Structure of the Earth PDF

Origin of the Earth & Internal Structure of the Earth  Course Information Aims: To understand of Origin of Earth To understand the internal structure of the Earth To understand the origin of Ocean and Continents and the internal processes under the Earth ...

Origin of the Earth & Internal Structure of the Earth  Course Information Aims: To understand of Origin of Earth To understand the internal structure of the Earth To understand the origin of Ocean and Continents and the internal processes under the Earth  Course Information Contents: Our Solar system and its different part Origin of the Universe Origin of the Solar System Birth of the Solar System Birth of our Earth Formation of the Core The Creation of Life Chemical Composition of the Earth Mechanisms of Plate Tectonics: Continental Rifting Sea-floor Spreading  Our Earth is a part of the Solar system Solar system is a part of the Universe Interesting things about the Solar System  Part of a our Galaxy- Milky way  Most of the mass of the Solar System is concentrated in the sun Solar System consists of the Sun and its planetary system of 8 planets, their moons, and other non-stellar objects (astronomical object in direct orbit around the Sun that is neither a dominant planet or comet) All 8 planets can be seen with a small telescope The planets and sun each have a somewhat different density suggesting different time and/or temperatures of origin Jupiter is more than twice as massive as all the other planets combined (the mass of Jupiter is 318 times that of Earth)  Saturn is the least dense of the planets; specific gravity (0.7) less than that of water  All planets spin on an axis nearly erpendicular to the plane of the ecliptic but Uranus' axis is almost parallel to the ecliptic  Origin of the Universe  The Universe began (why began?) about 14.4 billion years (Ba) ago  The BIG BANG THEORY states that, in the beginning, our Universe was all one place  All of its matter and energy were squished into an infinitely small point, a singularity Then it “exploded”  The tremendous amount of material blown out by the explosion eventually formed the stars and galaxies  After about 10 B, our Solar system began to form  Origin of the Solar System  We know how the Earth and Solar System are today… and this allows us to work backwards and determine how the Earth and Solar System were formed  Additionally we can out into the Universe — for clues on how stars & planets are currently being formed The Nebular Hypothesis In cosmogony, the Nebular Hypothesis is the currently accepted argument how a Solar System can form  The Nebular Hypothesis  The Solar System started out as a large nebular cloud (gas cloud) and then began to condense  Most of the mass in the center, there was turbulence in outer parts  The turbulent eddies collect dust and gases  Small chunks grow from the condensation of these materials and collide, eventually becoming large aggregates of gas and solid chunks  Cloud would have been 30-40 light years across  Mass of cloud would have been 2 -10 times the present Solar System mass  Pictures from the Hubble Space Telescope show newborn stars emerging from dense, compact pockets of interstellar gas called evaporating gaseous globules This undated NASA image taken by the Hubble Space Telescope shows spiral galaxy Messier 101 Expanding remnant of a star's supernova explosion. Located at a distance of about 6,500 light-years from Earth The nebula has a diameter of 11 ly and expands at a rate of ~ 1,500 km/s It was found to correspond to a bright supernova recorded by Chinese and Arab astronomers in 1054 Crab Nebula taken by Hubble Space Telescope  This monstrous object is actually a pillar of gas and dust - called the Cone Nebula because of its conical shape in ground- based images This giant pillar resides in a turbulent star- forming region This image shows the upper 2.5 light-years of the nebula, a height that equals 23 million round trips to the moon Taken by the Hubble Space Telescope in April’02 The entire nebula is 7 light years in length  Gravitational and magnetic attraction causes the mass of gas and dust to contract and it begins to rotate  The dust and matter slowly falls towards the center  The nebular theory of the origin of the solar system has been verified by images of circumstellar disks around very young stars  Formation of a Prostar As a result of contraction and rotation, a flat, rapidly rotating disk forms with a matter concentrated at the center that later becomes Proto-Sun (protostar) The multi-coloured area shows a dust disk surrounding a new born star …. This process is still going and every second a new star born The red-orange area at the center represents the brightest region, which contains the young star It is surrounded by the cooler, dusty disk, which appears as yellow, green and blue The diameter of the disk is about 20 times larger than our entire Solar System Proto sun core gets to about 10 million degrees kelvin After sufficient mass and density was achieved in the Sun and its temperature rose to 1 million C The pressures