Intrusive igneous rocks are formed when the molten lava insulate in to the country rock cools slowly to form a huge rock with a coarse texture. The extrusive rocks are formed when the molten lava fall in to the earth surface and cools fast to form fine grains. The points at which the earth is exposed to the coldness space are normally made from the igneous rocks. Igneous rocks are generally the cooled and hardened magma. An example of the igneous rocks is the Half Dome in Yosemite national Park. The classification of the igneous rocks is normally based on the texture and composition .
The fine grained igneous rocks are referred to the aphanites; their examples include the rhyolites, andesite and basalt. The intermediate grained igneous rock includes the dacite and the diabase. The coarse grained igneous rocks which are further referred to as the phaneritic include the granite, diorite, gabbro and peridotite. Obsidian is an example of glassy igneous rocks while the pumice and scoria are examples of frothy igneous rocks. The above classification is based on the texture of the igneous rocks. The photo below represents obsidian that is formed due to rapid cooling like that of the extrusive rocks.
There are four classes of pumice rocks in reference to classification according to the composition, these are 1. The felsic, e. g. the rhyolite, granite, obsidian and the pumice. 2. The intermediate, e. g. the andesite, dacite and diorite. 3. The mafic, e. g. the basalt, diabase, gabbro and scoria. 4. The ultramafic, e. g. the peridotite, its noted that the conditions necessary for the formation of these kind of igneous rocks do not exist now. The sedimentary rocks are formed underwater either in rivers, oceans or lakes; they form as a result of long time accumulation of this debris (mud and sand).
They are referred to as secondary rocks because they are formed from accumulation of debris of the already existing rocks. An example of the sedimentary rocks is the Bryce Canyon National Park in Utah. There are three types of sedimentary rocks; these are clastic, chemical and organic sedimentary rocks. Clastic are sedimentary rocks that are basic that are formed due to accumulation of the clasts. The photo below show sandstone that is one example of clastic sedimentary rocks formed due to cementation and compaction of quartz grains (sand).
Chemical sedimentary rocks are formed due to the evaporation of standing water leaving behind the dissolved mineral. They are frequent in arid and semi arid areas due to the high likely hood of the playa lakes in the depressions, this leads to thick deposition of salt and gypsum over time due to evaporations and repeated flooding. The organic sedimentary rocks are formed as a result of organic processes, like the depositing of calcium from animal bones, teeth and shells in sea floor and accumulate to form the organic sedimentary rocks.
The metamorphic rocks are as a result of pressure and high temperatures that cause the alteration of minerals in the igneous or sedimentary rocks leading to formation of new rocks. Metamorphosis occurs by breaking the bonds between the atoms of a mineral that end up rearranging them selves in to a new stable mineral form. Solid rocks undergo metamorphosis because bonds between the atoms are broken in to unstable mineral, the freed ions and atoms move to another position within the given mineral or bonds with atoms of different minerals.
The final result is that a mineral that is more stable in the environment that the metamorphosis occurred is formed. There is formation of new mineral with different texture from the original, unstable minerals, e. g. clay is normally broken down and form new stable mineral like the quartz. Some times due to the high temperature ions and atoms move to form a more stable element there only changing the texture whiles the chemical composition doest change. The rate of metamorphism is controlled by rate of conduction of the magma, temperature differences between the country rock and plutonic rocks and heat capacity of magma.
Below is a table showing the examples of metamorphic rocks, parent rock and the key minerals. rock parent rocks key minerals in the rock foliated slate shale graphite, micas, chlorite, clay minerals foliated gneiss shale, felsic igneous, sand stone talc, mica, hornblende, kyanite, garnet, staurolite foliated schist shale impure limestone, sandstone, hornblende, mica, garnite, talc, epdiodte non foliated marble pure limestone dolomite, calcite Non foliated quartzite chert, pure sandstone quartz The photo below shows non foliated quartz formed from sandstone.
Most of the rocks on the earth surface are not normally uniform throughout. On a scale that is usually measured in centimeters or millimeters, the rock structures are made up of single mineral grain that differ in shape sizes and composition. The rock structure is a geometric characteristic that exists between the large and small rocks. The rocks sizes vary from millimeters, centimeters, meters, to kilometers. It is the individual, larger scale, contrasted rock features that are referred to us the rock structures.
