Econstudentlog

Earth (2)

You can find my first post about the book here. Chapter 11 has a lot of stuff on wave dynamics which I found very interesting, but I won’t quote from that. I guess you can go watch Muller’s lectures on that subject if you’re interested – it’s a place to start. Generally there’s a lot of interesting stuff like this in the book which is just impossible for me to cover here (but I already mentioned that in the first post). Anyway, some more stuff from the book:

i. “The forces that shape coasts are essentially the same as those that form other topographies: the destructive processes of erosion operating in conjunction with currents that transport and deposit debris and the tectonic forces that cause uplift or subsidence of the Earth’s crust.”

ii. “The Sun, though much farther away [than the moon], has so much more mass that it, too, causes tides. The Sun tides are a little less than half the height of Moon tides. The two sets of tides are not synchronous. Those related to the Sun come every 24 hours, once each “solar” day. The time of rotation of the Earth with respect to the Moon is a little longer than the solar day – 24 hours and 50 minutes, because the Moon is moving around the Earth. In that 24 hours and 50 minutes, the “lunar day”, there are two high waters, with two low waters in between.
When the Moon, Earth and Sun line up, the combined gravitational pull of Sun and Moon reinforce each other and produce very high tides, the spring tides […] Such high tides come every two weeks at full and new Moon. The lowest tides, the neap tides, come between, at first- and third quarter Moons, when the Moon and Sun are at right angles to each other with respect to the Earth.
The above account describes the equilibrium tide, that is, one that is theoretically calculated for a uniform globe. The heights of the actual tides are very different in various parts of the oceans. Because the oceans are of various shapes and sizes, the water of the tide responds in complex ways.”

iii. Along shallow coasts, tidal movements give up energy through friction of the water with the sea floor—energy that must ultimately come from the rotation of the Earth and the Moon. That frictional loss is enough to slow the rotation of the Earth by a very small amount. […] The Earth must once have been rotating much faster, though the time of its revolution around the Sun was unaffected. The Moon’s rate of revolution around the Earth would also have been faster, and the Moon would have been closer to the Earth. That means that the tides would generally have been much higher, that there were many more days in a year, and that the days were shorter. […] Careful counting of […] layers in fossil corals has convinced many paleontologists that, 400 million years ago, there were nearly 400 days in a year.”

iv. “The surface currents of the oceans can be simplified into a pattern of large closed loops, called gyres […] The winds constitute the primary driving mechanism for the surface currents of the ocean. […]
The surface waters are warm near the equator—around 25°C or warmer—while those in the Arctic and Antarctic are quite cold—about 0°C or a little above. Because cold water is denser than warm, the polar waters tend to sink and slide along the buttom toward the equator […] As they do, they push in front of them the deeper waters, which tend to rise near the equator, being displaced from both directions. […] Because the dense, cold waters move slowly and mix with the surrounding waters very slowly, they tend to retain their original temperature and salinity […] The density-driven vertical circulation is much slower than the wind-driven horizontal circulation at the surface. Deep waters rise at the equator at speeds of only 2-5 m (6-15 ft) per year. Polar waters take about 1000 years to circulate […]”

v. “Physical sedimentation starts where transportation stops. When the wind dies down, dust settles; when water currents slow, sand settles. On Earth, physical transportation and sedimentation follow a general downhill trend in response to gravity, from rockfalls and mass movements downslope to river systems, and then down to the sea […] In running water, sedimentation is a one-way street, each temporary stage of transport and sedimentation carrying the sediment farther toward the bottom of the deep sea. Much of it is dropped along the way and never reaches the end of the line. Eolian sedimentation is different, for winds may blow material from low to high places and back again. But in the long run, eolian sedimentation is effectively a one-way street, too: Once windblown material drops to the ocean surface, it is trapped. It settles through the water and cannot be picked up again.
Chemical sedimentation is also a downhill process, but the driving force is chemical rather than gravitational. A major aspect of weathering and erosion is the chemical decay of rocks exposed to the water and carbon dioxide of the atmosphere. In the course of decay, ions from the rocks are dissolved, and rivers carry them to the sea […] The ocean may be thought of as a huge chemical reservoir: Water continually evaporates from the surface, and fresh river water runs in to replenish it. Although that keeps the amount of water fairly constant, it works also to enrich the sea in the dissolved ions: Evaporation takes away only the water; the ions do not evaporate.* Yet the sea maintains the same salinity. It does so because of sedimentation of the dissolved material as chemical precipitates. Totaled over all of the oceans of the world, those precipitates must balance the total inflow of ions released by weathering and brough in by rivers.”

vi. “Once a sediment is deposited and buried by other sediments, it is not immune to change. […] The many processes that produce the changes in a rock’s composition and texture after deposition are lumped together in the term diagenesis. Generally, they operate to harden the soft sediment into rock—that is, to lithify it. Diagenesis may also alter the mineral composition by dissolving some of the original minerals and precipitating new ones. The nature of oil, gas, and coal is almost completely the result of diagenesis of original sedimentary organic matter.
The major physical diagenetic change is compaction, a decrease in porosity caused by mineral grains being squeezed closer to each other by the weight of the overlying sediment […] Chemical diagenetic changes are the result of two general tendencies. The first is a gradual approach toward chemical equilibrium of the nonequilibrium mixture of diverse minerals that have been brought together as detritus in the sediment […] The second tendency is for a sediment to be buried more or less deeply in the crust. As a sediment is buried, it is subjected to increasingly high temperatures—on the average 1°C for each 30 m (100 ft) of depth on the continents […] and high pressures—on the average about 1 atm (atmosphere) for each 4.4. m of depth […] As minerals and the surrounding groundwater in pore spaces are heated and put under greater pressure, they tend to react chemically to form new minerals. This process, when carried far enough, becomes metamorphism, in which the entire character of the rock alters. The boundary between diagenesis and metamorphism is somewhat arbitrary, usually drawn at a temperature of about 300°C.”

vii. “there is such an immense amount of oxygen in our atmosphere and oceans […] that, even if all photosynthesis stopped tomorrow and all other respiring life went on as before, it would be several thousand years at least before oxygen would be significantly depleted by respiration and all of the other reactions in which oxygen is used”

viii. “The continents may be likened to rafts embedded in large plates. The rafts have grown through geological time. The oldest rocks found on Earth have been preserved on continents for nearly 4 billion years. Continents are difficult to destroy; they may be deformed, but they survive plate convergence because they are light enough to keep afloat. In marked contrast, the sea floor is created at mid-ocean ridges and destroyed in subduction zones on a time scale of 100-200 million years. […]
Oceanic heat flow is […] dominated by the process of cooling of the recently created oceanic lithosphere. Geophysicists believe that this form of convection may account for as much as 60% of the total heat flow from the Earth and that this may represent a major mode by which the Earth has cooled.”

August 13, 2012 - Posted by | books, Geology

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