and densities of hydrogen in the center ENERGY of the collapsed nebula become great enough and Nuclear Fusion starts at the center of the new star, converting hydrogen to helium and releasing lots of heat Just as our sun began to do since origin - and continues to do, to the great pleasure of us here on E We have already formed the Sun Now, let's make the planets The Sun Composition of the Sun Abundance of Light Elements Rarity of Lithium, Beryllium, Boron  Birth of the Solar System About 4.6 billion years ago, the Sun and the planets formed The enveloping disk of gas and dust forms grains that collide and clump together into small chunks or Planetesimals Possibly there was a supernova (explosion of a star) nearby to get things started Gravitational forces allow the terrestrial planets to accumulate and compact solid matter (including light and heavy atoms) by multiple collisions and accretion of Planetesimals Once particles join into larger units, their gravity attracts more particles, forming larger objects Eventually planet-sized objects appear Planets made of same material as Sun, minus elements that remain mostly in gases Solar radiation blew gases (primarily H and He) from inner planets Inner Rocky Planets (M, V, E, M): iron and magnesium silicates These gases were collected and condensed into the gas giants (J, S, U, N) Left-over debris from comets and asteroids We find this pattern in a certain class of meteorites  Birth of our Earth When planets got bigger, gravity got stronger, and planets ‘gather up’ surrounding debris – and started to spin on their axis due to collision of the debris Meteorites give us access to debries leftover from the supernova of the Solar System – We can date meteorites using radioactive – isotopes They also give us the information about the composition of the Planetesimals About 4.5-4.56 BYA, Proto-Earth formed from planetesimals  Early Earth Hot or Cold ? Up to 1940: Earth is hot inside, 1940-1970: Earth need not 1970-now: Earth did form hot so must have formed hot have formed hot after all Initial accretion of the Earth was cool Heat originated from collision (transfer of kinetic energy into heat, radioactive decay (U, Th and K) and compression Any molten object of > 500 km in size has sufficient gravity to cause gravitational The Heating, Cooling and Coalescing of the Earth separation of light and heavy elements thus producing a differentiated body Heavy elements (nickel and Iron) migrated to center to form core by gravity as material became molten Lighter material floated to the top to form crust, and material of intermediate density formed the mantle Earth began to cool, but the inside continues to be heated by radioactive decay Earth no longer has these craters due to weathering and movements of the Earth’s crust  Bombardment from Space For the 1st half Ba of its existence, the surface of the E was repeatedly pulverized by asteroids and comets of all sizes One of these collisions formed the Moon  Formation of the Moon The Giant Impact Hypothesis predicts that ~ 50 Ma after the initial creation of the E, a planet of ~ size of Mars collided with the E This idea was 1st proposed ~ 30 yrs ago, but it long time to calculate by modern high-speed computers to prove the feasibility This collision had to be very spectacular !!! A considerable amount of material was blown off into the space, but most fell back onto the E Part of the material from the collision remained in orbit around the E By the process collision and accretion, this The early Moon orbited very close to the E orbiting material united into the Moon  Formation of the Core ~ 100 Ma after the initial accretion, temperatures at depths of 400 - 800 km bellow the E’s surface reach the melting point of iron In a process called Global Chemical Differential, the heavier elements including melted iron, began to sink down into the core of the E, while the lighter elements such as Oxygen and silica floated up towards the surface The Global Chemical Differential was completed by ~ 4.3 Ba ago and the Earth had developed in an inner and outer core, a mantle and a crust Chemical composition of the Earth Each of the major layers has distinctive chemical composition, with the crust being quite different from the E as a whole  The First Billion Years Right after its creation, the E is thought with a thin atmosphere composed primarily of He and H gas The E’s gravity could not hold these light gases and they easily escaped into outer space Today, H and He are very rare in our atmosphere For the next several hundred Ma, volcanic out gassing began to create a thicker atmosphere composed of a wide variety of gases The Earth's surface was originally molten, as it cooled the volcanoes belched out massive amounts of CO2, STEAM, AMMONIA and CH4 The gases that were released were similar to gases of modern volcanic eruptions There is controversies that water might have come from the space but it is now well established that the STEAM/vapor escaping from the E’s interior via countless volcanic eruptions then condensed and produced shallow seas (this took 100s of