There are THREE Rock Formats :
|
| Igneous Rock. |
The result of boiling solid matter from the earths inner core being forced to the outer mantle or surface as larva or magma, and cooling down, forming a solid. |
| Metamorphic Rock. |
A secondary process of Igneous formation. The result of any rock being subjected to additional pressure, either by heat or force. This usually happens when an igneous formation is returned to the core below the mantle or is exposed to conditions akin to the conditions within the core. In some cases, the initial formation may even be older sedimentary formations. |
|
| Sedimentary Rock. |
A secondary process that follows on from the previous two types of formations. Sedimentary formations are the result of primary formations being exposed to external influences and resulting in this third type of formation, and is the subject of this Earth Cache. |
SEDIMENTARY ROCKS
Although most of the rock formation on planet Earth is Igneous and Metamorphic in origin, less than 5% is Sedimentary Rock, but here is the thing. Sedimentary formations cover over 95% of the surface of the planet. This means that our settlement on, and understanding of, this planet is directly built upon the Sedimentary Formations.
And the zinger is that early evidence from other planets, esp. Mars, is that there appears to be evidence of that planet also displaying a Sedimentary outer covering. But let us look toward our own piece of real estate three planets from the sun.
The Process of Rock Formation :
|
| Mechanical weathering – Existing rocks are broken up into smaller pieces by frost-wedging (the freezing and thawing of water inside cracks in the rock), root-wedging (tree and other plant roots growing into cracks), and abrasion caused by, for example, sand-blasting of a cliff face by blowing sands in the dessert, or the scouring of water transported sand, gravel, and boulders on the bedrock of a mountain stream. Mechanical weathering breaks rocks into smaller and smaller pieces but without otherwise altering the minerals. |
1. Weathering. |
| Chemical weathering– Rock minerals are changed into new minerals and mineral byproducts. Some minerals like halite and calcite may dissolve completely. Others, especially silicate minerals, are altered by a chemical process called hydrolysis. Hydrolysis is the reaction of minerals in weakly acidic waters. Most natural surface waters are slightly acidic because carbon dioxide from the air dissolves in the water. Some of the dissolved CO2 reacts with the water forming the chemical compound carbonic acid. One must also consider the very real possibility of there being an abundance of Volcanic ash in the air, which would influence the chemical properties of rain water, thus affecting the reaction with the rock mineral composition. |
|
|
|
| There are several modes of carrying the results of weathering to another location. Water is the most prolific, in liquid or solid form (as ice). |
2. Transport. |
| As a liquid the water carries the product of weathering either in suspension (smaller particles), or by force (larger samples rolled ). As a solid, ice includes the particles both large and small held in suspension, but often toward the base of the glacier. This in turn enhances the abrasive quality of the passing ice, which influences the first process of weathering. |
|
| Wind is another prolific conveyor of the weathering results. Small grains of sand are often transported great distances by the wind, particularly in areas of high sand concentrations. Gravity plays its part in the transporting of samples, ensuring that large unstable samples are drawn down the side of the mountain, and often come to rest alongside the mountain of origin. The moment wind or water no longer have sufficient energy to carry the particles, it is gravity that steps in and brings said particles to rest upon the ground. Both wind and water also carry some of the tools of Lithification, water with dissolved minerals and wind with airborne compounds emitted during volcanic activity, including emitted ash. |
|
| One further consequence of transportation is that the matter that is being carried, is sorted into concentrations of a similar size. Concurrently, contact with other particles happens, and abrasion occurs. This results in the rounding of the edges of the particle, so that the longer the particles are in transport, the closer each particle is to being a near perfect sphere. |
|
|
|
| The third step in the making of Sedimentary rock formations is the depositing of the materials. Sediments are transported only when there is enough energy in the transporting medium to carry such sediments. The moment that energy is diminished, the sediment is deposited. For example, when a flow of wind passes through a ravine or narrow saddle between two mountain formats, there is an amount of energy sufficient to carry grains of sand, and such particles are carried through the gap. The moment the wind flows over the flood plain or open ocean, in fact the moment there is not any restricting on the air flow, the carrying energy of the wind is diminished, and the particles being carried are no longer held by the wind flow, and the sediment is deposited. The same happens with water; In flowing down the side of the mountain it has enough energy to transport larger boulders, but when it slows down at the base of the mountain and enters the flood plain, the particles it carries are deposited as silt. So the largest sediments (boulders, cobbles, and pebbles) which survive the weathering process, tend to be deposited near to their source, for example at the point where a mountain stream flows out onto a valley floor. Sediments of a given size are deposited whenever they move into an environment with insufficient energy to transport them. A transporting medium that is saturated with the product of weathering will deposit such particles when the medium encounters another saturated medium. Gypsum, Halite, and other salts, precipitate out of water in arid areas, like the Dead Sea in the Jordan Valley, where evaporation is high (thus increasing the salinity) and influx of fresh seawater is low. Sediments are deposited layer upon layer. This is called bedding, or stratification. The bedding is mostly horizontal. Often cross stratification would occur, usually when the deposition happens within a medium that has regular patterns such as currents. Sorting. A significant result of the Deposition process is the aspect known as sorting. Particles of a similar composition would tend to precipitate from the transporting medium at the same time. Conversely a sudden loss of energy in the transporting medium (such as a river flowing into an open and large pan) would result in diverse particles being deposited simultaneously. |
3. Deposition |
|
|
As the sedimentation formation process continues, and the particles are deposited as part of the formation process, the earlier sediments are laden with an increasing burden of later sediment deposits. The more deposits in an area results in a greater mass the lower layer of sediment has to support. This pressure increases with each successive round of sediment deposit. The lower layers are squashed together, and the applied pressure increases as the additional layers increase. It must be remembered that the increasing mass does not have to depend on successive layers of sediment. Increased water volume contained within the same parameters increases the compaction pressure, as does the build up of solid water (ice).
