Lecture14 TheEarth

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Published on March 11, 2008

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ASTR 330: The Solar System:  ASTR 330: The Solar System Announcements Dr Conor Nixon Fall 2006 Homework #3 due today. Early warning grades. Mid-term #1 results delayed til Thursday. Homework #4 out today - due back 10/31/06. Feedback assessment forms. ASTR 330: The Solar System:  ASTR 330: The Solar System Lecture 14: The Earth Dr Conor Nixon Fall 2006 “Mostly Harmless” – The Hitch-Hikers Guide to the Galaxy, by Douglas Adams. Image: METEOSAT ASTR 330: The Solar System:  ASTR 330: The Solar System The Earth As A Planet Dr Conor Nixon Fall 2006 Most of the fields of human study, including all the humanities, social sciences, geography, even biology and much of geology are concerned with the thin layer of activity which occurs at the Earth’s surface. In this we will examine the Earth from a whole-planet perspective, including every layer from the core to the top of the atmosphere. There is far too much information to sum up what we know about the Earth in a single lecture, so we will focus on processes which we want to compare between the Earth and its hot and cold cousins: Mars and Venus, especially: volcanism and geology impact cratering greenhouse warming in the atmosphere. ASTR 330: The Solar System:  ASTR 330: The Solar System Major Concentric Zones of Earth Dr Conor Nixon Fall 2006 MAGNETOSPHERE: region of charged particles above the atmosphere, from 200 km to 100,000 km above the surface. ATMOSPHERE: gas layer, from surface to 200 km altitude. HYDROSPHERE: or ocean, including ice caps, which covers 2/3 of the surface. CRUST: the solid surface of the Earth, about 10-30 km thick. MANTLE: the solid but plastic rock layer extending to 2900 km below the surface. CORE: metal sphere, divided into outer (liquid) and inner (solid). ASTR 330: The Solar System:  ASTR 330: The Solar System Important Earth Facts Dr Conor Nixon Fall 2006 From the perspective of the planetary system, we need to take note of the following: HYDROSPHERE AND ATMOSPHERE: Earth is the only planet with liquid water on the surface. The Earth is partially obscured by clouds, the extent of which varies considerably. Polar caps exist, which change their coverage seasonally. The atmosphere is 21% oxygen, a highly reactive gas not abundant elsewhere in the planetary system. CO2 is a minor fraction, 1% of the Earth’s atmosphere. ASTR 330: The Solar System:  ASTR 330: The Solar System Facts: contd. Dr Conor Nixon Fall 2006 SURFACE About 1/3 of the surface is covered by land, which has some high mountain ranges (9 km). The seabed can be 11 km below sea level. There is geologic activity: volcanoes, earthquakes, hot springs. MAGNETOSPHERE Electrons and protons are trapped in magnetic fields surrounding the Earth. CORE Due to its high density, the core is probably metal. ASTR 330: The Solar System:  ASTR 330: The Solar System Seismology as a probe of the interior Dr Conor Nixon Fall 2006 Seismic waves are very similar to sound waves, and are produced by earthquakes. Figure credit: Univ. Tenn. Knoxville These low-frequency waves (several km wavelength) occur in two types: Compression (longitudinal) – can pass through solids and liquids. Shear (transverse) – transmitted by solids only. Can be demonstrated with a spring. Seismic waves can be detected with seismometer. ASTR 330: The Solar System:  ASTR 330: The Solar System The Crust Dr Conor Nixon Fall 2006 The crust is the layer of solid igneous rock between the mantle beneath and the hydrosphere/atmosphere above. The ocean crust and continental crust are fundamentally different: Ocean crust (55% of crust): thin crust (6 km) composed mainly of denser basalts. young (200 Myr or less), somewhat analogous to lunar maria. formed by plate movements: where plates move apart magma rises and solidifies, forming new crust. Continental crust (45%): mainly granites: also igneous but formed at depth and pressure. thickness is 20 km to 70 km. includes some sedimentary, metamorphic rocks. rocks can be much older than ocean crust: up to several Gyr. ASTR 330: The Solar System:  ASTR 330: The Solar System The Mantle Dr Conor Nixon Fall 2006 Below the crust lies the mantle, which is composed of