What's the difference between oil, coal, dirt, and limestone?

Question

What's the difference between oil, coal, dirt, and limestone? Obviously I'm talking about how they're formed.

Answer 1

The other answer is excellent and informative, but doesn't really address how these materials are formed.

Oil is formed from buried organic material. In most examples oil forms from the remains of phytoplankton (microscopic plants) and zooplankton (microscopic animals) which sink to the ocean floor and are buried by mud, clay, and silt, which later forms shale. As the shale is buried deeper and deeper (often several miles) it warms up and when the temperature exceeds about 140 F. the organic material begins to form oil. This process can take millions of years for the burial stage, and thousands of years for the chemical alteration of the organics to become oil. This process is known as catagenesis.

Coal is formed from the remains of larger plants and trees which have been preserved by falling into swamps in which there was not enough dissolved oxygen in the water to allow the plant material to rot or decompose. These accumulated in thick deposits in some places in the world. As these accumulations of plant material were covered by other deposition of shale they were compressed and heated, undergoing a process called coalification that made them into coal.

Dirt is formed primarily from weathered rock material, with some organic content contributed by plants and animals. There are many types of soil but they are generally divided into twelve soil orders. This website has good illustrations and explanations of all of the soil orders, along with maps:
http://soils.ag.uidaho.edu/soilorders/orders.htm

Limestone, as you can guess, is my specialty. Limestone is formed by living organisms in almost all cases. In rare examples limestone can form by direct precipitation of calcium carbonate from sea water or even fresh water, but in most cases it is the remains of living organisms. The organisms that form limestone include microbes, bacteria, algae, foraminifera, plankton, sponges, molluscs, corals, bryozoans, brachipods, echinoderms, arthropods, and many more organisms that all secrete calcium carbonate as either part of their body or as a shell. As the bodies of all of these organisms accumulate in a marine or fresh water environment they gradually become cemented together by a process known as diagenesis. Some organisms form deposits of calcium carbonate that remain in place within the eventual limestone bed. Corals can be seen in their original growth position in some outcrops of limestone (Barbados for example). Again, some degree of burial and pressure helps with the process of cementation and diagenesis. However, it has been proven that some limestone can be formed in only a few decades, as pop-tops from 1960's cans (which are no longer in existence) have been found within some deposits of limestone along tropical beaches. The texture of limestone is highly variable based on what type of organism made up the bulk of the calcium carbonate. Some very fine-grained smooth limestone is formed from the remains of green-algae that create sort of a lime mud before it forms limestone. Accumulations of oysters and clams can form very porous rough limestones in what is commonly known as coquina. Travertine is a form of limestone that is formed by both evaporation of hot spring water and the action of microbes that help precipitate the limestone.

Here is a website that has some information on both the formation of limestone and coal:
http://gpc.edu/~pgore/geology/historical_lab/sedrockslab.php

Answer 2

Oil is liquid and easily seperated into its components (easy meaning relative to the other substances in your list). This is accomplished by heating and cooling (distillation) in enclosed vessels and heat exchangers. There are many follow on processes like catalytic cracking to improve the yield on each barrel of oil.

Coal has hydrocarbons similar to oil but it also contains a much large percentage of other ingredients like sulphur, mercury, etc. It is much more difficult to seperate coal into its various components. Because it is a solid, it cannot be pumped in to enclosed vessels and heat exchangers. There are coke ovens that heat the coal and break it down into its components but some of these compounds escape during the charging and unloading and re-charging cycle. Its an environmental nightmare anywhere downwind of large scale coke ovens.

Dirt may have any different numbers of components with varying quality. It usually has zero energy available but does contain minerals and nutrients to promote plant growth along with sunshine, water and CO2. This plant life may be compared to oil in coal in that it is hydrocarbons and can release energy although the energy density is much less than oil and coal. Dirt depending on its chemical composition can be made into gypsum, concrete, iron ore, bricks, tiles, etc.

Limestone as far as I know is largely inert, contains zero energy and does not promote plant life. IT can be ground into powder and applied with fertilizer to raise pH and create favorable conditions for dirt to support plant life. However, it does have other commercial value- it can be used as a building material and has great aesthetic value as stone work, or tile, or wall covering. It is also useful in iron making as it is mixed with iron ore (dirt), coke (primary ingredient of coal after breakdown in oven) to produce iron. When heated to 2700 degrees, it acts as a flux to maintain pH and helps the impurities seperate from the primary iron pool.