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Chapter 5

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Physics and Astronomy
PHYS 1470
Paul Delaney

Chapter 5 – The Earth Moon System 2/2/2013 4:53:00 PM How The Earth Cooled  Buffon measured how long it took globes to cool  How time depended on the volume of the sphere  Buffon pondered the basic question "How old is our planet?" He suspected that the Earth was a sphere of rock and metal, and he wanted to determine its history  He produced a 36-volume encyclopedia called the Theory of Nature  Buffon addressed the question directly. He assumed that Earth had formed in conjunction with the much hotter Sun, and that the Earth had therefore started in a molten state  Buffon published his conclusion that the planet was 74,832 years old  That the real age of the Earth is still greater Early Estimates of the Earth’s Age  In the Middle Ages, scholars thought they could calculate Earth's age by finding out how long humans had lived on Earth  Ussher deduced that the cosmos formed on Sunday, October 23, in 4004 B.C., and that humanity was created on Friday, October 28 the same year  This hypothesis suggested a simple question: how long would a molten Earth take to cool to present-day temperatures?  First, they studied sedimentary layers exposed in canyons in many parts of the world and realized that the total depth of sediments is immense  Second, geologists found evidence that mountains had gone through many cycles of erosion, subsidence, and uplift Ages Using Radioactivity  The discovery of radioactivity happened by accident  A radioactive atom is an unstable atom that spontaneously changes (usually into a more stable form) by emitting one or more particles from its nucleus  The original atom thus becomes either a new element (change in the number of protons in the nucleus) or a new form of the same element, called an isotope (change in the number of neutrons in the nucleus)  The original atom is called the parent isotope and the new atom is called the daughter isotope  The time required for half of the atoms of any original radioactive parent isotope to decay into daughter isotopes is called the half-life of the radioactive element  rubidium-87 decays to strontium-87 with a half-life of 49 billion years  uranium-238 decays (in a series of steps) to lead-206 with a half- life of 4.5 billion years  potassium-40 decays to argon-40 with a half-life of 1.25 billion years  carbon-14 decays to nitrogen-14 with a half-life of only 5570 years  This means that the exact time when an individual atom decays is impossible to determine – random  Suppose we could determine the original number of parent and daughter isotope atoms in a rock meaning at the time when the rock first formed  This can be done by counting the relative numbers of different isotopes in the minerals of the rock  Then, if we simply count the present numbers of parent and daughter isotope atoms in the rock, we can tell how many parent atoms have decayed into daughter atoms and hence tell how old the rock is  For instance, if half the parent atoms have decayed, the age of the rock equals one half-life of the radioactive parent element being studied. This technique of dating rocks is called radioactive dating  Choose a parent isotope that is matched in half-life to the approximate age of the phenomenon we want to measure Radioactive Half-Life  Give the age of a rock sample in terms of the number of atoms that have decayed  After 2 million years, half of that amount would be left. This would be ½ × ½ = 1/4 of the original number. How many would be left after 3 half-lives, or 3 million years? It would half that number again, or ½ × ½ × ½ = 1/8  F = (1/2)N  log F = N log (1/2) = -0.301 N  A particular radioactive form of potassium decays with a half-life of 1.25 billion years (known to 3 significant digits), yielding a certain form of argon atoms. Suppose we measure the argon and potassium in the rock crystal, and we find that 58% of the radioactive potassium has already decayed into argon, while 42% of the original radioactive potassium atoms are left in the crystal. How old is the rock? Our measurement has told us that F is 0.42, and so our equation gives -0.376 = -0.301 N. Thus, N = 1.25 half- lives. That would mean that the rock is 1.62 billion years old Ages of the Earth and Moon  Using radioactive dating of rocks, we can measure the time since the rock was last melted  By knowing the rate of decay processes, and measuring the ratio of parent and daughter isotopes, it's possible to place constraints on the age of a rock  The best estimate for the total age of the Earth is 4.6 billion years  Scientists add about 100 million years to this age for the time it took the molten Moon to solidify  We've been around for less than a tenth of one percent of the history of our planet! Internal Heat and Geological Activity  Heat is the ultimate source of energy that drives geological activity on the planet  Temperature is a measure of particles’ speed, so increased motion results in a higher temperature  Radioactive material is therefore an energy source, and it heats the interior of the planet in which it’s trapped  Planets produce heat according to their size  Radioactive atoms decay in the interior, and conduction and convection transport this heat from the interior to the surface  Bigger planets have more