The Earth Contents

The Earth - Rock Cycle - Page 13

Question 1 answer

The formation, breakdown, transportation and re-deposition of rocks can be represented as a cycle. The main components of the rock cycle are cycling between sediments, biogenic depositis, sedimentary rocks, metamorphic rocks, magma and igneous rock and interactions with the hydrosphere, biosphere and atmosphere.

Physical and chemical weathering processes operate in tandem on rocks of all types to separate their chemical constituents according to their solubility. Both the residual solids and the dissolved components are ultimately transported via surface water flow and rivers into the oceans. Here they create marine sediments. Burial then converts sediments into new sedimentary rocks, which may in time become uplifted and again exposed to weathering processes on land. In addition, buried rocks may be subject to high temperatures enough to melt them, producing metamorphic rocks. Volcanic activity returns melted rock to the Earth's surface, and thereby initiates the erosion process once more.

Question 2 answer

The separation of dissolved load from solid material depends on the extent to which each element can interact with water. Water is a powerful solvent because of the dipole it possesses which allows the water molecule to attach itself to both negative and positive ions, and hence facilitate the separation of both types of ion in an ionic bond.

Question 3 answer

Both alkali elements and alkaline Earth elements have a high degree of ionic character which allows them to be readily solubilised by water. Many other rock minerals contain bonds which are a mixture of ionic and covalent character, with the result that these elements are only weakly dissolved by water. Calcium, magnesium, potassium and sodium all have high % ionic character, 79%, 71%, 87% and 83% respectivel, compared to Aluminium (60%) and silicon (48%), which posses both ionic and covalent bond characteristics. Thus, this is why calcium, magnesium, sodium and potassium nearly always predominate over aluminium and silicon in natural waters derived by the weathering of silicate rocks, even though these rocks typically consist of 5-10 times more aluminium and silicon than the other elements.

Question 4 answer

The ionic potential is defined as the ratio of ( (ionic charge)/( ionic radius) ). Ionic charge represents the number of electrons lost or gained by an atom, or group of atoms. If the ion has more electrons than protons, it is negatively charged, and vice-versa. The ionic radium is the distance from the centre of an ion to the outermost eleectron(s). Negative ions have a greater radius whilst positive ions have a smaller radius than the corresponding neutral atom.

Question 5 answer

The removal of soluble species leaves behind residual partially solubilised rock minerals, which are now relatively enriched in the insoluble elements, in particular silicon and aluminium.

Question 6 answer

In some cases the residues and the new clays may remain on land to constitute a soil, and in others they may be transported by rivers into the oceans. The journey into the oceans may involve many intermediate resting places, such as flood plains and mud flats, often for prolonged periods and particles are often combined into soils. The particulates found at these intermediate depositional sites reflect a number of factors. For example, river velocity affects the maximum size of a solid particle which can be transported, so that faster flows tend to correlate with higher sediment loads.

Question 7 answer

Rivers vary in velocity through their flow paths, and sites where the flow slows down will accumulate sediment, at least until the next flood is able to move them onward. River flow paths are closely related to the topograhy of the land over which the river is flowing. Rivers may sometimes change course, leaving behind sediments. Throughout the length of each river, material is being exchanged with the land, the water flow erodes soils and rocks, sediments are picked up and may be deposited further downstream.

Question 8 answer

Large quantities of material are eroded and transported as glaciers move over their bedrock floors. These materials are eroded from continental masses, and deposited downstream as moraine as the ice melts, from where they are subject to further weathering and erosion processes.

Question 9 answer

The movement of wind over bedrock and substrate can erode material from the surface, as the wind lifts and move loose particles of dust and silt. These airborne particles can then attack and wear away larger surfaces of solid rock through natural sandblasting.

-

Most particular material produced during weathering is deposited in the oceans, simply because gravitational processes always tend to bring particulates down to the lowest possible point. Coarser elements (such as sand) are deposited first, as the rivers slow down on entering the estuaries. Finer material, such as silt, is transported to the continental shelves, whilst only a tiny fraction of the sediment (clay, the very finest particles) are able to stay in suspension long enough for transportation away from the land and into the deep oceans to deposit on the abyssal plain.

Question 10 answer

Most particular material produced during weathering is deposited in the oceans, simply because gravitational processes always tend to bring particulates down to the lowest possible point. Coarser elements (such as sand) are deposited first, as the rivers slow down on entering the estuaries. Finer material, such as silt, is transported to the continental shelves, whilst only a tiny fraction of the sediment (clay, the very finest particles) are able to stay in suspension long enough for transportation away from the land and into the deep oceans to deposit on the abyssal plain.

Question 11 answer

Differences in the type of mineral found at different locations arise because different sediment components have different origins. The material derived from land (or from the weathering of undersea volcanic material) is termed lithogenous. Material in marine sediments which has been derived by biogenic processes, principally the from the skeletons of marine organisms (usually silica and calcium carbonate) is termed biogenous. Hydrogenous clays are derived by inorganic precipitation from sea water. An example would be the new clay minerals formed by the reaction between sea water and basalt, as well as evaporite minerals.

Question 12 answer

Marine sediment patterns reflect the complex interplay between the rates at which lithogenous material is supplied from the continents, compared to the rates at which either biogenous or hydrogenous materials can be generated in situ.