This pressure is occasionally enhanced by the upward pressure from the earth mantle, which is being pressured by Teutonic pressures. Furthermore, Earthquakes and shudders from within the shifting plates beneath the crust of the Earth result in greater compaction.
|
4. Compaction. |
|
|
|
|
| At the same time as the Compaction process, and in harmony with it, is the process of Cementation. The water held in the spaces between the sediment particles is expelled as the particles are forced together with increase in pressure due to compaction. Dissolved minerals in this water precipitate (crystallize) to form mineral crusts, which gradually bond the particles, thus creating the rock. The degree of sorting, as well as the consistency of rounding, of the component particles, would aide this cementation process. Calcite (calcium carbonate), silica, and hematite (red iron oxide) are the most common cementing agents. An example of Calcite (commonly called lime) encrustation is often found around older plumbing fittings and old shower-heads. The Compaction and Cementation processes are often refered to as Lithification. |
5. Cementation. |
|
|
Thus is the process in creating Sedimentary Rocks.
All Sedimentary Rocks undergo these processes to some degree of the other, and in defining a rock these processes help to determine the nature of the sample. But the actual composition also determines the Rock type.
We find that there are only four types of Sedimentary Rocks. By understanding the rock type, and the process involved in creating it, gives us an intimate exposure to the environment when these rocks were born. And thus, we can know a little bit more about this planet we live on.
The FOUR different TYPES of Rock composition
|
| Clastic Sediment. |
| Shale. |
Clay particles too small to observe with the naked eye. |
| Siltstone. |
Silt particles visible only under a microscope. |
| Sandstone. |
Sand particles measure from under one tenth of a millimeter to Two millimeters diameter. The subdivisions within Sandstone include very fine, fine, medium, coarse and very coarse. |
| Conglomerate. |
Any particles measuring from more than 2 millimeters diameter to large boulders. Conglomerate Rocks are usually poorly sorted, containing sand, gravel and boulders all in one rock. If the rock contains many sharp edges, and shows little evidence of being rounded, it is said to be angular (un-rounded) and is termed Breccia. |
It can be seen that the size of the constituent particles determines the end product rock. Usually the larger the particle, the less distance the sediment has had to be transported. This allows for a greater understanding of the near environment.
Conversely, the longer the transportation cycle, the more the particles are rounded, creating a more evenly textured rock at the end. Importantly, a regular structured sediment – which means a sediment of well rounded particles which is also sorted well; this sediment will support another layer of sediment, or sediment bed, with greater distinction than a bed of Breccia. |
|
|
|
|
We spoke of Compaction and Cementation, the final two steps in the process of creation, and noted that these two steps are referred to as Lithification.
|
|
|
Lithification of sediment into sedimentary rocks takes place after the sediment has been deposited and buried. The processes by which the sediment becomes lithified into a hard sedimentary rock is called diagenesis and includes all physical, chemical and biological processes that act on the sediment.
The first step in diagenesis is the compaction of the sediment and loss of water as a result of the weight of the overlying sediment. Compaction and burial may cause recrystallization of the minerals to make the rock even harder. Fluids flowing through the rock and organisms may precipitate new minerals in the pore spaces between grains to form a cement that holds the sediment together. Common cements include quartz, calcite, and hematite.
Other conditions present during diagenesis, such as the presence or absence of free oxygen may cause other alterations to the original sediment. Where there is an abundance of Oxygen, known as an Oxydising Environment, Iron will change from Fe2+ to Fe3+, and will change the color of the sediment to a deep red (rust) color.