gravity, and the pressure due to gravity helps to create a molten interior that can drive geological activity  The bigger the planet, the longer it takes internal heat to reach the surface  Therefore, the surface of a large planet will be young and show fewer impact craters  The strong temperature difference between the center and surface induces convection in the mantle - sluggish mass movement of the hot, slightly plastic material  If they’re hot enough, the inner rock layers of a planet can actually flow slowly  Hot plumes of convecting material rising in the asthenosphere create "hot spots" under the lithosphere  These hot spots produce volcanoes  Planets in order of increasing size o Deimos - Cold throughout - Heavily cratered o Phobos - Cold throughout - Heavily cratered o Moon - Mostly cold - Heavily cratered, some lava flows (3.5 billion years old) o Mercury - Mostly cold - Heavily cratered, some old flows (3.5 billion years old?) o Mars - Warm interior? - Half of the planet cratered; half is covered by young lava flows (1.4 billion years old?). Several very young volcanoes (500 million years old, possibly still active?) o Venus - Still hot - Few impact craters. Intense volcanism, lava-covered surface (mean age 0.7 billion years) o Earth - Still hot - Few impact craters. Active volcanism and plate tectonics (mean age of crust 400 million years)  The Sun is the dominant heat source only for the top meter or so of the surface - this layer warms in the day and cools off at night  The Sun's heat doesn’t penetrate more than a few meters into the planet  Another source of thermal energy that’s independent of size is tidal heating Internal Structure of the Earth and Moon  Seismic waves passing through the Earth  These waves can be generated by earthquakes, volcanism, or artificial explosions  Seismic waves bend according to the density of the rock they pass through  Seismic waves slow down when they enter denser rock  Through seismic studies we can map the transitions from dense core, to overlain mantle of moderate density, to a crust of the least dense rocks at the surface  The crust ranges from about 5 km thick under the oceans to about 30 km thick on the continents  The boundary between the crust and the mantle is marked by a seismic discontinuity called the Mohorovičić (or Moho) discontinuity  This occurs at about 5 to 10 km below the oceanic crust, and about 30 to 60 km below the continents  At this depth, seismic waves suddenly increase their velocity  We can tell from the density of the core that it can't be pure iron  The outer part of the core is a cauldron of molten metal  The inner part is even hotter, but it's under such high pressure that it remains solid, in spite of the heat  Earth's iron core has a radius of about 3500 kilometers, just over half of the planet's radius.  Lunar seismology data  Mean density of the Moon much lower than Earth's Basic Rock Types  A rock is a combination of two or more minerals that have either been fused together by heat or pressure, or that have cooled out of a liquid together  As an example, rubies are just aluminum oxide molecules arranged into a crystalline lattice  Three basic varieties of rock o Igneous rocks, forms when molten material cools and solidifies, either above ground (―extrusive‖) or below ground (―intrusive‖) - granite, basalt, and all other volcanic rocks o Sediments can solidify by compression due to overlying material, or by the formation of cement when liquid solutions percolate through the sediments, most common  Two common terrestrial sedimentary rocks are sandstone, which is formed from compressed sand grains, and limestone, which is produced when deposits of calcium carbonate (CaCO3) precipitate out of solution in water o Metamorphic rocks, form when igneous or sedimentary rocks are modified by heat, pressure, or hot water containing acidic solutions or dissolved minerals  Marble, for example, forms when limestone is subjected to heat and pressure  Impact events  Breccia: a type of rock formed when when minerals are cemented together in the high-heat, high-pressure environment of an impact  The oldest rocks from the Moon are 4.4 to 4.5 billion years old Layers of the Earth and Moon  The layers are not distinguished by their composition, but instead by how the rock moves in response to a force  The brittle surface layer is called the lithosphere  The Earth’s lithosphere extends about 100 kilometers down through the crust into the upper mantle  The asthenosphere, can deform plastically - the regions below the lithosphere  Moon’s lithosphere is about 1,000 kilometers deep - much thicker than the Earth’s lithosphere  This is because the Moon is smaller than the Earth, and so it cooled more quickly and completely The Evolving Earth  Uniformitarianism, refers to the long timescales over which the surface of the Earth transforms  ―Catastrophism:‖ a relatively short geologic history, characterized by sudden cataclysmic events  Churning convection cells in the mantle drive plate tectonics, constantly creating new surface rock and destroying old rock at subduction zones  A fossil is the remains of a living organism, after minerals have seeped in and hardened  Lyell also studied fossils, and speculated that climate change had caused the extinction of many species. Like rock, life too was subject
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