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Lecture 2

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Department
Earth Sciences
Course
Earth Sciences 2240F/G
Professor
Prof
Semester
Winter

Description
Week 2 Layout: Intro: differentiation of Earth into zoned structure, sources of internal heat, concept of plates driven by heat energy Chapter 3: theory of plate tectonics from the concept of continental drift (Wegener’s hypothesis), mechanisms occurring inside Earth that were caused by evolution of heat Unit 2- Internal Energy Processes Introduction  Internal energy processes are sudden release of large amounts of energy internal to the Earth  volcanic eruptions, earthquakes, but NOT landslides Earth’s Internal Structure  Parts: o Inner core of nearly pure metal o Outer core of metal, with an ill-defined outer boundary that’s likely characterized by pools of a mixed composition of metals and silicates o Core mantle of dense silicate and oxide minerals  The very upper-most part of the mantle is relatively cold and brittle, and reacts to large-scale processes, as does the crust  The term to identify the crust +brittle upper mantle is the lithosphere o Crust, thin outer shell  part of the lithosphere (with the brittle upper mantle) o Oceans and atmosphere  These are older names, and the newer names include the mesosphere (part of mantle, ‘stiff plastic’), asthenosphere (‘soft plastic’), lithosphere, hydrosphere and atmosphere o Mesosphere: very dense rock, behaves as a solid o Asthenosphere: above the mesosphere, thin zone of very dense rock, but since it is under less pressure than the mesosphere, it behaves like a hot plastic, and may even be fluid  The internal structure of Earth developed at the point in time when all of Earth became sufficiently hot and liquid, so that material could move relatively freely  the process of heavy material moving (due to gravity) to the planet’s core is called differentiation Earth’s Internal Heat Energy  Accretion: the growth of a body as smaller objects stick to it, Earth grew with accretion of cold material. Now, the interior is 7200-7500C, more than 1000C higher than the Sun’s outer layer. How did that heat originate? Accretion and Impact Heating  Earth grew in mass as more and more materials accreted (asteroids, dust), and the kinetic energy which was liberated during impacts became critical in the thermal energy of the planet  Kinetic energy = ½ mv^2  The overall thermal effects of impact and accretion are difficult to quantify o 1979: research estimated that for any body of 1000km radius, the energy acquired from impacts in the early period of the solar system would have raised the temperature of the body by 1000C  this means that simply from the heat of impacts, early Earth would have melted completely  The moon was formed from the debris when a Mars-sized planet (Theia) hit Earth Core Formation Heat  When initially homogenous planets sort themselves into core-mantle-crusts, energy is liberated, as differentiation involves the release of gravitational potential energy  Once the heat of accretion has melted Earth, gravity takes over and starts to segregate heavier material from lighter material o This took no more than a couple of million years for Earth o The phase of Earth’s formation is sometimes called ‘the iron catastrophe’ Heat from Dissipation of Tidal Energy  Tidal energy is an important source of heat, especially in small satellites where the content of radioactive isotopes might be small  The sun and moon raise large tides in Earth’s oceans. Similar but smaller tidal distortions take place in the interiors of both Earth and Moon o The highest tide in Earth’s asthenosphere/mantle has a height of only 11cm, so it’s not easy to detect  Over the course of geologic history, however, the effect of tides on Earth has been to slow down its rotation rate, and to cause the Moon to slowly recede from Earth  all the energy used in ‘braking’ Earth’s rotation about its axis is manifested as heat o It is tidal forces which keep one hemisphere of the moon facing Earth (dark side of the moon), as Io does to Jupiter o Io is the most volcanically active object in the solar system, it’s about the same size as the moon Heat from Radioactivity  Virtually all of Earth’s current internal energy is derived from the energy of radioactivity. As more and more radioactive isotopes decay to the vanishing point, Earth will cool significantly  Long-lived radioactivity: long-lived radioactive isotopes such as potassium (K), uranium (U) and thorium (Th) have been producing heat energy all through Earth’s early history, and will continue to do so for a long time  Short-lived radioactivity: in early Earth history, the heat provided by radioactive decay processes was likely very high, simply because there was an abundance of short-lived radioactive isotopes (which are no virtually gone) o For example, Al26 decays to Mg26, with a half-life of 720,000 years, so it Al26 has now been reduced to the vanishing point. BUT at one time in the past the heat of radioactive decay must have added substantially to the heat budget of Earth Heat Flow: Past, Present and Future  Heat flow is the heat escaping per unit time from the interior of Earth across each unit of surface area  Early on, the surface was too hot to hold liquid water oceans (for a long time after Theia’s impact)  there would have been a direct transfer of heat from hot magma to the atmosphere o As cooling progressed, solid rock crust formed and oceans gradually stabilized, heat flow decreased as the crust and ocean acted as insulation between the atmosphere and the hot mantle  Short-lived radioactive elements declined in importance, and long-lived radioactivity is the only major heat source remaining  It is Earth’s internal heat that drives the processes within our planet, that lead to earthquakes and volcanism Chapter 3: Plates and Tectonics  Most earthquakes and volcanic eruptions are associated with edges or boundaries of plates (segments of Earth’s crust, this realization was made 100 years ago)  these plates interacted, ‘tectonics’, on these boundaries, in response to escaping heat energy from inside the Earth  Plate tectonics makes Earth so unique from other planets  continents, ocean basins and mountain ranges are all produced by tectonics  Theory of Plate Tectonics: explains why and where significant deformation of Earth’s surface occurs, also explains the type and location of many metallic mineral deposits, explains the global distribution of earthquake and volcanic hazards Early Development of the Theory  Alfred Wegener: continental drift, published in 1912 o He was interested in the ‘jigsaw fit’ of continents across the Atlantic Ocean o Glacial terrain of continents in the southern hemisphere when those land masses were fitted, continuity of old geological structures, fossil stratigraphy from continent to continent o Supercontinent called ‘Pangaea’ o BUT he had no idea of the mechanism that made continents drift  In the early 1940s, there was a need to detect sunken ships and submarines, so magnetometers (which measure magnetic field strength) were deployed in ‘pods’ which could be towed behind ships back and forth across particular sections of ocean o Scientist detected repetitive magnetic records in ocean floor rocks, but they couldn’t process this data until the war was over  Following WWII, two lines of research (hunt for natural mineral resources, map the geographic features of ocean floors) began to produce the first comprehensive maps of the ocean floor o Discovered a huge volcanic ridge down the ocean, and the volcanoes were active for almost its whole length  the largest volcanic system  Harry Hess, Princeton University, 1960: these ridges represented spreading centres where Earth’s crust was moving in opposite directions, like conveyor belts, allowing new ocean floor to be built from volcanic rock at the ridges o He calculated that due to this activity, the Atlantic Ocean was widening by about 2.5cm/year, which means that the Americas and Europe/Africa were in contact about 180 million years ago Paleomagnetism and Earth’s Magnetic Field  Paleomagnetism: study of Earth’s magnetic field through the analysis of rock magnetism  Earth has a strong magnetic field which regenerates continuously, protects us from space radiation and solar storms  we study effects close to the surface, and so try to figure out what’s happening internally  Remember- the positions of geographic poles are the poles of planet rotation; positions of the magnetic poles are NOT the same  Dynamo Model: in a hydroelectric generating station, water turns turbines (or dynamo
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