higher-density materials. There are three distinct layers of the mantle: Figure credit: Univ. Tenn. Knoxville Upper Mantle (0-100 km depth): This is attached to the crust, and the two move together and are called the lithosphere. Below the upper mantle is a plastic boundary layer called the aesthenosphere. Convective Mantle (100-700 km depth): Heat is transported upwards from the core by slow overturning motions. Same composition as the upper mantle, but different mechanical properties. Lower Mantle (700-2900 km depth): solid, no convection. ASTR 330: The Solar System:  ASTR 330: The Solar System The Core Dr Conor Nixon Fall 2006 The mantle-core boundary marks a major change in composition: from rock, to iron, with traces of nickel, sulfur and some other elements. The boundary occurs at a pressure of 1.3 million bars (1 bar is about the pressure at sea level) and a temperature of 4500 K. Iron is liquid at these conditions: so the change in composition also marks a transition from solid to liquid phase. Turbulent flows in this liquid generate the Earth’s magnetic field, which can change over time. The core is 7000 km across: bigger than the whole of Mercury! And accounts for 1/3 of the Earth’s mass. At 5200 km depth, the core itself changes from liquid to solid, due to the massive pressure of 3.2 million bars. At the very center, the pressure reaches 4 million bars, and T=5000 K. ASTR 330: The Solar System:  ASTR 330: The Solar System Geologic Timescale Dr Conor Nixon Fall 2006 As we have noted before during our discussion of the Moon, there are two principal ways to date rock layers: Stratigraphy: gives a relative timescale. Radioactive dating: gives an absolute chronology. The chronology of the Earth can be divided into three eons: Archaeozoic: from 4500 to 2500 Myr ago Proterozoic: from 2500 to 590 Myr ago Phanerozoic: from 590 Myr to present. The first two eons together are known as the Precambrian. ASTR 330: The Solar System:  ASTR 330: The Solar System Phanerozoic: Eras, Periods, and Epochs Dr Conor Nixon Fall 2006 The Phanerozoic eon is further sub-divided into a bewildering number of eras, periods and epochs! The Paleozoic saw the arrival of fish and reptiles. The Mesozoic was the age of dinosaurs. The Cenozoic has been the age of mammals, inc. humans. Figure credit: Dr Bobs Geology, Eastern Illinois Univ. ASTR 330: The Solar System:  ASTR 330: The Solar System Earth-Moon Comparison Dr Conor Nixon Fall 2006 It is useful to emphasize here that most of the ‘action’ on the Earth occurs after the Moon had more or less reached its final form. On the Earth, the oldest rocks are from 3.8 Gyr ago, but much younger than the lunar highlands. Indeed, the youngest lunar basins were formed then! The mare volcanism on the Moon ceased about 3.2 Gyr ago, long before the first single-celled life forms arose on Earth. At the time the last major lunar impact occurred (Copernicus), 1 Gyr ago, dinosaurs had not yet walked the Earth! Indeed the dinosaurs could have seen the Tycho impact on the Moon, 100 million years ago, long before mammals dominated the Earth. ASTR 330: The Solar System:  ASTR 330: The Solar System Volcanism Dr Conor Nixon Fall 2006 Volcanism (named after the Roman god of the forge, Vulcan) occurs when heat within the mantle drives magma through the lithosphere to the surface, where it may erupt in several ways. Lava plains: vast outflows similar to the lunar maria. A volcano, i.e. a volcanic mountain, which may be either: A shield volcano: a gently-sloping mountain caused by repeat outflow of fluid lava from a vent, or A composite volcano: a steep-sided, conical mountain, caused by the eruption of more viscous lava. Geologists have many more types! Let’s look at some examples. ASTR 330: The Solar System:  ASTR 330: The Solar System Lava Plains Dr Conor Nixon Fall 2006 Lava plains are vast outflows similar to the lunar maria. Examples on Earth are the Columbia River Basalts and Deccan plains of India. Both occurred about 60 million years ago, much younger than the maria. Figure and picture: North Dakota State Univ; Thor Thordarson ASTR 330: The Solar System:  ASTR 330: The Solar System Shield Volcanoes Dr Conor Nixon Fall 2006 Mauna Loa in Hawaii is the largest volcano on the planet, rising 9 km from the sea bed. At the top is a large, shallow