) and carbonic acid (H 2 CO 3 ) by inorganic processes and are much less common.
Practice problems of this concept can be found at: Sedimentary Rock Practice Test
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