Ma) The cooling down of the primordial world to the point where the out-gassed volatile components were held in an atmosphere of sufficient pressure for the stabilization and retention of liquid water There was NO OXYGEN  Supporting evidence includes the following facts  Enough water comes out of Volcanoes to have filled the ocean basins 20 time during the history of the E The earliest evidence of surface water on E, dates back ~ 3.8 Ba A Billion Year Old Earth By 3.5 Ba ago, when the E was a Ba old, it had a thick atmosphere composed of CO 2, Methane, water vapor and other volcanic gases By human standards this early atmosphere was very poisonous It contained almost no oxygen as we require (today it is ~ 21%) By 3.5 Ba ago, the E also had no extensive oceans and seas of salt water, which contain many dissolved elements such as Fe  But most important, by 3.5 Ba Evidence - bacteria flourishing 3.5 Ba ago ago, there was life on the E life got under way about 1000 Ma after the Earth Life existed in the shallow oceans close to thermal vents These vents - source of heat and minerals These 3.5 Ba old fossilized algae mats, which are called Stromatolites, are considered to be the earliest known life on Earth They are found in Glacier National Park, Western Australia Sromatolites (a structure) formed in shallow seas or lagoons The First Continent on Earth when millions of Cyanobacteria (a primitive type of bacteria) live together in a colony By 2.5 Ba ago, the first continent had been formed  The Creation of Life How to create cyanobacteria ? The composition of the early atmosphere and oceans were conductive to the creation of primitive amino acids which are the building blocks of protein molecules as demonstrated in the next slide  Miller-Urey Experiment 50 years ago, Stanley Miller, a graduate student working with Cosmologist Halord Urey, was able to create amino acids by exposing a gas that simulated the early E atmosphere to UV Radiation and water  Oxygen in the Atmosphere The ability of cyanobacteria to perform oxygenic photosynthesis is thought to have converted the early E atmosphere into a oxidizing one Which dramatically changed the life forms on E and provoked an explosion of biodiversity By ~ 1 Ba ago, the E had developed an atmosphere i.e., very similar to today’s atmosphere (O2 and N2) How do we know that there was no O2 in the early E atmosphere ? The strongest evidence is geological Mineral deposits Iron is almost completely insoluble in water when free oxygen is present, but is highly soluble when free oxygen is not present All over the world, we find large deposits of iron oxide that are all just about the same age After photosynthetic algae evolved and became abundant All that iron fell out of solution and settled to the bottom of all the oceans as iron oxide when free oxygen became available on the Earth's surface These so called banded iron formations are the source of almost all our iron ore, so they are very well studied Oxygen oixides native Fe and created minerals e.g. hematite (Fe 2O3) Simply put water and oxygen you can find the rust which is nothing but Fe 2O3 Banded Iron Formation (BIFs) are a distinctive type of rock often found in primordial sedimentary rocks It consists of repeated layers of iron oxides (hematite) alternating with bands of iron poor silica rich shales or cherts BIFs are primarily found in very old sedimentary rock ranging from ~3 to 1.8 Ba in age It is hypothesized that the banded Fe layers were formed in sea water as the result of free O2 released by photosynthetic cyanobacteria Combining with dissolved iron in oceans to form insoluble Fe oxides, which precipitated out forming a thin layer on the seafloor "Great Oxidation Event" (GOE), nearly 2.3 billion years ago, when oxygen made any measurable dent in the atmosphere, stimulating the evolution of air-breathing organisms and, ultimately, complex life as we know it today.  Chemical Composition of the Earth The internal structure of the earth cannot directly be observed (the deepest ever drilled into the earth was 12km, only a scratch on the surface) Studies regarding the interior of the earth depend mostly on the indirect observations They depend on the inferences made by the seismic studies, meteorites and to some extent on the surface rocks In the fig. below, the left diagram shows the variation of seismic wave velocity with depth The velocity is not constant anywhere except at the outer core for S-wave where it goes to 0 km/s This variation in the speed of P- and S- waves indicates that the interior is not made up of the same stuff everywhere An increase in the wave velocity indicates denser material and decrease indicates a comparatively rarer material Seismic studies have led to many significant discoveries about the earth and its make-up They revealed that the earth has several distinct layers These layers are distinguished by their physical and chemical properties like thickness, depth, density, temperature and metallic content The earth is divided into eight main layers: 1. Inner core, 2. Outer core, 3. D”, 4. Lower mantle, 5. Transition