{"id":2988,"date":"2016-07-25T18:56:08","date_gmt":"2016-07-25T18:56:08","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/geo\/?post_type=chapter&p=2988"},"modified":"2020-01-12T00:31:09","modified_gmt":"2020-01-12T00:31:09","slug":"plate-tectonics-assessment","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/chapter\/plate-tectonics-assessment\/","title":{"raw":"Earth Density, Isaac Newton, And Internal Layering","rendered":"Earth Density, Isaac Newton, And Internal Layering"},"content":{"raw":"OK,, not everyone is a math whiz.\r\nBut Isaac Newton (back in the late 17th century) was certainly both great scientist and great mathematician.\u00a0 He worked out the whole science of \"mechanics,\" i.e. how things move!\r\nAnd he developed a whole new kind of math to help describe the business of objects in motion (calculus).\r\n\r\n\"\"<\/a>\r\n\r\nIt turns out that it doesn't require calculus to do some great stuff with the LAW OF GRAVITY.... which just says that the FORCE of gravity, is directly related to the masses of objects in question, and the distance between them.\u00a0 Perhaps you've seen the equation,\u00a0 F=GmM\/r2<\/sup>,\r\nIn this equation,\u00a0 m can be the mass of an object falling towards earth's surface, and M is the mass of the earth.\r\nG is a constant that was determined in the 18th century (courtesy of a rather eccentric wealthy English scientist, Henry Cavendish) and\u00a0 r is the radius of earth.\r\n\r\nOnce G was worked out by Mr. Cavendish, and since we know the acceleration of objects at earth's surface,, about 10m per second squared, it's possible to evaluate the mass of the earth!\r\nIt's about 6x1024<\/sup> kilograms (6 followed by 24 zeros).\r\nThat's a lot of kilograms!\r\n\r\nOK, now here is the cool part--- if you know the mass of the earth, and you know the size of the earth (which provides us the volume),\r\nTHEN it's easy to get the density--\u00a0 remember?-- density is just mass divided by volume.\r\n\r\nThe earth's AVERAGE density works out to about 5.5 g\/cm3\u00a0<\/sup> (0r 5500 kg\/m3<\/sup>)...\r\nAnd,\r\nThis is kind of weird, because rocks have average denisty of around 2.5 or 2.6 g\/cm3<\/sup> ..... SO, this suggests that there is something really dense deep in the earth, which bumps up the average density of our planet.\r\nWhat could it be?\r\nWell, at this point, you probably know-- the metallic Fe-rich core!\u00a0 (which has a density of around 8 g\/cm<\/span>3<\/sup>)<\/span>\r\n\r\nAnyway, watch the video if you like,, and fast forward through it, if math just isn't your thing!\r\n(You will not be tested on your ability reproduce the math.)\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=p1U-gzcqC5s\r\n