crater called a caldera, produced by subsidence. Typical slope is about 10°. Pictures: R.W. Decker, J.D. Griggs (USGS) ASTR 330: The Solar System:  ASTR 330: The Solar System Composite Volcanoes Dr Conor Nixon Fall 2006 Also called stratovolcanoes, after the strata or layers of material which are built up by successive eruptions, which can be very destructive (e.g. Vesuvius destroyed Pompeii and Herculaneum in AD 79). Examples include Mt St Helens, Mt Fuji, Mt Rainier. Mt. Shishaldin (below right) is the largest volcano in the Aleutian Islands of Alaska, and very active. Figure credit: Michael Ritter Image: R. McGimsey ASTR 330: The Solar System:  ASTR 330: The Solar System Other Types Of Geologic Activity Dr Conor Nixon Fall 2006 Other important processes on the Earth which may change the landscape and eradicate the geologic record include: Erosion: the mechanical process of wearing down the landscape, by glaciers, wind, rain, etc. This tends to reduce the height of mountain ranges over time, and carve features such as the Grand Canyon of Arizona, or the fjords of Norway. Weathering: the chemical destruction of the landscape, by acid rain, dissolution, etc. Sedimentation: particles which are transported by erosion (e.g. river silts) eventually settle out (e.g. river deltas) and begin to form layered deposits. This is the process by which sedimentary rocks ultimately are formed. ASTR 330: The Solar System:  ASTR 330: The Solar System What About Impacts? Dr Conor Nixon Fall 2006 The geological record of the Earth is conspicuously lacking in large impact craters, such as we see on the Moon and Mercury. We might think that the atmosphere has some part to play in protecting us. In fact, we are only protected from objects with masses less than 50,000 tons, but anything bigger will not be substantially slowed. Until recently, we did not worry much about large impacts on the Earth, partly due to the lack of crater evidence. On the Earth, the geologic processes we have just described, not to mention the growth of vegetation, quickly mask impact craters. Only now we have planes and spacecraft can we spot the blurred outlines of ancient impacts from above. ASTR 330: The Solar System:  ASTR 330: The Solar System Tunguska Dr Conor Nixon Fall 2006 The largest impact of recent times was the Tunguska event in Siberia, on June 30th, 1908. Witnessed by very few people, over 1000 km2 of forest was flattened, and hundreds of animals died. The diameter of the stony projectile was probably about 60m, about the size of an office block. The explosive force was equal to 10-20 megatons, the same as a modern hydrogen bomb. It has been estimated that the same size of explosion over a European city of the time could have killed 500,000 people, and burned a city to the ground. Figure credit: Dennis J Ramsey, tmeg.com ASTR 330: The Solar System:  ASTR 330: The Solar System Chicxulub Dr Conor Nixon Fall 2006 The 180-km Chicxulub impact crater of the Yucatan peninsula of Mexico is believed to be the site of the event which caused the massive K-T extinction 65 Myr ago, including the dinosaurs. The impact is believed to have been a roughly 10-km size asteroid, with a blast of 100 million megatons (5 million H-bombs). The blast deposited a global layer of the rare metal iridium. Gravity Map: Alan Hildebrand, Geological Survey of Canada ASTR 330: The Solar System:  ASTR 330: The Solar System Continental Drift Dr Conor Nixon Fall 2006 Until the 1960s, geologists mostly believed that the continents had remained in the same positions since the Earth formed. The first serious challenge to this notion came from Alfred Wegener (1880-1930) who noticed reports of identical fossils found on either side of oceans: such as Africa and South America. Photo: UCMP Berkeley Wegener’s 1915 book postulated that 300 million years ago, the continents had all been joined together in a super-continent, which he called Pangaea. However his theory did not gain general acceptance at the time: he did not know the mechanism by which the drifts occurred, and his rates were much too fast. ASTR 330: The Solar System:  ASTR 330: The Solar System Dr Conor Nixon Fall 2006 Figure credit: Michael Pidwirny, Okanagan Univ. Coll. The mechanism for continental drift is now understood by the theory of plate