Oceanography-Chapter 3.pdf

Oceanography Crust-ocean interactions Genesis of an ocean Continental drift has reconfigured the Earth's oceans, joining and splitting ancient oceans to form the current oceans. Ancient oceans include: Bridge River Ocean, between the ancient Insular Islands and North America. Iapetus Ocean, between...

Oceanography Crust-ocean interactions Genesis of an ocean Continental drift has reconfigured the Earth's oceans, joining and splitting ancient oceans to form the current oceans. Ancient oceans include: Bridge River Ocean, between the ancient Insular Islands and North America. Iapetus Ocean, between Baltica and Avalonia. Panthalassa, the vast world ocean that surrounded the Pangaea supercontinent. Rheic Ocean, between Gondwana and Laurentia Slide Mountain Ocean, between the ancient Intermontane Islands and North America. Tethys Ocean, between the ancient continents of Gondwana and Laurasia. Khanty Ocean, between Baltica and Siberia. Mirovia, the ocean that surrounded the Rodinia supercontinent. Paleo-Tethys Ocean, between Gondwana and the Hunic terranes. Pan-African Ocean, the ocean that surrounded the Pannotia supercontinent. Superocean, the ocean that surrounds a global supercontinent. Ural Ocean, between Siberia and Baltica. Genesis of an ocean Genesis of an ocean Genesis of an ocean Genesis of an ocean In the early 1900's Alfred Wegener proposed the idea of Continental Drift. His ideas centered around continents moving across the face of the earth. The idea was not quite correct - compared to the plate tectonics theory of today - but his thinking was on the proper track. Continental Drift and Plate-Tectonics Theory According to the theory of continental drift, the world was made up of a single continent through most of geologic time. That continent eventually separated and drifted apart, forming into the seven continents we have today. The first comprehensive theory of continental drift was suggested by the German meteorologist Alfred Wegener in 1912. The hypothesis asserts that the continents consist of lighter rocks that rest on heavier crustal material—similar to the manner in which icebergs float on water. Wegener contended that the relative positions of the continents are not rigidly fixed but are slowly moving— at a rate of about one yard per century. According to the generally accepted plate-tectonics theory, scientists believe that Earth's surface is broken into a number of shifting slabs or plates, which average about 50 miles in thickness. These plates move relative to one another above a hotter, deeper, more mobile zone at average rates as great as a few inches per year. Most of the world's active volcanoes are located along or near the boundaries between shifting plates and are called plate-boundary volcanoes. EVIDENCE THAT THE CONTINENTS WERE ONCE JOINED: The continents seem to fit together like a giant jigsaw puzzle Identical plant and animal fossils of the same age have been found in rocks in Africa and South America. This strongly suggests that the two were once joined. For instance the fern 'Glossopteris' can be dated at 270 million years ago and can be found in banded areas of South America, Africa, India and Australia. The types of rocks found on each continent today show similar strata and ages. Mountain range on the coast of West Africa disappear then suddenly reappear again on the coast of South America. This is an example of a divergent plate boundary (where the plates move away from each other). The Atlantic Ocean was created by this process. The mid-Atlantic Ridge is an area where new sea floor is being created. As the rift valley expands two continental plates have been constructed from the original one. The molten rock continues to push the crust apart creating new crust as it does. As the rift valley expands, water collects forming a sea. The Mid-Atlantic Ridge is now 2,000 metres above the adjacent sea floor, which is at a depth of about 6,000 metres below sea level. The sea floor continues to spread and the plates get bigger and bigger. This process can be seen all over the world and produces about 17 square kilometres of new plate every year. In the diagram you can see that the continental crust is beginning to separate creating a diverging plate boundary. When a divergence occurs within a continent it is called rifting. A plume of hot magma rises from deep within the mantle pushing up the crust and causing pressure forcing the continent to break and separate. Lava flows and earthquakes would be seen. The yellow dot shows a massive underwater chain of mountains that stretch right down the Atlantic. It is called the Mid-Atlantic Ridge and it is an area where new plate is constantly being created. Oceanic surveys found that such mountain chains extend all over the world. The ocean floor rocks are made from magma that has been erupted into the water at the mid-ocean ridge. The two plates are moving away from each other at the ridge and new plate is created. The rock is called Basalt. These rocks contain minerals that are magnetic. One such mineral is called magnetite. If magnetite is heated over 500°c (called its Curie temperature) it loses its magnetism. So the magma pouring out at the ridge will not be magnetic until it cools to form new Basalt. The key point is that when magnetite is cooled it once again becomes magnetic (a phenomena called remanent magnetism). Plate tectonics (from Greek τέκτων, tektōn "builder" or "mason") describes the large scale motions of Earth's lithosphere. The theory encompasses the older concepts of continental drift, developed during the first half of the 20th century, and seafloor spreading, understood during the 1960s. The outermost part of the Earth's interior is made up of two layers: Above is the lithosphere, comprising the crust and the rigid uppermost part of the mantle. Below the lithosphere lies the asthenosphere. Although solid, the asthenosphere has relatively low viscosity and shear strength and can flow like a liquid on geological time scales. The deeper mantle below the asthenosphere is more rigid again due to the higher pressure. The lithosphere is broken up into what are called tectonic plates — in the case of Earth, there are seven major and many minor plates. The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: convergent or collision boundaries, divergent or spreading boundaries, and transform boundaries. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 50—100 mm/a. Asthenosphere, hot, partially molten layer of Earth, lying below the lithosphere (the rocky outer layer of Earth). The top layer of the upper mantle is called the asthenosphere. The asthenosphere (from an invented Greek ἀσθενός a + ''sthenos "without strength" and Greek word σφαίρα (sphera) meaning globe) is the region of the Earth between 100 and 200 km (~ 62 and 124 miles) below the surface — but perhaps extending as deep as 400 km (~ 249 miles) — that is the weak or "soft" zone in the upper mantle. The asthenosphere lies just below the lithosphere, which is involved in plate movements and isostatic adjustments. In spite of its heat, pressures keep it plastic, and it has a relatively low density. Isostasy Isostasy takes place on the Earth wherever a large amount of weight is present. This weight might be due to a large mountain, ice from an ice age, or even from manmade structures, such as the weight from large manmade lakes. Isostasy also takes place when a large amount of weight is removed from an area, causing that portion of the Earth’s crust to rise, such as when ice caps melt. Isostasy (Greek isos = "equal", stásis = "standstill") is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. On a geological scale, isostasy can be observed where the Earth's strong lithosphere exerts stress on the weaker asthenosphere which, over geological time, flows laterally such that the load of the lithosphere is accommodated by height adjustments. Isostatic effects of deposition and erosion When large amounts of sediment are deposited on a particular region, the immense weight of the new sediment may cause the crust below to sink. Similarly, when large amounts of material are eroded away from a region, the land may rise to compensate. Therefore, as a mountain range is eroded down, the (reduced) range rebounds upwards (to a certain extent) to be eroded further. Some of the rock strata now visible at the ground surface may have spent much of their history at great depths below the surface buried under other strata, to be eventually exposed as those other strata are eroded away and the lower layers rebound upwards again. An analogy may be made with an iceberg - it always floats with a certain proportion of its mass below the surface of the water. If more ice is added to the top of the iceberg, the iceberg will sink lower in the water. If a layer of ice is somehow sliced off the top of the iceberg, the remaining iceberg will rise. Similarly, the Earth's lithosphere "floats" in the asthenosphere. Plate tectonics is the study of the lithosphere, the outer portion of the earth consisting of the crust and part of the upper mantle. The lithosphere is divided into about a dozen large plates which move and interact with one another to create earthquakes, mountain ranges, volcanic activity, ocean trenches and many other features. Continents and ocean basis are moved and changed in shape as a result of these plate movements. Interactions The Mid-Oceanic Ridges are entirely volcanic; here, plates are moving apart (‘diverging’), and magma is rising to fill the gaps. The Oceanic crust is subducted beneath the Continental crust. The Oceanic plate sinks underneath another Oceanic plate, and the rising magma creates a chain of explosive volcanoes as an island arc. Diagenesis In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism. These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture. There is no sharp boundary between diagenesis and metamorphism, but the latter occurs at higher temperatures and pressures. Hydrothermal solutions, meteoric groundwater, porosity, permeability, solubility, and time are all influential factors. After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution. Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis. Porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals. The study of diagenesis in rocks is used to understand the tectonic history they have undergone; the nature and type of fluids that have circulated through them. There is not a clear, accepted distinction between diagenesis and metamorphism, although metamorphism occurs at pressures and temperatures higher than those of the outer crust, where diagenesis occurs. Diagenesis: the physical, chemical or biological processes that turn sediment into sedimentary rock by modifying the mineralogy and/or texture. Diagenesis occurs where the mineralogy of the rock becomes unstable as a result of changes in the conditions or chemistry. Instability usually occurs at grain contacts and in pore space between the grains. Changes in pressure and temperature cause new minerals to form or preexisting minerals to become modified as the sediment (or rock) adjusts to new equilibrium conditions. There are 7 main diagenetic processes: compaction recrystallization solution cementation authigenesis replacement bioturbation Compaction is the process by which the volume of a sediment is reduced as the grains are squeezed together. The weight of the overlying sediment and rock causes a reorganization of the packing of grains and the expulsion of intergranular fluid. As a result, the porosity of the sediment is reduced. The degree of compaction is controlled by such factors as grain shape, sorting, original porosity, and the amount of pore fluid present. Recrystallization is a process in which physical or chemical conditions induce a reorientation of the crystal lattices of mineral grains. These textural changes cause the sediment to become lithified. It occurs in response to such factors as pressure, temperature, and fluid phase changes. It also occurs as a result