G103
Week Four
Sedimentary
Rocks and
Processes
Click on the link below to hear a mini-lecture
audio segment.
Introduction
There are two types of sedimentary rocks, chemical and clastic.
- Chemical sedimentary rocks form by precipitation of minerals from an aqueous solution or by some other chemical or biochemical reaction.
-Clastic sedimentary rocks form when existing rocks are weathered into small fragments called sediment, eroded and transported to another site, and deposited, compacted and lithified.
Sedimentary Processes
Weathering
The creation of sedimentary rocks involves a series of processes that begin with weathering. There are two types of weathering.
Mechanical Weathering
Mechanical (or Physical) Weathering involves the mechanical breakdown of a rock by several processes into smaller pieces. The forces that cause mechanical weathering are water, wind, ice, and gravity.
There are a number of ways in which mechanical weathering may occur.
- Spheroidal weathering occurs when rock which is blocky is rounded by a combination of water infiltration and chemical weathering. This type of weathering is both mechanical and chemical.
- Frost heave occurs when water seeps into fractured rock, freezes, expands, and breaks the rock apart.
- Pressure Release occurs when rock which has been buried moves towards the surface as the overlying sol and sediment is removed. As the rock gets closer to the surface the pressure on it is released and it expands. Sheet fractures appear as the pressure is released and the rock is spalled off into sheets.
Chemical Weathering
Chemical Weathering occurs when rock is exposed to water and air and is broken down or transformed into another material. The abundance of oxygen and water on the earth, two highly reactive substances enhances the chemical weathering of rocks.
Oxygen breaks down materials by a process called oxidation. An example of oxidation occurs when you leave an iron nail out in the elements. The iron is transformed into iron oxide (rust) by the reaction between the iron and the air.
4Fe +3 + 3O2 ----> 2Fe2O3
iron + oxygen -----> iron oxide
Water also plays a significant role in chemical weathering because it is such an effective solvent. When water is acidic (<7.0 pH), it tends to break down many of the common rock materials. This process is called solution weathering.
Calcite often goes into solution when it is placed in slightly acidic water or water that contains carbon dioxide, as is shown in the link below.
Minerals and rocks may often be transformed into another rock or mineral in the presence of water. A good example of this process is the chemical weathering of feldspar into clay, as shown in the link below.
2KAlSI3O8
K-feldspar
Al2SI2O5 (OH)4
Clay
Sedimentary Rocks
There are two types of sedimentary rocks, chemical and clastic.
- Chemical sedimentary rocks form by precipitation of minerals from an aqueous solution or by some other chemical or biochemical reaction.
-Clastic sedimentary rocks form when existing rocks are weathered into small fragments called sediment, eroded and transported to another site, and deposited, compacted and lithified.
Chemical Sedimentary Rocks
- Chemical sedimentary rocks may form by precipitation of minerals from seawater when the concentrations of the dissolved minerals is higher than the water's capacity for the ions of which the minerals are composed. Limestone forms when the Ca+ and CO3- precipitate in the form of calcite. Chert forms when Si+ and O2- precipitates for form the mineral quartz.
- Chemical sedimentary rocks may also form when water containing dissolved mineral species evaporates, resulting in the precipitation of the minerals as the volume of water is decreased. Rock Salt forms when Na+ and Cl- precipitate from evaporating water. Gypsum forms when Ca+ and SO4- precipitate from evaporating water.
- Some chemical sedimentary rocks, such as coal, form when carbon-based organic material decomposes in the absence of oxygen. If enough pressure and temperature is exerted on the coal, the formation of diamonds may result.
- Clastic sedimentary rocks are the result of a long process in which existing rocks are broken down into fragments, transported, and reconstituted as sedimentary rocks. The process begins with the weathering of existing rocks (igneous, sedimentary, or metamorphic) by physical or chemical processes. Chemical weathering is the breakdown by dissolution of rock materials when they are exposed to water. Physical weathering occurs when rock materials are abraded or fragmented by collision with other particles or by frost wedging (when water in a crack in a rock freezes, expands, and breaks the rock apart). The fragments of rock produced by physical weathering are called sediment. The sediment is eroded and transported by water, gravity or ice down slope to a new location where it is deposited. After deposition, the sediment is buried compacted and cemented together, called lithification.
- Clastic sedimentary rocks are classified in terms of texture and mineral content, much like igneous rocks, except the texture classification is based on sorting and grain size. If a rock is well sorted, the grains in the rock are all approximately the same grain size. If there is great variability in grain size, the rock is said to be poorly sorted.
Common Sedimentary Rocks
Put
on your 3D glasses and click on the link below.
In this week's lab you will be identifying unknown sedimentary rocks. In the links below you will identify unknown rocks in virtual laboratory exercises.
To test your understanding of the characteristics of clastic and chemical sedimentary rocks, click the exercise link below.
Environments in which Sedimentary Rocks Form
Clastic Sedimentary Rock Environments
Chemical Sedimentary Rock Environments
Let us complete our study of sedimentary rocks with a look at three different environments in which sedimentary processes that are important to the formation of sedimentary rocks occur. You will go on three virtual field trips using virtual reality technology. To move around in a scene, place your cursor in the image, click your mouse down and move your mouse.
The first is Naples Beach, Florida, as shown in the link below.
Try to summarize the features that you observe at this site. Are sedimentary rocks being created here?
The next site we will visit will be the Grand Canyon of the Yellowstone, as shown in the link below.
Try to summarize the features that you observe at this site and compare them to the Naples panorama. Are sedimentary rocks being created here?
The last site we will visit is near the campus, out at Karst Farm Park. Try to summarize the features you see here and compare them to the other two locations. Are sedimentary rocks forming here?
Sedimentary Rock Formation Exercises
The links below are some exercises that will allow you to test your knowledge of sedimentary rocks and how they form.