tectonics. We find that the Earth is divided into six major and ten smaller sections (plates). Plate Tectonics The plates are the lithosphere: they float on the aesthenosphere. Convection currents in the mantle force the plates apart: new material forms in the gap or rift. ASTR 330: The Solar System:  ASTR 330: The Solar System Tectonic Processes Dr Conor Nixon Fall 2006 Plate movements give rise to four types of boundaries: Rift Zone: this is an area where two plates are moving apart, such as the mid-Atlantic ridge. Subduction Zone: occurs where one plate slides under another (typically the ocean crust goes under the continental crust), e.g. the deep trenches off the coast of Asia. Fold Mountain Range: e.g. the Himalayas. Occurs where two plates collide and neither one goes under, typically two continental plates. Fault line: where two plates scrape against each other in a parallel direction, e.g. the San Andreas fault of California. ASTR 330: The Solar System:  ASTR 330: The Solar System Summary of Tectonic Processes Dr Conor Nixon Fall 2006 This figure shows most of the tectonic processes. ASTR 330: The Solar System:  ASTR 330: The Solar System Lifetime of Oceanic Crust Dr Conor Nixon Fall 2006 There are 60,000 km of active rift zones in the ocean, where new basaltic crust is forming by outpouring magma. The average rate of spreading is 2-3 cm/year (Wegener guessed 100 cm/year, much too fast). By multiplication we find the about 2 km2 of new crust is created each year. As the ocean has about 260 million km2 of crust, we can divide to find that the lifetime of a piece of ocean crust is about 100 million years. Oceanic crust is destroyed at subduction zones, where is it pulled under the continental crust, and eventually melts at 200-300 km depth. Volcanoes can form along this line, e.g. Pacific ‘arc of fire’. ASTR 330: The Solar System:  ASTR 330: The Solar System Dr Conor Nixon Fall 2006 Diagram: wikipedia Animation: UCMP Berkeley A Super-Continent Wegener noticed that the shapes of the continents fit together like a giant jigsaw puzzle. North America to Eurasia, South America to Africa. The original Pangaea super-continent broke apart 200 Myr ago in the Jurassic period, into two parts: Gondwanaland and Laurasia. ASTR 330: The Solar System:  ASTR 330: The Solar System The Hydrosphere Dr Conor Nixon Fall 2006 The term hydrosphere means all the water on the surface of the Earth, including freshwater. However, most of the mass is salt water. The salt in the ocean is mostly sodium chloride (NaCl), but there are other salts. The six elements Na, Mg, Cl, Ca, S, K make up 90% of salts. Calcium provides a lesson about cycles. 108 tons of Ca enter the oceans every year, yet there are only 1014 tons in the ocean, a million years worth of inflow. What has happened to all the Ca? The Ca combines with CO2 and O2 to form CaCO3: limestone rock. Oxygen and carbon dioxide are dissolved in the oceans as well as present in the air; 60 times more CO2 in the oceans than atmosphere! The average temperature of the oceans is just 4°C, although the surface temperature can vary from 30°C to less than zero. Far below the surface, the temperature is almost constant. ASTR 330: The Solar System:  ASTR 330: The Solar System Ocean Currents Dr Conor Nixon Fall 2006 Warm surface waters and cooler sub-surface waters form a giant circulation pattern of current, transporting nutrients and salts round the world. Figure credit: Michael Pidwirny, Okanagan Univ. Coll. In this figure, warm near-surface currents are in red, while colder deep currents are in blue ASTR 330: The Solar System:  ASTR 330: The Solar System Composition of the Atmosphere Dr Conor Nixon Fall 2006 The atmosphere is mostly composed of two gases: nitrogen and oxygen. Water, CO2, CH4 and the other gases marked with a ‘*’ are quite variable. Note that the atmospheric pressure, or weight, at sea level is 1 bar, the same weight as a 10-m thick layer of water. Pressure decreases as we ascend a mountain or in a plane. Table: Michael Pidwirny, Okanagan Univ. Coll. ASTR 330: The Solar System:  ASTR 330: The Solar System Production and Loss of Gases Dr Conor Nixon Fall 2006 Nitrogen and argon are fairly un-reactive, and because they cannot escape, they tend to accumulate over geologic time. In contrast, oxygen and carbon