Lesson 1 Notes
Lesson 1 Notes ISNS 2359
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Date Created: 01/30/16
Matter, earth and sources of energy ISNS 3359 Earthquakes and Volcanoes How We Understand the Earth • Must of us think in terms of geologic time rather than human time – thousands, millions and billions of years • In 1788, Hutton introduced concept of geologic time: – “No vestige of a beginning, no prospect of an end.” – Everyday changes over millions of years add up to major results • Uniformitarianism: natural laws are uniform through time and space; present is the key to the past • Contrast to previously believedcatastrophismm • Currently modified actualism: rates of Earth processes can vary Energy Sources Four primary energy sources fuel Earth processes: • Impact of extraterrestrial bodies – Asteroids and comets; abundant in early Earth history, rare now • Gravity – Mass of Earth pulls objects (glaciers, hillsides) downhill • Earth’s internal heat – As Earth cools, heat flows from interior to surface – Volcanic eruptions, earthquakes – Plate tectonics; formation of continents, atmosphere and oceans • The Sun – Evaporation of water into atmosphere produces weather – Hailstorms, lightning, tornadoes and hurricanes Origin of the Solar System • Solar system began as rotating spherical cloud of gas, ice, dust and debris • Gravitational attraction brought particles together into bigger and bigger particles • Cloud contracted, sped up and flattened into disk Origin of the Solar System • Formation of Sun – Greatest accumulation of matter (H and He) at center of disk – Temperature at center increased to 1 million °C – Nucleosynthesis begins via nuclear fusion of hydrogen (H) and helium (He) , producing solar radiation – Smaller mass stars (like the Sun), • “Burn” slowly • Live longer (10 Ga) • Create lighter elements up to carbon ( C) – Larger mass stars (10-100x the mass of the Sun) • “Burn” rapidly • Are shorter lived (10s of Ma) • Create heavier elements up to iron ( Fe) Origin of the Sun and • Formation of planetsPlanets – Rings of concentrated matter formed within disk – Particles within rings continued to collide to form planets – Inner planets (Mercury, Venus, Earth and Mars) lost much gas and liquid to solar radiation, becoming rocky (terrestrial) – Outer planets retained gas and liquid, as gas planets • Impact origin of the Moon – Early impact of Mars-sized body with Earth – Impact generated massive cloud of dust (from Earth’s crust and mantle) and gas which condensed to form Moon – Lightweight gases and liquids lost to space – Lesser abundance of iron (from Earth’s core) in Moon Origin of the Moon? Solar System Formation • Most favored theory is that a small planetoid collides with Earth • Debris forms a ring around the Earth • The debris coalesces and forms the Moon Mare Basalt -- This sample of mare basalt, ~3.7 billion years old, was collected by Apollo 17 astronauts. The small holes in the sample are vesicles, which formed by the degassing of the lava as it cooled. http://solarsystem.nasa.gov/multimedia/gallery/Lunar_Maria.jpg Lunar Thin Sections Rocks similar to Basalts From the Earth http://www.cas.usf.edu/~jryan/ Solar System Formation • The atmosphere develops from volcanic gases • When the Earth becomes cool enough – Moisture condenses and accumulates – The oceans are formed Magnetic Field • The Earth has a prominent dipolar magnetic field • Like a bar magnet, the field has North and South ends • Magnetic flux has a direction that flows • Magnetic field lines… – Extend into space – Weaken with distance – Create a shield around Earth (the magnetosphere) The Van Allen Belts + The solar wind is deflected by the magnetosphere + Narrest deadly cosmic radiationgnetic field forms the Van Allen belts, which Earth History • 3.9 billion years ago: large oceans, small continents • 3.5 billion years ago: life (photosynthetic bacteria) • 2.5 billion years ago: large continent • 1.5 billion years ago: plate tectonics • This last point - actually a very interesting debate ==> concept of uniformitarianism and how far back does it apply A Layered Earth • We live on the thin outer skin of Earth • Early perceptions about Earth’s interior were wrong – Open caverns filled with magma, water and air – Furnaces and flames • We now know that Earth is comprised of layers – The Crust – The Mantle – The Core • Outer Core • Inner Core The Layered Earth • Differentiated into layers of increasing density • Center of Earth: Iron-ricoreore – Inner core is solid – Outer core is liquid and has viscous convection currents, responsible for Earth’s magnetic field • Surrounding core is Earth’mantlele, 2,900 km thick – Stony in composition • Low-density