There are many different rock structures but geologists conveniently classify the structures in to two major structures, the secondary and primary structure. The primary structures are formed before or when a material is being formed into rock, this could for example when the sediments are accumulating or when the magma is crystallizing, secondary structures are formed when the rock is already formed, it may be as a result of the pressure or compression of the existing rocks. The sedimentary rock structures are determined by factors such as the water depth, current velocity, and the direction during their formation.
It is of key importance to know the various sedimentary structures so that we can determine which factors were behind their formation. The sedimentary rocks could also have been formed by the faulting and folding after the processes of deposition had ended. It is important to know which rock was initially on top before the deposition occurred. The layered structure is one of the obvious structures of the sedimentary rocks. The layers are manifested because of the differences in the clast size, degree of assortment, mineral composition and color in the unlike layers.
In some rock the degree of difference differ because of the differences in level of resistance to weathering. Layering is based on thickness as expressed in the table below. Thickness of the layers in CM Name of the layer greater then 300 cm massive 100cm-300cm very thickly bedded 30cm-100cm thickly bedded 10cm- 30cm medium bedded 3cm-10cm thinly bedded 1cm-3cm very thinly bedded 0. 3cm-1cm thickly laminated less than 0. 3cm thinly laminated Stability of the rock slopes is normally controlled by the structures of the rocks masses especially the surfaces that have soft structure.
The analysis of the rock masses structures is of paramount importance for the establishment of geological models, the numerical simulation and the foundation that can be used for the assessment of the stableness of the rocks slopes. The weaker rock structures consist mainly of the schistocity planes, joints, faults, bedding planes and the contact zones in the intrusive bodies. Excavating method involves three major steps, namely: 1. Mapping and recording. 2. Excavation strategies. 3. publication of findings Archeologists usually have some information about the place of excavation with assistance of the maps.
They usually have several tools to aid in mapping and recording. The mapping of the place of excavation must be very accurate such that it shows the true values and precision in the map is usually reflected in the consistency within which the measurements can be repeated. The sequence of events must be recorded while still noting the soil texture, and color and presence and the size of rock and stone if any. The sequence of excavation depends on the objective to be achieved. If the objective is to retrieve small pieces of artifact, the recording of the data will be finer than if the aim will be to retrieve large objects.
Some methods used for recovering the artifact include water screening and elaborate screening Initially excavation was simply mining but today things have undergone metamorphosis. Today during excavation, any thing that is found must be recorded and a horizontal and vertical relationship established. These relationships explain both the cultural relations and reasons why the site was selected. There is further recording and testing as the excavation proceeds to deeper strata with keen attention to the variances in the soil color and texture.
Generally, excavation proceeds till the objective is achieved. Convectional excavation apparatus include magnifying glass, pruning shears, whiskbroom, bamboo stick, dustpan, tape measures, trowel, army shovel, grapefruit, and shovel and may be bulldozer. Since the work of excavation mostly involves destruction of the original site, its important to record the data for use by the future generation. The data should be recorded and analyzed in chronological, strategraphic biological and physical manner. The materials that are excavated have got many uses since they vary in many aspects.
These materials include soil, rocks, mineral, artifacts, and water. First the soil used may be used for agricultural purposes in area which were not arable due to the composition of the soil. The soil can also be used to fill in dam that were left uncovered and have turned to be breeding places for mosquitoes and other pests like in east Africa. The soil can also be used in learning institution for carrying out educational experiment. The soil can also be studied to know its chemical composition there for determine the history of that area by the archeologist.
The rocks excavated can be used for economic purposes like construction purposes. The rocks are very important in most construction purposes since they form a very strong base in the most of the building. The mineral discovered are very useful depending with their composition, minerals like uranium is used in manufacturing of atomic bombs while other like the coal is used in making of various products like cement upon combination with other mineral.
REFERENCES What are the 3 basic types of rocks? Retrieved on 29th august, 2007, available at WWW. jersey. uoregon.
edu/~mstrick/AskGeoMan/geoQuerry13. html Analysis of the Modes of Rock Mass Structure on Slope Stability, retrieved on 29th august, 2007, available at www. pku. edu. cn/academic/xb/2002/_02e216. html Types of Rock, retrieved on 29th august, 2007, available at www. sand4students. net/en/textbook/text04. html Rock structure, retrieved on 29th august, 2007, available at www. core/topics/rocks/rock_structure/rock_structure_intro. html Metamorphic Rocks, retrieved on 29th august, 2007, available at www. uwsp. edu/geo/faculty/ritter/geog101/textbook/earth_materials_structure/metamorphic_rocks. html