dioxide are highly reactive chemically, so their current abundances reflect a balance between production and loss mechanisms. For example, CO2 is absorbed by plants, which release O2 after photosynthesis. When plant material is burned or consumed as food, CO2 is released again. We also note that if the all the water in the oceans was to evaporate, then we would have an atmosphere of 300 bars of water vapor. Water would be the primary constituent of the atmosphere. In that case, all the oceanic CO2 would also be released into the air. ASTR 330: The Solar System:  ASTR 330: The Solar System Carbon Cycle Dr Conor Nixon Fall 2006 Figure credit: Michael Pidwirny, Okanagan Univ. Coll. ASTR 330: The Solar System:  ASTR 330: The Solar System Biological Effects On Atmosphere Dr Conor Nixon Fall 2006 In the absence of life, we would expect the Earth’s atmosphere to be mainly carbon dioxide, nitrogen and argon: like Mars and Venus. CO2 would the main atmospheric constituent, with a surface pressure of several tens of bars. Instead we find much more O2, due to life. This transition must have taken place in the past, an estimated 2 billion years ago. At this time, plants were able to remove much of the carbon from the air and lock it up in sediments. The atmosphere became oxidizing. We believe that life could not have arisen in a reactive oxidizing environment, but once life had formed, it was able to adapt to the changing conditions, and evolve the metabolism of modern animals, using oxygen as a fuel. ASTR 330: The Solar System:  ASTR 330: The Solar System Lower Atmosphere: Troposphere Dr Conor Nixon Fall 2006 The atmospheric temperature initially decreases as we ascend, until about 11 km altitude. This region (0-11 km), called the troposphere (or sphere of ‘turning’), is dominated by convection. Surface air is heated by the Sun, becomes warmer and less dense. Due to buoyancy, the warm air rises, while denser cooler air descends elsewhere. Convection maintains a lapse rate (temperature drop-off rate) of about 6 degrees Celsius per kilometer. All our weather conditions and most of the mass is contained in the troposphere. ASTR 330: The Solar System:  ASTR 330: The Solar System Atmospheric Structure Dr Conor Nixon Fall 2006 Figure credit: Michael Pidwirny, Okanagan Univ. Coll. Above the troposphere, temperature reaches a minimum at the tropopause, and then ascends through the stratosphere. The warming is due to ozone (O3) which reaches a maximum between near 50 km altitude, the stratopause. The stratosphere is thin and dry compared to the lower atmosphere. ASTR 330: The Solar System:  ASTR 330: The Solar System Stratosphere and Ozone Dr Conor Nixon Fall 2006 Ozone absorbs solar UV radiation which would otherwise be harmful to surface life. The energy is re-radiated in the infrared as heat, creating the stratospheric warming. The ozone layer was being damaged by man-made gases called CFCs – chlorofluorocarbons, used in refrigeration and packing materials. Although harmless to humans, CFCs are very long-lived (100s of years), and make their way to the stratosphere where they attack ozone. This effect was discovered by researchers who noticed a ‘hole’ appearing in the ozone concentration annually over Antarctica. CFCs have now been phased out, and will slowly break down in the atmosphere. ASTR 330: The Solar System:  ASTR 330: The Solar System Ozone Hole Dr Conor Nixon Fall 2006 This figure shows the record-breaking ozone hole over Antarctica of Sepember 8th, 2000: 28.3 million km2. The ‘hole’ covers an area three times larger than the continental US. The hole has now stabilized and does not seem to be growing further. Figure credit: Richard McPeters, NASA GSFC ASTR 330: The Solar System:  ASTR 330: The Solar System Mesosphere and Thermosphere Dr Conor Nixon Fall 2006 The mesosphere (50-80 km altitude) is a region of decreasing temperatures, as the ozone levels fall. The temperature reaches a minimum at the mesopause, 80 km and 190-180 K. The temperature then gradually rises again due to absorption of solar radiation by the small number of O2 molecules. The temperature eventually reaches 1000 K, although the molecules are now far apart. Figure credit: NASA GSFC ASTR 330: The Solar System:  ASTR 330: The Solar System Summary-Quiz (contd) Dr Conor Nixon