elements extracted over time (mainly through volcanic activity) from the mantle to forcrust,crust, atmosphere and oceans A Layered Earth: The Evidence • Earthquakes – Seismic waves pass through Earth – Seismic waves have been used to probe the interior • Wave velocity changes with density • Velocity changes give depth of layer changes • Changes with depth – Pressure – Temperature The Layered Earth Lithosphere = crust + rigid part of upper mantle Mantle convection Driving force of plate tectonics http://wapi.isu.edu/Geo_Pgt/Mod04_Earth/EarthFig2.htm Earth’s Interior Layers – Crust • Continental • Oceanic – Mantle • Upper • Lower – Core • Outer - Liquid • Inner – Solid © W. W. Norton The Core • An iron-rich sphere with a radius of 3471 km • 2 components with differing seismic wave behavior – Outer core • Liquid iron-nickel-sulfur • 2255 km thick. • Density – 10-12 g/cm 3 – Inner core • Solid iron-nickel alloy. • Radius of 1220 km. 3 • Density – 13 g/cm • Flow in the outer core generates the magnetic field Earth’s Mantle • Solid rock layer between the crust and the core • The mantle is 82% of Earth’s volume • Mantle composition is the ultramafic rock peridotite • Below ~100-150 km, the rock is hot enough to flow • It convects: hot mantle rises, cold mantle sinks • Three subdivisions: Upper, transitional and lower Behavior of Materials • Gas, solid and liquid are obvious terms, but should be considered with respect to time – Over longer time periods, solids may behave as liquid • A glacier is solid ice, yet flows downhill as ultrahigh viscosity liquid over time span of years • Elastic deformation is recoverable – object returns to original shape • Ductile deformation is permanent – stress applied over long time and/or at high temperatures • Brittle deformation is permanent – stress applied very quickly to shatter or break object The Layered Earth • Asthenosphere is plastic – but strong enough not to yield all at once”ield to an influence, • About 250 km thick • Comes to surface at mid-ocean ridges (and other basaltic volcanoes) • Lies more than 100 km below surface elsewhere • Allows Earth to be oblate spheroid (flattened during rotation) • Allows continents to ‘float’ atop the mantle, by principles of isostasy Surface Features • Earth’s surface: continents are high; oceans are low • Due to the differing buoyancy of each type of crust Distribution • Most land lies within 1 km of sea-level • Most ocean floors are close to 5 km depth • Extremes of depth or height are rare • The 2 dominant “levels” reflect continental vs. oceanic crust The Crust • The outermost “skin” of Earth with variable thickness – Thickest under mountain ranges (70 km) – Thinnest under mid-ocean ridges (3 km) • The Mohorovicic discontinuity is the lower boundary – Separates the crust from the upper mantle – Discovered in 1909 by Andrija Mohorovicic – Marked by a change in the velocity of seismic P waves Two Types of Crust • Continental crust – Underlies the continents 3 – Average rock density about 2.7 g/cm – Average thickness 35-40 km – Granitic in composition • Oceanic crust – Underlies the ocean basins – Density about 3.0 g/cm3 – Avg. thickness 7-10 km – Basaltic in composition Two Types of Crust • Crustal density controls surface position – Continental crust • Less dense; “floats higher” – Oceanic crust • More dense: “floats lower” Lithosphere-Asthenosphere • Lithosphere – The outermost 100-150 km of Earth – Behaves as a non-flowing, rigid material – The material that moves as tectonic plates – Made of 2 components: crust and upper mantle • Asthenosphere – Upper mantle below lithosphere – Shallower under oceanic lithosphere – Deeper under continental lithosphere – Flows as a soft solid Isostasy • Isostasy: Less dense materials float on top of more dense materials (i.e. iceberg floating in ocean) • Earth is a series of density-stratified layers • Core – densities up to 16 g/cm 3 3 • Mantle – densities from 5.7 to 3.3 g/cm • Continents (crust) – densities around 2.7 g/cm 3 3 • Oceans – densities around 1.03 g/cm • Atmosphere – least dense Isostasy Examples of isostasy: • Impoundment of water in Lake Mead behind Hoover Dam caused area to sink 175 mm over 15 years • Scandinavia is currently rising (about 200 m so far) – Had been depressed under weight of ice sheets during last glacial period (since 10,000 years ago) – Ground ruptures and earthquakes are present – Viking ship buried in the harbor mud of Stockholm was lifted above sea level – Another 200 m of uplift is likely http://www.museum.state.il.us/exhibits/larson/glaciers.html http://www.uwgb.edu/dutchs/EarthSC202Notes/folds.htm Earth History • Earth began as aggregating mass of particles and gases – Aggregation