of solution and reprecipitation of mineral phases already present in the rock. Solution refers to the process in which a mineral is dissolved. As fluids pass through the sediment, the unstable constituents will dissolve and are either transported away or are reprecipitated in nearby pores where conditions are different. Cementation is the process in which chemical precipitates (in the form of new crystals) form in the pores of a sediment or rock, binding the grains together. Some common cements are quartz, calcite and hematite, but a wide variety of cements are known, such as aragonite, gypsum, and dolomite. Cementation reduces porosity by filling in the pore spaces between the grains. Uncrossed polars Crossed polars Photomicrograph of a dolomite-cemented siltstone in crossed and uncrossed polars. The cement between the grains can be easily seen. Authigenesis (neocrystallization) is the process in which new mineral phases are crystallized in the sediment or rock during diagenesis. These new minerals may be produced by reactions involving phases already present in the sediment (or rock) through precipitation of materials introduced in the fluid phase, or from a combination of primary sedimentary and introduced components. Replacement occurs when a newly formed mineral replaces a preexisting one in situ. Replacement may be: neomorphic: where the new grain is the same phase as the old grain, or is a polymorph of it (i.e. albitization; replacing a grain with a more Na-rich plagioclase grain). pseudomorphic: where the old grain is replaced with a new mineral but the relict crystal form is retained, allomorphic: an old phase is replaced with a new phase with a new crystal form Although there are many replacement phases, dolomite, opal, quartz, and illite are some of the most important phases. Bioturbation refers to the physical and biological activities that occur at or near the sediment surface which cause the sediment to become mixed. Burrowing and boring by organisms in this way, can increase the compaction of the sediment and usually destroys any laminations or bedding. During bioturbation, some organisms precipitate minerals that act as cement. Diagenetic processes include purely physical ones that involve rearrangement of the sediments such as compaction, slumping, bioturbation by organisms, infiltration, and soft sediment deformation Biochemical or organic processes such as particle accretion, flocculation, boring, and decomposition Physiochemical processes such as cementation, authigenesis (formation of new minerals), inversion, recrystallization, grain growth, replacement, and interstratal solution Most of these processes involve a reduction in porosity and permeability, which are two important sediment properties in considering the migration of subsurface fluids and the accumulation of oil and gas and certain types of mineral deposits in subsurface rock units. However, one diagenetic process, interstratal solution, is of major importance in creating secondary porosity. The degree to which each of these processes contributes to the diagenesis of any given sediment is controlled by such factors as composition pressure (due to burial) temperature the composition and nature of the pore fluids grain size porosity, permeability the amount of fluid flow Any sediment that has been deposited is subject to diagenesis The three main types, or classes, of rock Sedimentary Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils. Examples of this rock type include conglomerate and limestone. Metamorphic Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbonlike layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface. Examples of this rock type include gneiss and marble. Igneous Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock. Examples of this rock type include basalt and obsidian. 0 m – 200 m PRESSURE 0-10 ATM 13°C Mobula ray Eats tiny fish and small sea animals Has fins that extend up to 17 feet 0 m – 200 m PRESSURE 0-10 ATM 13°C Bull kelp Grows up to 80 ft long Many sea animals use bull kelp for both food and shelter 0 m – 200 m PRESSURE 0-10 ATM 13°C Humpback whale Female measures 49 to 52 feet long; male measures 43 to 46 feet long Flippers can grow up to 16 feet long Tails can grow up to 18 feet wide 0 m – 200 m PRESSURE 0-10 ATM 13°C 200 m – 1000 m PRESSURE 50 ATM 4°C Mesopelagic zone (twilight zone) Gets very faint sunlight Temperature varies in this zone due to the thermocline (temperature gradient) Twinkling lights of bioluminescent sea animals are visible 200 m – 1000 m PRESSURE 50 ATM 4°C Lanternfish Swims close to the surface to find food at night Has special organs to produce light (bioluminescence) 200 m – 1000 m PRESSURE 50 ATM 4°C 1000 m – 4000 m PRESSURE up to 398 ATM 4°C Bathypelagic zone (midnight zone) Sunlight doesn’t reach this zone Temperature is constant at 4°C Most sea animals in this zone are black or red because lack of sunlight leads to low visibility 1000 m – 4000 m PRESSURE up to 398 ATM 4°C Flapjack octopus Has fins on its head that look like ears Has 8 to 10 tentacles Giant tube worm Grows over 2 meters long Lives near hydrothermal vents on the floor of the Pacific Ocean Can tolerate extreme pressure and rapid changes in water temperature 1000 m – 4000 m PRESSURE up to 398 ATM 4°C 4000 m – 6000 m PRESSURE up to 750 ATM 2°C Abyssopelagic zone (abyssal zone) Sunlight doesn’t reach this zone Temperature is fairly constant near 2°C In most areas of the ocean, this zone is near the ocean floor 4000 m – 6000 m PRESSURE up to 750 ATM 2°C Common fangtooth Measures about 6 inches long Swims to the upper zones of the ocean to find food at night 4000 m – 6000 m PRESSURE up to 750 ATM 2°C 6000 m – 11,000 m PRESSURE up to 1,219 ATM 2°C Hadalpelagic zone (the trenches) Sunlight doesn’t reach this zone Pressure is 8 tons per square inch Animals in this zone lack color pigments due to complete lack of sunlight 6000 m – 11,000 m PRESSURE up to 1,219 ATM 2°C Supergiant amphipod Measures almost a foot long Eats sunken woods and plants