Fall 2006 What the three major layers of the solid Earth? What are the two type of seismic waves, and which can travel through the core? What the main differences between the continental and oceanic crusts? Describe the three layers of the mantle. What are the main types of volcanic outflows? How does viscosity determine which one occurs? Give an example of each type. Describe n major impact event on the Earth. Why does the Earth show less impact record than the Moon and Mercury? ASTR 330: The Solar System:  ASTR 330: The Solar System Summary-Quiz (contd) Dr Conor Nixon Fall 2006 What was the evidence for continental drift, and what were the problems? How was continental drift finally explained? What types of boundaries can occur when plates meet? What is the hydrosphere, and why does calcium not accumulate further? What are the main variable constituents in the atmosphere? What sort of atmosphere would we have now without plant life? Describe the temperature structure of the atmosphere. Why do we have an ozone hole? ASTR 330: The Solar System:  ASTR 330: The Solar System Sunlight At The Earth Dr Conor Nixon Fall 2006 In an earlier lecture we calculated the amount of sunlight which intercepts a square meter of area at the Earth’s distance from the Sun: about 1420 W/m2. Of course, the Earth is not a flat wall facing the Sun, it is a ball, and hence only the center of the dayside receives this amount: other parts of the dayside receive less. The night side receives none! About 30% of sunlight reaching the Earth is reflected, either by clouds, or by the surface (ice caps, snow etc). The other 70% is absorbed in the atmosphere or at the surface. The average amount of energy absorbed by the Earth, averaged over all latitudes is 240 W/m2: 100 billion megawatts for the whole planet! ASTR 330: The Solar System:  ASTR 330: The Solar System Dr Conor Nixon Fall 2006 Figure credit: NASA LARC ASTR 330: The Solar System:  ASTR 330: The Solar System Greenhouse Effect Dr Conor Nixon Fall 2006 The atmosphere is mostly transparent to visible sunlight. When sunlight strikes the surface, it is absorbed, and re-emitted again as heat: infrared radiation. However, the atmosphere is much more opaque to IR light: it can’t very easily escape. It tends to get trapped in the atmosphere. The planet warms up more than it would have without an atmosphere: this is called the ‘greenhouse effect’. Figure credit: IPCC 1990 ASTR 330: The Solar System:  ASTR 330: The Solar System Greenhouse Gases: Effectiveness Dr Conor Nixon Fall 2006 Table: seda.nsw.gov.au Different gases have different greenhouse effectivenesses: i.e. how much they contribute to warming. Another factor is the lifetime of these gases in the atmosphere: how long they stick around after they are released. The greenhouse effect (you can see the analogy to a gardener’s greenhouse) warms the planet up by about 25°C, making the difference between a chilly -15°C average temperature, and our actual +10°C ! Without the greenhouse effect, we’d be in a major ice-age, there would probably never have been seas, and life may not have arisen. ASTR 330: The Solar System:  ASTR 330: The Solar System Carbon Dioxide Emissions Dr Conor Nixon Fall 2006 CO2 is the most important greenhouse gas: even at 0.04% of the atmosphere it has a huge effect. Since the industrial revolution of the 19th century, we have been burning fossil fuels (oil, gas, coal) at ever increasing rates, pumping more and more CO2 into the atmosphere. We estimate that this has already caused global temperatures to rise by 2°C: and temperatures are expected to rise several more degrees by the middle of the century. If we do not curb our emissions, the ice caps may melt, and eventually many coastal cities may be flooded. Not all scientists agree on the amount of global warming, and whether it is an immediate threat or not. ASTR 330: The Solar System:  ASTR 330: The Solar System Greenhouse Effect Dr Conor Nixon Fall 2006 This graph shows the increasing emissions of CO2 to the atmosphere, from various sources. Graph: Woods Hole Oceanographic Institute ASTR 330: The Solar System:  ASTR 330: The Solar System Greenhouse Gases: Changes Over Time Dr Conor Nixon Fall 2006 Table: Michael Pidwirny, Okanagan Univ. Coll.

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