Increased sediment beds will increase the pressure on the lower laying sediment bed, and this will result in the temperature increasing. Gradually this increase of pressure and thus temperature leads the diagenesis toward the formation of Metamorphic Rock. |
|
| Biological Sediment. |
This Rock has as its foundation the presence of Carbon. Biogenic or Bio chemical Sediment is the rock that is derived from once living organisms. When an organism such as man or a tree or a mollusc for example; when these living organisms die, their decaying remains will accumulate to become sediment or sedimentary rock. |
|
| Limestone. |
Calcite (CaCO3) is precipitated by organisms usually having a shell or other skeletal structure. Accumulation of these skeletal remains results in a limestone. Sometimes the fossilized remains of the organism are preserved in the rock, other times recrystallization during lithification has destroyed the remains. Limestones are very common sedimentary rocks. |
| Chert. |
Tiny silica secreting planktonic organism like Radiolaria and Diatoms can accumulate on the sea floor and recrystallize during lithification to form biochemical chert. The recrystallization results in a hard rock that is usually seen as thin beds. |
| Diatomite. |
When diatoms accumulate and do not undergo recrystallization, they form a white rock called diatomite as seen in the White Cliffs of Dover. |
|
| Organic Sediment. |
When living matter is covered, or specifically when there is a depletion of the Oxygen necessary for it to continue living, it dies but does not decay. The organic matter has not decayed before being lithified. The leading factor in this is a depletion of oxygen (known as a Reducing Environment). The resultant sediment is coal. Due to the conditions of formation in that the energy loss due to decomposition has been denied at the point of Lithification. The Carbon rich sediment is high in latent energy. Further compaction results in the chemical composition of the matter further breaking down into structurally simple Carbon chains, which modern man favours for the production of petroleum. This form of Rock has meant that these sediments are valued sources of energy, so valuable in our world today. |
| Chemical Sediment. |
Chemical compounds dissolved in the water (usually) result in a water that is rich in ions. This water flows into streams or groundwater tables. Eventually these dissolved ions end in up in the ocean or a large body of stagnant water such as a pan or lake. This, as an aside, is why seawater is noticeably salty. Typically the water evaporates which increases the concentration of salts – ( ion rich medium is referred to as a salt) This increase in ion concentration reaches a point of saturation whereby the ions are forced to bond with each other, precipitating in a chemical mineral that accumulates and becomes a sediment. Thereafter the process of Lithification follows, as already detailed. |
|
Examples of Chemical Sedimentary Rocks:
| Evaporites. |
Formed by evaporation of sea water or lake water. Produces halite (salt) and gypsum deposits by chemical precipitation as concentration of solids increases due to water loss by evaporation. This can occur in lakes that have no outlets (like the Great Salt Lake) or restricted ocean basins, like has happened in the Mediterranean Sea or the Gulf of Mexico in the past. |
| Travertine. |
Groundwater containing dissolved Calcium and bicarbonate ions can precipitate calcite to form a chemically precipitated limestone, called travertine. This can occur in lakes, hot springs, and caves. |
| Dolostones. |
Limestones that have been chemically modified by Mg-rich fluids flowing through the rock are converted to dolostones. CaCO3 is recrystallized to a new mineral Dolomite CaMg (CO3)2. |
| Chemical Cherts. |
Groundwater flowing through rock can precipitate SiO2 to replace minerals that were present. This produces a non-biogenic chert. There are many varsities of such chert that are given different names depending on their attributes, For example:
Flint – Black or gray from organic matter.
Jasper – Red or yellow from Fe oxides.
Petrified wood – Wood grain preserved by silica.
Agate – Concentrically layered rings
|
|
And so, after plodding through the above, you should now have an understanding of what makes up the predominant covering of our planet Earth. As mother earth might say, “Sedimentary Rock, you cover me almost completely!”
References:
EENS 1110 Physical Geology
Tulane University Prof. Stephen A. Nelson
Anhaeusser, C.R. Viljoen, M.J. & Viljoen, R.P. (eds) 2016
Africa's Top Geological Sites Struik Nature Cape Town.
THERE IS NO PHYSICAL LOG TO SIGN.
In order to claim this as a find, you are expected to consider the description above, and apply it to the surroundings at the co-ordinates.
Below are several Tasks that should be undertaken, the results of which should be sent to the CO's Geocaching profile.
If the CO has not received mail submission via the message centre within 10 days of the find being logged, the log will be deleted. Interpretation of the submitted evidence rests entirely with the CO alone.
Group responses will not be acceptable.
It is not mandatory to include any photographs or visual media.
Tasks to be undertaken in relation to this Earth Cache.
Task 1:
At the co-ordinates, facing you from across the water, is a rock face. What sort of rocks make up this outcrop, and why do you think that?
Task 2:
Explain the texture (to the naked eye) of the rock face before you.
Task 3:
There is a noticeable difference in the rock face colouring, both before you and upon the rock face behind you. Why do you think that this is so?
Task 4:
In your opinion, what would this environment have been like about 30 to 40 million years ago?
Task 5: (On site verification) Behind you is a form of remembrance. What year is indicated upon this remembrance?
Task 6: OPTIONAL Include in your log a picture of the formation with yourself or friends in the picture.