took 30 to 100 million years – Occurred about 4.57 billion years ago • Process of aggregation created huge amounts of heat • As temperature rose above 1,000 centigrade, iron melted – Liquid iron is denser than remaining rock, so sank toward center of Earth to form inner and outer core – Release of gravitational energy produced additional heat – Remaining rock melted, allowing low-density material to rise – Low-density material formed crust, oceans and atmosphere Internal Sources of Energy • Impact energy – Tremendous numbers of smaller bodies hit the Earth early after its formation, converting energy of motion to heat • Gravitational energy – As Earth pulled to smaller and denser mass, gravitational energy was released as heat Heat from both of these early sources is still flowing to the surface today, as heat conducts very slowly through rock Also, loss of heat through advection - hydrothermal circulation at the mid-ocean ridges Gravity • Gravity is attraction between objects: – Two bodies attract each other with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them – Planetary mass can be considered concentrated at its center • Gravitational effects of Sun and Moon on Earth – Moon’s mass is about 1/80 of Earth – Sun’s mass is 332,000 times that of Earth – Moon is 386,000 km from Earth – Sun is 150,000,000 km from Earth • Tidal energy is generated by interactions of gravity between Earth, Sun and Moon • Tides are caused by differences in gravitational pull from one part of Earth to another – Sun’s pull in mostly uniform throughout Earth, so small tides – Moon’s pull is much greater on facing side than on opposite side, so Moon tides are about twice magnitude of Sun’s tides twice Gravity • Tides affect land, water and air – most obvious in ocean • Moon’s tidal bulges move relative to Sun’s tidal bulges – Both tides coincide twice monthly (spring vs neap tides) • Tidal motions within the Earth cause: – Earth’s and Moon’s rotations to slow down – Earth and Moon to move farther apart – Days become longer – Years have fewer days • 370 million years ago, Earth day was 22 hours and year had 400 days Internal Sources of Energy Radioactive elements • Unstable radioactive atoms decay and release heat • Early Earth had much larger amount of short-lived radioactive elements and therefore much greater heat production than now • Radioactive decay process: – Measured by half-life: length of time for half the present number of atoms of a radioactive element (parent) to disintegrate to decay (daughter) product – Half-lives against time is negative exponential curve Internal Structure of Atom • Atom = protons, neutrons, electrons. • Composition of an atom is expressed as the number of protons and neutrons in the nucleus. The number of protons is the atomic number (Z) and deﬁnes the location on the periodic table. N = number of neutrons and A = Z+N = mass number. • Nuclide = atom with a unique number of protons and neutrons. Can thus deﬁne a nuclide by specifying Z and A e.g.: Table of isotopes: Internal Structure of Atom Internal Structure of Atom Internal Structure of Atom Internal Structure of Atom Internal Structure of Atom • Isotopes of an element - same # of protons (Z) but different # of neutrons (N) i.e., same atomic number, diff. atomic weights -27 • Mass of proton = 1.6726231 x 10 kg -27 • Mass of neutron = 1.6749286 x 10 kg • Mass of electron = 0.91093897 x 10-30kg • Stable isotopes = if not involved in radioactive decay scheme • Radiogenic isotope = if formed by decay but not involved in a decay scheme • Radioactive isotope = if undergoes radioactive decay Internal Structure of Atom Internal Structure of Atom Internal Sources of Energy • Sum of internal energy from impacts, gravity and radioactive elements (plus tidal friction energy) is very large • Internal temperatures have been declining since early Earth maximum, but still significant enough to cause plate tectonics, earthquakes and volcanic eruptions Age of the Earth • Oldest Solar System materials are 4.57 billion years old – Measured using radioactive elements in Moon rocks and meteorites • Oldest Earth rocks (found in northwest Canada) are 4.055 billion years old • Oldest Earth materials (zircon grains from Australian sandstone) are 4.4 billion years old Age of the Earth http://www.geology.wisc.edu/zircon/Earliest%20Piece/Earliest.html How Does Radioactive Dating Work? • Parent-Daughter Ratio measured in precise instruments – SIMS • SHRIMP – USGS SHRIMP-RG at Stanford – MC-ICP-MS • LA-MC-ICP-MS • Carefully prepared, pure mineral samples – Mica, Amphibole useful for 40Ar/39Ar dating method – Zircon, Baddelyite useful for U/Pb, Th/Pb methods • Mineral must have remained closed system – Possible to detect thermal disturbance through U/Pb concordia methods • Many systems useful – U/Pb, Th/Pb, Rb/Sr, Sm/Nd • New systems being developed~Lu/Hf for example • System must have geologically useful half-life • Carbon dating only for organic compounds < 60,000 years Mass Spectrometry • Elements of interest ionized – TIMS, SIMS, ICP for example • Ion beam focused through magnet lenses • Ion beam sent through large magnetic field – Physical magnets in some instruments, electronic quadrupoles in other types of instruments – Heavy ions are deflected less than lighter ions • Detection device measures isotopic ratio of elements of interest – 206/238, 207/235, 208/232 Th-U-Pb for example SHRIMP-II-RG How it Works Results Age of the Earth • Earth must be younger than 4.57 billion years old materials that formed the planet – Seems that Earth has existed as coherent mass since about 4.54 billion years ago – Probably took 30 million years (0.03 billion years) for Earth to form – NOTE; new estimates for gas giants (Jupiter and Saturn) - may be as little as a few hundred years • Collision that formed the Moon seems to have occurred between 4.537 and 4.533 billion years ago • Earth must be older than 4.4 billion years old zircons (which indicate low T surface conditions of formation) Radioactivity Disasters • Chernobyl disaster of 1986, in Ukraine – Explosion released 185 million curies of radioactive atoms affecting everyone from Scandinavia to Greece – 31 workers were killed and 165,000 later deaths blamed on accident, with more to come • Can such a thing occur in nature? Depends on relative amounts of U-238 and U-235: – U-235 makes up 0.7% of uranium ore – Uranium ore used in reactors is enriched to 2-4% U-235 – Because U-235 decays more rapidly than U-238, at some point in the past all uranium ore would have had about 2-4% U-235 – Sites in West Africa were natural nuclear reactors about 2 billion years ago, at about 400 degrees centigrade temperatures External Sources of Energy All sources of internal heat flow on Earth are dwarfed by 5,300 times greater heat flow to Earth frSunSun • Solar radiation: –43% visible wavelengths –49% near-infrared wavelengths –7% ultraviolet wavelengths • Sun’s energy: –30% reﬂected back to space at short wavelengths –47% absorbed as heat –23% evaporates water and begins hydrologic cycle Hydrologic Cycle • Sun’s heat evaporates water and plants transpire water into atmosphere • Atmospheric moisture condenses and precipitates • Sun’s energy is stored in water and water vapor • Equatorial regions receive excess solar radiation, whereas in polar regions radiation reflected back to space exceeds that incoming • Imbalance between equatorial and polar regions cause ocean currents and winds, transferring heat Hydrologic Cycle Hydrologic Cycle • 97.1% of Earth’s water is in oceans • Of remaining 2.9%: – 68% in glaciers – 21% underground – 10% in atmosphere – 1% in rivers, lakes, inland seas and soil moisture • Water in atmosphere holds, transports and releases solar energy, distributing heat around the Earth Water – The Most Peculiar Substance on Earth? • Present in liquid, solid and gas states • Highest heat capacity except liquid ammonia • Bipolar molecule easily able to bond with ions • Highest dielectric constant of liquids • Highest surface tension of liquids • Expands 9% when frozen (most substances shrink) Processes of Construction vs. Destruction Rock Cycle: • Internal heat melts rock to magma, which rises and cools to form igneous rocks – Process of Construction • External heat from Sun drives hydrologic cycle that weathers and erodes rocks into sediments, which are transported and deposited – Process of Destruction The Rock Cycle Processes of Construction vs. Destruction Rock Cycle: • Continents could be eroded to sea level in 45 million years (1% Earth history), but processes of construction keep raising continents and forming new landmasses Volcanoes and the Origin of the Ocean, Atmospheres and Life • Volcanic gases (H, O, S, C, Cl etc..) combine to make: – Water (2 O), carbon dioxide2(CO ), sulfur di2xide (SO ), hydrogen sulfide2(H S), carbon monoxide (CO), hydrogen (2 ), hydrochloric acid (HCl), meth4ne (CH ), and others…… • Dominant volcanic gas is water vapor – more than 90% • Are volcanoes responsible for the oceans? Volcanoes and the Origin of the Ocean, Atmospheres and Life • Volcanic rocks: – Oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), potassium (K) • 4.5 billion years of volcanism has brought light weight elements to the surface to make up – Continents – Oceans – Atmosphere – CHON (carbon, hydrogen, oxygen, nitrogen) elements of life
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