Last but NOT LEAST, WHY LAYERS?<\/h2>\r\nIn science, some of the best questions are the most obvious ones!\r\nEver since elementary school science classes, we've learned that our planet has layers.\r\nWhy?\r\n\r\nFirst off, the layers are concentric spheres.\r\nThe layers might be based on physical properties like liquid\/solid, brittle\/ductile, and hence describing things like inner\/outer core and lithosphere\/asthenosphere; or they might be based on chemical composition like felsic\/mafic, rocky\/metallic, and hence describing things like\u00a0 crust\/mantle, or core\/mantle.\r\nBut no matter what, the layers are basically concentric spheres.\u00a0 This is undoubtedly due to gravity.\u00a0 Objects with enough mass tend to pull themselves into spherical shape.\u00a0 Admittedly, things like planets and stars are not perfectly spherical, but they are pretty close.\u00a0 Lower mass objects like asteroids are rarely spherical, as there is just not enough mass to gravitational generate sphericity.\r\n\r\nThe second aspect is trickier-- why layers at all?\r\nDid the earth form in layers, with an iron core serving as a starting point to which rocky material was added, rolling-snow-ball-style?\r\nOr did the earth initially form as a homogeneous mass and later \"differentiate\" into its current heterogeneous layered form?\r\nIn general, scientists believe that the latter is most likely the case.\r\nIn fact, one of the key features of our models for planet formation (particularly the rocky inner \"terrestrial\" planets) is that the process of formation results in very high temperatures.\r\nYoung planets are particularly \"hot\" because:\r\n
    \r\n \t
  1. Heat from accretion.\u00a0 Lots of pieces of space junk slamming in to each other and sticking together but also transforming kinetic energy into heat.<\/li>\r\n \t
  2. Heat from gravitational pressure.\u00a0 So much material, condensing and squeezing together, releases heat.<\/li>\r\n \t
  3. Heat from radioactive decay.\u00a0 Later we'll learn a bit about radioactivity and its usage in determining rock ages, but radioactive elements are simply present in rocky \"terrestrial\" worlds and the associated radioactive decay releases heat.<\/li>\r\n<\/ol>\r\nAs the proto-earth heated up, melting of material took place, and the denser material sank towards the core.\u00a0 This is why iron (a relatively dense metal) makes up our core.\u00a0 This is also why crustal rocks are enriched in somewhat lower density material like silica, aluminum, and potassium.\r\n\r\nHere is a good (short and sweet) synopsis, via fun video, from Public Broadcasting, PBS-- and, I might add, pretty spot on!\u00a0 Check it out,\r\n\r\n[embed]https:\/\/www.youtube.com\/watch?v=WwiiOjyfvAU[\/embed]","rendered":"

    OK,, not everyone is a math whiz.
    \nBut Isaac Newton (back in the late 17th century) was certainly both great scientist and great mathematician.\u00a0 He worked out the whole science of “mechanics,” i.e. how things move!
    \nAnd he developed a whole new kind of math to help describe the business of objects in motion (calculus).<\/p>\n

    \"\"<\/a><\/p>\n

    It turns out that it doesn’t require calculus to do some great stuff with the LAW OF GRAVITY…. which just says that the FORCE of gravity, is directly related to the masses of objects in question, and the distance between them.\u00a0 Perhaps you’ve seen the equation,\u00a0 F=GmM\/r2<\/sup>,
    \nIn this equation,\u00a0 m can be the mass of an object falling towards earth’s surface, and M is the mass of the earth.
    \nG is a constant that was determined in the 18th century (courtesy of a rather eccentric wealthy English scientist, Henry Cavendish) and\u00a0 r is the radius of earth.<\/p>\n

    Once G was worked out by Mr. Cavendish, and since we know the acceleration of objects at earth’s surface,, about 10m per second squared, it’s possible to evaluate the mass of the earth!
    \nIt’s about 6×1024<\/sup> kilograms (6 followed by 24 zeros).
    \nThat’s a lot of kilograms!<\/p>\n

    OK, now here is the cool part— if you know the mass of the earth, and you know the size of the earth (which provides us the volume),
    \nTHEN it’s easy to get the density–\u00a0 remember?– density is just mass divided by volume.<\/p>\n

    The earth’s AVERAGE density works out to about 5.5 g\/cm3\u00a0<\/sup> (0r 5500 kg\/m3<\/sup>)…
    \nAnd,
    \nThis is kind of weird, because rocks have average denisty of around 2.5 or 2.6 g\/cm3<\/sup> ….. SO, this suggests that there is something really dense deep in the earth, which bumps up the average density of our planet.
    \nWhat could it be?
    \nWell, at this point, you probably know– the metallic Fe-rich core!\u00a0 (which has a density of around 8 g\/cm<\/span>3<\/sup>)<\/span><\/p>\n

    Anyway, watch the video if you like,, and fast forward through it, if math just isn’t your thing!
    \n(You will not be tested on your ability reproduce the math.)<\/p>\n