region, 6. Upper mantle, 7. Oceanic crust and 8.continental crust The velocity of P-wave gradually increases but suddenly drops at the outer core  Inner Core Depth: 6378 - 5150 km with the radius of ~ 1,220 km The inner core is made of iron and nickel (Ni- Fe or Nife) in a solid state; also contains enough gold, platinum and other Siderophile elements Siderophile elements: high-density transition metals which tend to sink into the core because they dissolve readily in iron either as solid solutions or in the molten state) It is suspended in the molten outer core It is believed to have solidified as a result of pressure-freezing which happens to most liquids under extreme pressure The Earth's inner core is slowly growing (1 mm/yr) as the liquid outer core at the boundary with the inner core cools and solidifies due to the gradual cooling of the Earth's interior (about 100 degrees Celsius per billion years) To be about the same temperature as the surface of the Sun: approximately 5700 K (5430 °C) Inner core rotates and research published in Science in 2005 and more recently in the February 2011 issue of Nature Geoscience confirms that Earth’s inner core does indeed rotate faster than the rest of the planet Outer Core Depth: 5,150 – 2,890 km with the radius of ~ 2,266 km Made of mainly iron and nickel, temperature of the outer core ranges from 4400 °C in the outer regions to 6100 °C near the inner core It is a hot electrically conducting liquid This conductive layer combined with earth’s rotation causes a dynamo effect that maintains a system of electrical currents creating the earth’s magnetic field This layer is not as dense as pure molten iron indicating the presence of lighter elements Scientists suspect that about 10% of the layer is composed of sulphur and oxygen because these elements are abundant in the cosmos and dissolve readily in molten iron  Earth's Core and the Geodynamo The Earth's magnetic field is mostly caused by electric currents in the liquid outer core, which is composed of highly conductive molten Fe and Ni Convection of liquid metals in the outer core creates the Earth's magnetic field This magnetic field extends outward from the Earth for several thousand kilometers, and creates a protective bubble around the Earth that deflects the solar wind Without this field, a larger proportion of the solar wind would directly strike the Earth's atmosphere The presumed effect would be to strip the Earth's atmosphere away slowly This is hypothesized to have happened to the Martian atmosphere, rendering the planet effectively lifeless  D” Layer Depth: 2,890 – 2,700 km This layer is 200 to 300 km thick It is often identified as part of the lower mantle But a strong change of density characterizes the core-mantle interface, it looks very abrupt so that this boundary acts as a quite perfect reflector for the seismic waves But seismic evidence suggests the layer might differ chemically from the lower mantle lying above it Seismologically, this part is known as the Gutenberg Discontinuity  Lower Mantle Depth: 2,700 – 650 km The lower mantle is composed of mainly silicon, magnesium and oxygen It probably also contains some iron, calcium and aluminum Scientists make these deductions by assuming the earth has a similar abundance and proportion of cosmic elements as found in the sun and the primitive meteorites The high temperatures within the mantle cause the silicate material to be sufficiently ductile that it can flow on very long timescales Because of the temperature difference between the Earth's surface and outer core and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle  Above the lower mantle there is a transition between Upper part of the Mantle & Crust – two distinct layers are there namely Asthenosphere and Lithosphere Asthenosphere From Greek asthenēs 'weak' + sphere The upper mantle of the E; highly viscous, mechanically weak and ductilely-deforming region due to temperature and pressure Covers the depths between 100 and 200 km but perhaps extending as deep as 700 km It flows like a convection current, radiating heat outward from the E's interior Moving at rates of deformation measured in cm/yr over lineal distances eventually measuring thousands of kilometers  This figure is a snapshot of one time-step in a model of mantle convection. Colors closer to red are hot areas and colors closer to blue are cold areas. The flowing asthenosphere carries the lithosphere of the Earth, including the continents and oceans, on its back.  