Final Exam Geology
Patterns of Surface Ocean Currents
Surface currents are found in the upper four hundred meters of an ocean body and they form major patterns such as eddies, upwelling, and gyres (Garrishon, 2007). Surface currents result from wind energy that is concentrated in both north and south hemispheres forming trade winds which are easterlies and westerlies (Garrishon, 2007). Waves on the sea surface transfer some of the energy from the moving air to the water by friction. Pulling of wind on the surface of ocean causes an accumulation of water with the water displaced underneath the wind forming the movement.
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Moving water ‘piles up’ in the direction the wind is moving to resulting into a higher pressure on the ‘pile up’ side. At the same time, gravity force pulls the water down the slope against a pressure gradient which the Corlolis effect intervenes. The Corlolis effect is a force created as a result of rotation of the earth (Kershaw & Cundy, 2000). In the intervention, a gyre is formed near the two poles of the earth which is a system of rotating ocean currents. They are mainly five and include: Indian Ocean, North Atlantic, South Atlantic, North Pacific, and South Pacific gyres.
Eddies are also caused by Corlolis effect and is as a result of water spinning away from a surface ocean current (Kershaw & Cundy, 2000). The centre of eddies is either warm or cold and are seen to form when a bend in a surface ocean current lengthens finally forming a loop that lasts for months. In the northern pole warm eddies rotate clockwise and cold counter clockwise, while in southern pole warm eddies rotate anticlockwise and cold clockwise. Upwelling occurs as a result of perpendicular movement of water from deep levels. The pattern happens when wind blows along a coastline causing surface water to move perpendicular to the coast line in a process called Ekman transport (Garrison, 2007). Movement of water away from coast results in water from deeper levels moving up to fill the gap.
Analysis of Waves: swell, breaking surf, and tsunami
Swell refer to any waves that spread away from a region in which they are produced (stormy area) travelling for longer periods, hence are hard to be predicted on the basis of local conditions. As the waves propagate their height somewhat decays due to friction, air resistance, and more importantly angular spreading (Garrison, 2007). Frequency dispersion also occurs in the waves resulting in longer waves moving faster than shorter ones. However, in cases where direction and frequency is narrow then the wave travels long without decay. The wave is characterised by long periods—above 8 seconds—and relatively low wave height between to 50 - 100 meters (Garrison, 2007). “The speed of a swell is obtained by multiplying the period by 1.5 hence the minimum speed is 12 nm per hour (Garrison, 2007, p. 233).”
Breaking Surfs are wind waves that undergo breaking, that is, the wave’s amplitude reach a critical level where a mechanism results in wave energy being transformed into kinetic energy that is turbulent (Kershaw & Cundy, 2000). Breaking surf forms when a waves formed by wind arrive at shallow waters, or when two waves merge or counter each other. They have been seen to reach maximum heights of between 12- 18 meters and a maximum speed of 18nm per hour hence are seen to have less wavelength and travel much slower than swells (Kershaw & Cundy, 2000).
Tsunami waves are extremely large waves that result from seismic activity in the ocean flow. Earthquakes and movements in the ocean floor result in waves that have long wavelengths between 6 to 300 miles (Garrison, 2007). The depth of the wave mainly depends on the ocean floor since the entire oceans are mostly shallow in comparison to mist tsunamis. Hence the speed will largely depend on water depth only making the speed to be 384 nautical miles per hour if the depth of 2.5 miles (Garrison, 2007). This makes the waves be extremely dangerous to human life.
Causes of Tides
Tide is a term used to refer to the systemic rise and fall of ocean levels in comparing with land. The process depends on gravitational force of the sun and moon, and the centrifugal force (Garrison, 2007). In the case of gravity all masses have gravity force whose strength varies with the distance between the two or more masses. This means that as distance between planetary bodies increase then the tide-raising force decreases by a factor of eight (Garrison, 2007). Therefore, the only body near the earth that could exert considerable tide-raising force is the moon. Lunar gravity force on the ocean will result in ocean waters to be drawn towards the earth’s face in front of the moon. Consequently, on the opposite end of the earth, a bulge results as a result of centrifugal force which occurs as a result of the moon circumventing around the earth.
“As a result of sun, moon, and earth interaction different tides occur where in instances of the full and new moon the spring tide occurs (Garrison, 2007, p. 254).” Spring tides occur when the sun and moon gravitational forces coalesce resulting to maximum levels of high and low tides. In quarter moons, the moon and sun are at perpendicular positions which result in cancellation of gravity force resulting in neap tides characterised by minimum levels of high and low tides.