Lithosphere Comes from the greek word, lithos, (rock, and the word, sphere) It comprises the crust and the uppermost mantle Constitute the hard and rigid outer layer of the E Lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure Oceanic lithosphere: Associated with Oceanic crust and exists in the ocean basins Oceanic lithosphere is typically about 50–100 km thick The major part of the earth’s crust was made through volcanic activity The density of these rocks is ~3.0 g/cm3 Continental lithosphere: Associated with Continental crust Continental lithosphere has a range in thickness from about 40 km to perhaps 200 km At the base of the crust is the Mehorovicic discontinuity Crust Uppermost layer of the Earth, is not always of same thickness Crust under the oceans is only about 5 km thick while continental crust can be up to 65 km thick Ocean crust is made of denser minerals than continental crust It is composed of a great variety of igneous, metamorphic, and sedimentary rocks  Temperature, Pressure & Density The earth is a sphere with a radius of 6378 km The temperature, pressure and density increase with depth as can be seen from the figure below The temperature at the core is believed to be an incredible 5000-6000 0 C The pressure is around 7500 kbar The density is around 13.5  The P-wave velocities, density and thickness of different layers Thickness P-wave velocity Layer Density (g/cm³ ) (km) (km/sec) Continental crust avg. 35 2.6 - 2.8 6 Oceanic crust 5 - 10 3.0 - 3.5 7 Mohorovicic discontinuity (Moho) Mantle 2885 4.5 - 10 8 – 12 Gutenberg discontinuity Core (average) 3470 10.7 or 12 - Outer core (liquid) 2250 - 8 – 10 Inner core (solid) 1220 13.5 11 – 12  Plates The lithospheric outer shell of Earth is not one continuous piece but is broken, like a slightly cracked eggshell, into about a dozen major separate rigid blocks, or plates There are two types of plates, oceanic and continental; Oceanic plates are heavier than continental plates There are 12 major plates in the Earth The Pacific, N American, S American, Eurasian, Australian-Indian, African and the Antarctictic are the 7 major plates The Philippine, Juan de Fuca, Nazca, Cocos, Caribbean, Arabian and Scotia are the minor plates They differ in size, direction of motion and the type of crustal rocks present in the plate Some plates such as the N American Plate carry continents and adjoining pieces of the ocean floor Others such as the Pacific Plate, in contrast, are completely covered by oceans and are made of oceanic crust The speed of movement is estimated at 1 to 10 cm per year Most of the earth’s seismic activity occurs at the plate boundaries as they interact The plates are interconnected at global scale and the activity of one plate can profoundly change the behaviour of the other plates  Plates Tectonics Tectonics is concerned with the movements in the earth and the forces that produce movement A scientific theory that describes the large-scale motions of E's lithosphere The model builds on the concepts of Continental Drift, developed during the first decades of the 20th century This theory explains the movement of the plates and also the cause of the earthquakes, volcanoes, oceanic trenches, island arcs and many other geologic phenomena Where plates meet, their relative motion determines the type of boundary: Convergent, Divergent, and Transform How Plates Move ?  Mechanisms of Plate Tectonics: 1 Ridge-Push 2 Slab Pull Mantle Drag 3  Convective flow of mantle It is believed that the force for the movement of the plates is the convection currents Heat generated from the radioactive decay of elements deep in the interior of the earth is the source for convection currents in the asthenosphere Convection currents in the asthenosphere transfer heat to the surface, where plumes of less dense magma break apart the plates at the spreading centers creating divergent plates These plates float on the hot ductile mantle (asthenosphere) like slabs of ice on a pond Much of the earth’s history is the result of plates rifting into pieces to form new ocean basins and converging back together to form mountains and giant continents The plates and their relationships are described by the 3 tectonic regimes: Cratons and Ocean Basins, Hot Spots and Plate Boundaries (divergent boundaries, ocean basins, convergent boundaries, and transform boundaries)  The 3 tectonic regimes are the individual components that interact in plate tectonic theory Cratons (stable continents): An old and stable part of the continental lithosphere Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates Those portions of the earth most of us live on Modern cratons are, for example, the interior of North America east of the Rocky Mountains and west of the Appalachian mountains (e.g. the mid west), and central and western Australia. Ocean Basins: Not everything that is below sea level is ocean basin, but ocean basins are always below sea level When you see a picture of the earth from space, most of that area underwater is ocean basin Ocean basins compose the largest surface area on Earth Unlike continents, ocean basins form and disappear quickly; the oldest we have is only about 150 Ma old (compared to the oldest continent at 3.9 Ba)  Plate Boundaries Plates interact in three ways. They diverge, converge and slide past each other These three plate interactions form three types of boundaries and they are: 1. Convergent boundaries – Where one 2. Divergent boundaries – Mid-oceanic spreading lithosphere dives under another in a process ridges that generate new oceanic crust. Because the called subduction. These are “compressional” plates are pulling apart, these are “extensional” boundaries. These are destructive plate margins boundaries. These are constructive plate margins 3. Transform boundaries – Where plates slide horizontally past each other along giant faults. California’s San Andreas Fault is the best known transform plate boundary. These are conservative margins  Convergent Plate boundary Continental vs. Oceanic Plate Convergence In a contest between a dense oceanic plate and a less dense, buoyant continental plate, guess which one will sink? The dense, leading edge of the oceanic plate actually pulls the rest of the plate into the flowing asthenosphere and a Subduction Zone is born! Where the two plates intersect, a deep trench forms The oceanic plate sinks before it completely melts, but remains solid far beyond depths of 100 km beneath the earth's surface When the subducting oceanic plate sinks deeper than 100 km, huge temperature and pressure increases make the plate ‘sweat‘ The uncomfortable conditions force minerals in the subducting plate to release trapped water and other gasses The gaseous sweat works its way upward, causing a chain of chemical reactions that melt the mantle above the subducting plate This hot, freshly melted liquid rock (magma) makes its way toward the surface Most of the molten rock cools and solidifies in huge sponge-like magma chambers far below the E's surface Some molten rock may break through the E's surface, instantly releasing the huge pressure built up in the gas-rich magma chambers below Gasses, lava and ash explode out from the breached surface. Over time, layer upon layer of erupting lava and ash build volcanic mountain ranges Large intrusive rock bodies that form the backbones of great mountain ranges such as the Sierra Nevada form by this process Oceanic vs. oceanic plate convergence In a contest between a dense oceanic plate and a less dense, buoyant continental plate, you know that it’s the dense oceanic plate that sinks What happens when two dense oceanic plates collide? Once again, density is the key! Little by little, as new molten rock erupts at the mid- ocean ridge, the newly created oceanic plate moves away from the ridge where it was created The farther the plate gets from the ridge that created it, the colder and denser ('heavier') it gets When two oceanic plates collide, the plate that is older, therefore colder and denser, is the one that will sink The rest of the story is a lot like the continental vs. oceanic plate collision we described above Once again, a subduction zone forms and a curved volcanic mountain chain forms above the subducting plate Of course, this time the volcanoes rise out of the ocean, so we call these volcanic mountain chains island arcs Volcanoes of the Eastern Caribbean Island Arc is an excellent example of a very volcanically-active island arc Lesser Antilles Subduction Zone, where oceanic crust of the South American Plate is being subducted under the Caribbean Plate  Nazca Plate The Caribbean Plate lies in a complex area with two major plates and two minor plates bordering it The plate includes oceanic and continental Crust The Caribbean Sea covers most of the plate with Central America and volcanic islands covering the rest The edges of the plate have intense seismic activity, frequent earthquakes and volcanic eruptions The area also contains seventeen active and dangerous volcanoes 2 notorious volcanoes in the area are Soufriere Hills on Montserrat and Mount Pelee on Martinique The plate is bounded by 2 active subduction zones which have two associated active trenches and volcanic arcs, the Lesser Antilles (to the E) and the Central American (to the W) trenches/arcs, and by 2 complex transform-trascurrent fault zones, the Motagua-Polochic-Swan-Bartlett/Oriente to the N, and the northern S. American fault system to the S, where subduction of the Caribbean plate below S. America is active at present Continental vs. Continental Plate Convergence By this time, you understand enough about plates to guess that when the massive bulk of two buoyant continental plates collide there is bound to be trouble! When two huge masses of continental lithosphere meet head-on, neither one can sink because both plates are too buoyant It is here that the highest mountains in the world grow At these boundaries solid rock is crumpled and faulted Huge slivers of rock, many kilometers wide are thrust on top of one another, forming a towering mountain range The Himalayan mountain range provides a spectacular example of cont, vs. cont. collision The pressure here is so great that an enormous piece of Asia is being wedged sideways, slipping out of the way like a watermelon seed squeezed between your fingers  Divergent Plate boundary Most are located along the crests of oceanic ridges called “Mid Oceanic Ridge” and can be thought of as constructive plate margins Some also located in the continent called “Continental Rifting” Mid oceanic ridges or Continental rifts are the centers of divergence or spreading apart of two plates As molten rock material moving upward (asthenosphere) by convection reaches the surface along these ridges, the entire oceanic lithosphere moves away from