Latitudes which signify locations on the globe, determine the period of tides on the earth. When the earth rotates a point on the earth passes into and out of the bulges of water creating tides. Depending on the angle of declination, when a point enters a water bulge then tide rises and lowers when it leaves. Therefore different latitudes will pass through various parts of tidal bulges—inside or near. From this system diurnal tides occur at high latitudes, mixed tides at mid-latitudes, and semi-diurnal tides at low latitudes (Kershaw & Cundy, 2000). Seasons also influence tide formation—more so declination tides—when the sun is 23.5 0 during the summer and winter solstice resulting in a more diurnal sun tide during the seasons of winter and summer (Kershaw & Cundy, 2000).
Depositional and Erosion Coasts
Depositional Coasts result from accumulation of sediments whose sources are from: local sources; or by sediments transported by rivers, glaciers, or by ocean currents (Kershaw & Cundy, 2000). The generally recognizable facet in the coast is a beach which is an area of loose sediments covering part or all of the shore. “All coasts next to the edge of Lithosperic plates that is trailing tend to have extensive relief and low plains which are characterised by estuaries, river deltas, lagoons, and barrier Islands that accumulate various types of sediments. Erosion is also experienced but the rate is not as that of deposition (Kershaw & Cundy, 2000, p.67).”
Waves, currents, and tides considerably influence these coasts by dictating which landform will be in the beach. Wave dominated coasts are seen to contain advanced sand beaches usually formed on long barrier Islands containing less tidal spaced inlets having wide spaces. Examples of this coast are Texas and North Carolina coasts. Tide dominated coasts are rare and result in coasts having numerous funnel-shaped embayments, tidal creeks and flats, and salt marshes. Example is the West German Coast of the North Sea. Mixed coasts having both tidal and wave influences have landforms such as many tidal inlets and short barrier Islands. An example is South Carolina Coasts
Erosion coasts are coasts where rate of erosion is more than deposition and depict high relief and topography that is rugged (Garrison, 2007). They are found in the leading edge of lithosperic plates. They also result from glacial erosion and examples are coasts in north of New England. These coasts have common facets such as sharp slopes, bare bedrocks, and high altitudes at the shore. The most common feature is sea-cliffs, wave-cut platforms, and sea stacks and arches (Garrison, 2007). The different features are as a result of differential rates of erosion due to resistance from rocks at the coast. This feature causes intermittent mass movements characterised by the shape of the coastline and the features listed.
Survival of Marine Organisms
“The pelagic zone in the ocean refers to any water region that is neither near the bottom nor the shore (Kershaw & Cundy, 2000, p. 96).” The region occupies approximately 1500 million cubic meters, a minimum depth of 4 kilometres, and a maximum depth of 12 kilometres (Kershaw & Cundy, 2000). The zone is divided into three categories in accordance to conditions that persist in them. Conducive conditions for organisms’ life decreases as one goes deeper in the zone. Aspects such as pressure increases deeper, light decreases, and temperature decreases. The three zones are: top zone called euphotic that allows light penetration; dyspotic or twilight zone having little light; and aphotic zone where no light reaches (Garrison, 2007).
Life is also affected as you go deeper with decreased levels of nutrients and oxygen being experienced. In the top zones there are many organisms and the food chain starts with planktons and ends with predators. Examples of pelagic fish in this zone are jelly-fish, tuna, dolphins, and sharks. Organisms in the second zone have specialised gills that absorb more oxygen and also move up the higher zone at night to feed. Organisms in this region are swordfish and squids. The last zone has no light, little dissolved oxygen, and very high pressure. Organisms have adapted by having no eyes and they survive by consuming debris that falls from higher zones. Examples of organism are dumbo octopus and giant squids.
In the Benthic zone entails the bottom waters of oceans and lakes, ocean floor, and sub-layer of ocean floor (Garrison, 2007). Organisms living in this region are called benthos and are adapted to living in these extreme conditions. Since there is no light, the only source of energy is from organic matter from higher zone. Some adapt to extremely cold conditions by living in the floor of the ocean. Some organisms such as Hatchet fish produce their own chemical light and prey on whichever organism that comes about. Furthermore because of the large oxygen concentrations in this zone, organisms in this region are larger than their counterparts in higher zones (Garrison, 2007).
References
Garrison, T. (2007). Oceanography: an invitation to marine science (6th ed.). California: Thomson Higher Education. Pp. 233-537
Kershaw, S. & Cundy, A. (2000). Oceanography: an earth science perspective. Cheltenham: Stanley Thornes. Pp. 31-111