the spreading center In this way new ocean floor constantly forms and slides away from either side of the ridge as solid plates of the lithosphere Longest topographic feature on the E’s surface representing 20% of E’s surface with the width of 1000-4000 km Ridges are often perpendicular to the motion between the two plates on each side Creating new sea floor !!! Typical spreading rate ~ 5 cm /yr No ocean floor is older than the Jurassic (150 Ma) !!! This process is responsible for the phenomenon known as “Seafloor Spreading” Example of Continental Rifting Iceland: An example of continental rifting The divergent zone splits the Iceland into two parts A rift between the N. American (right) and Eurasian (left) continental plates in Iceland  Interestingly both theory of Continental Drifting and Sea Floor Spreading came well before Plate Tectonics theory Continental Drift – Evidences and Finding Alfred Wegener First proposed his continental drift hypothesis in 1915 Published The Origin of Continents and Oceans Continental drift hypothesis Continents "drifted" to present positions Evidence used in support of continental drift hypothesis Fit of the continents Fossil evidence Rock type and structural similarities Similarities in rock age Paleoclimatic evidence/ Glacial evidence Paleomagneitc evidence  Early Observations Leonardo da Vinci and Francis Bacon wondered about the possibility of the American and African continents having broken apart, based on their shapes Even if we try … we can roughly juxtapose all the land parts of the world into a single land mass Wegener’s matching of mountain ranges on different continents  Fossil Record The same kinds of fossils are found from areas known to be adjacent to one another in the past The fossil record had revealed that the geology and paleontology match on opposite sides of the Atlantic Ocean In fact, there are matching fossil records that span across all of the continents Without plate tectonics, this is hard to explain Fossils of the same types of ancient amphibians, arthropods and ferns are found in South America, Africa, India, Australia and Antarctica Sometimes the descendants of these organisms can be identified and show unmistakable similarity to each other, even though they now inhabit very different regions and climates  Similarities in rock age, rock type and structural similarities Even before geochronology, the relative framework of rock ages showed strong correlation across the Atlantic, as did mountain ranges of similar age  Glacial Evidence Large ice masses carve grooves in the rocks over which flow Such masses tend to flow outward (generally downhill) from a central locality Paleomagnetism Earth’s magnetic field A hot magma is not magnetic As a magma cools and solidifies, the iron-bearing minerals (such as ferromagnesian silicates) crystallize Eventually, the minerals cool below the Curie temperature and the iron-bearing minerals become magnetic Like tiny compass needles, these magnetic minerals align themselves parallel to the lines of force of the Earth’s magnetic field This remnant magnetism, which is also called Paleomagnetism, points to the north pole like a sign post Rocks that were formed at different places on the Earth's surface have different magnetizations Using this information, when rock layers were uncovered with a magnetization that did not agree with it's position on the Earth In fact, with the help of the crystallized magnetic minerals we can draw the movement of the continents over long geologic time Sea Floor Spreading –Evidence and Finding Harry Hess combined his observations with the earlier ideas of Wegener and the mechanism of Holmes into the concept of sea floor spreading, which lead to plate tectonics. Paleomagnetism on the Sea Floor In the 1950s, the Atlantic Ocean seafloor was found to consist of alternating stripes of normal and reversely magnetized rocks The maps showed parallel magnetic ‘stripes’ that were perfectly symmetrical across the ridge axis Colored stripes represent rocks with present-day magnetic orientations (‘normal polarity’), grey represents rocks with reversed polarity An amazing discovery was made when the magnetic profile of the sea floor around the Mid-Atlantic Ridge was mapped Vine and Matthews interpreted the magnetic stripes as products of steady creation of new ocean crust over geologic time, supporting the hypothesis of Hess.  Ages of Sediment and Rocks In the early 1960s, massive programs for drilling into the seafloor began Extracted cores of seafloor showed that sediments are thicker on top of seafloor basalt near the continents while MOR sediments are thin Cores of both sediments found that near the continents the oldest sediments are at the bottom and young sediments are at the top MOR sediments are all of recent age – When the ages of rocks are measured, the continental rocks are billions of years old, while seafloor rocks are less than 200 million years of age Rocks of the oceanic crust increase in age as their location extends from the MOR, and at the MOR they are new.  History of Continental Rifting Present-day continents were the fragmented pieces of preexisting larger landmasses(“Supercontinent") The diagrams below show the break-up of the supercontinent Pangaea (meaning "all lands" in Greek), which figured prominently in the theory of continental drift Pangea broke up into two smaller super continents separated by the Tethys Sea The creation of Pangea meant that only one large continent existed on the Earth balanced by one large ocean called Panthelessa (all seas) Beginning 230 my ago, the present Atlantic Ocean formed and began spreading Pangea broke apart completely and the continental fragments are now scattering across the globe  Before Pangea, another super continent called Rodinia existed between 1.2 Ba and 750 Ma ago There is growing evidence that even older supercontinents viz., Vaalbara (~ 3.1 Ba ago), Kenorland (~ 2.7 Ba ago) and Columbia (during a period of 1.8 to 1.5 Ba) predated Rodinia early in Earth’s history Vaalbara  In the Triassic Period, Pangea began to break up again Heat loss from the mantle through the thin ocean floor is more rapid than through the thick continental crust, which acts as a blanket Accumulation of heat expanded the continental crust, so that its surface was uplifted and the relatively brittle upper part was fractured Upwelling currents in the mantle (mantle plumes) caused upraised blisters that broke into three rifts at 120 to each other Rifts continued to widen, pulled apart by the movement of mantle material beneath the crust Rifts continued to extend to link with similar rifts above other mantle plumes, so building a lengthy rift system which eventually widened to become an ocean The Red Sea, the Gulf of Aden & the north end of the African Rift in Ethiopia would be amodern example  The rift valley filled with volcanic rocks and with sediments eroded off its flanks The block of crust between two parallel faults dropped down, making a steep-sided valley-like depression called a Graben Half-grabens (down-faulted on only one side) were developed when Permian and older thrust faults and transverse faults were reactivated, such that the original compressive forces were released and extensional forces allowed the blocks to slide back down the fault plane A series of such reactivated thrust and transverse (strike-slip) faults transected the province, and adjacent to these faults were formed several new sedimentary basins These grabens gradually filled with thick sequences of continental-type sediments and volcanic rocks During this period of sustained tension, magma welled up some fractures adjacent to the sedimentary basins and either solidified as basaltic dykes and sills within the rock strata or was extruded at the surface as lava flows This cycle of opening and closing ocean basins is the Wilson Cycle HOT SPOTS When convection rises as a single plume rather than along a linear spreading ridge, the result is a “Hot Spot” On the surface hot spots erupt as volcanoes of dark basaltic rock material The source of the molten rock is at the asthenosphere The Hawaiian islands are the best known modern example of hot spots derived from mantle plumes Hot spot volcanoes often form long chains that result from the relative motion of the lithosphere plate over the hot-spot source Hot spots are single plumes of molten rock ascending from the (lower) mantle into the overriding lithosphere Such plumes have built the Hawaiian Islands and the volcanic features of Yellowstone National Park World map showing the locations of selected prominent hotspots  Mariana Trench Philippine Plate Pacific Plate Trenches form at the subduction zone Trenches are deep features of the earth The depth of the Mariana trench is 11,033 m or 36, 198 feet Whereas the height of the highest mountain (mt. Everest) is only 8,848m or 29,029’  Transform Boundary When two plates slide past each other, transverse fractures or transform faults take place Along these boundaries no plate is formed or destroyed and so they are called conservative plate margins These represent most of the oceanic fracture zones Mid Oceanic Ridge System all over the globe is effected by series of Transform Fault Boundary This is to adjust the stress developed in the MOR And are generally defined by shallow earthquakes They commonly offset active spreading ridges producing zigzag plate margins A few occur on land. The San Andreas Fault zone in California is a transform fault The San Andreas Fault zone is 1,300km long It slices through two thirds of the length of California Along it, the Pacific Plate has been grinding horizontally past the North American Plate for 10 my at an average rate of about 5 cm/year Land on the west side of the zone is moving in a northwesterly direction relative to the land on the east side of the fault zone  California Earthquake April 4 2010 The most famous example is the San Andreas Fault Zone in California The portion of California in blue is heading northwest to Alaska This is the most studied fault zone in the world Notice how many have occurred in California They are not kidding when theytalk about the “BIG ONE” Each red dot marks an earthquake