ERS120 Term Test 1 Review Sheet.doc

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Earth Science
Lindsay Schoenbohm

Term Test 1 Review Sheet Lecture 2 Formation of solar system: 1. A nebula forms from hydrogen, helium and helium left over from big bang 2. Nebula condenses into a swirling disc, with a central ball surrounded by rings 3. Ball at centre grows dense and hot enough for fusion → becomes the sun 4. In the rings, dust particles collide and stick together, forming planetesimals 5. Planetesimals grow into the proto-Earth; interior heats up and becomes soft 6. Gravity reshapes proto-Earth into a sphere; interior differentiates into a core and mantle 7. Soon after Earth forms, a small planet collides with it → blasts debris forming a ring around Earth 8. The moon forms from the ring of debris 9. Atmosphere develops from volcanic gases; when the Earth becomes cool enough, moisture condenses and rains to create the oceans. Some gases may be added by passing comets Formation of Inner & Outer planets: • Planetesimals exerted gravity to pull in other objects nearby • Later grew into protoplanets • Eventually incorporating all the debris within its orbit, it became a full-fledged planet Differentiation of Earth – Internal layering • Large planetesimals heated up: o Transformation of kinetic energy into thermal energy during collisions o Decay of radioactive elements o Bombarded by meteorites • Iron alloy separated out and sank to the centre of the body; rocky materials remained in a shell surrounding the centre Shaping of Earth – sphere • When a protoplanet gets big enough, gravity can change its shape • Rock composing a small planetesimal is initially cool and strong • Once the planetesimal reaches a critical size (1000 km), interior becomes warm and soft enough to flow in response to gravity o Protrusions pulled inward toward the centre o Spherical shape is formed o Gravity is nearly the same at all points on its surface • Shape is supported because mass is evenly distributed around the centre in a sphere Formation of the Moon • Mars-sized protoplanet slammed into newborn Earth • Colliding body disintegrated, along with large part of Earth’s mantle • Ring of debris formed around Earth quickly coalesced to form the moon Differences between gaseous outer and rocky inner planet Rocky Inner Gaseous Outer Terrestrial planets; consist Gas-giant planets; mass of a shell of rock consists of gas and ice surrounding a ball of metallic iron alloy Closer to the Sun Further away from the Sun Relatively small Very large Mercury, venus, earth, Jupiter, Saturn, Uranus, mars neptune Major layers of the Earth Basic Composition Thickness Crust 10-70 km • Oceanic: thin, consists of basalt and gabbro • Continental: varies in thickness and rock type (0.5%) o Mafic and felsic • Oxygen = must abundant element Mantle • Entirely of peridotite 2880 km (67%) • Mantle rock can be soft enough to flow (100-150 km) Outer Core • Liquid iron alloy 2260 km (30.8%) Inner Core • Solid iron-nickel alloy 1220 km (1.7%) Meteorites: some meteors (stony: carbonaceous chondrites) are asteroids derived from planetesimals that never underwent differentiation into a core and a mantle Stony Earth’s Crust Stony-Iron Earth’s Mantle Iron Earth’s Core Age of the Earth • 4.57 billion years • Meteorites appear to be 4.54 Ga and carbonaceous chondrites are 4.57 Ga Kimberlite Pipes: carry up mantle rocks (xenoliths) and diamonds embedded in a special kind of igneous rock called kimberlite • Pipes are formed by high velocity volcanic eruptions P Waves: causes particles of material to move back and forth parallel to the direction in which the wave itself moves S Waves: causes particles of material to move back and forth perpendicular to the direction in which the wave itself moves • S waves don’t travel through liquid o Ability to travel through a certain material and the velocity at which it travels depend on the character of the material o Particles in liquids aren’t close enough to support the movement of particles moving perpendicular to the direction the wave is moving Seismic Waves • Waves travel at different velocities in different rock types; they can accelerate or slow down if they pass from one rock into another • Only P waves can travel through a liquid • In general, seismic waves travel more slowly in a liquid than in a solid • Detect the depths at which velocities suddenly change = determined boundaries Earth’s magnetic field • Largely dipole – has a North and South pole • Generated by convection in the Earth’s liquid outer core • Solar winds interacts with the magnetic field, distorting it into a huge teardrop pointing away from the sun • Deflects solar winds and keeps the atmosphere intact Lecture 3 Thickness Density/Rigidity Continental Crust ~35 km thick Relatively low density (2.56 g/cm3) Oceanic Crust ~10 km thick Higher density; heavier (3.2 g/cm3) Lithosphere – ~100-250 thick Relatively rigid Crust & Upper Mantle Asthenosphere – 200 km thick Relatively ductile (easily Upper Mantle deformed) Basic Tectonic Model • Lithosphere is thin, cool and hard • Asthenosphere is hot and weak • Lithosphere can be broken into large fragments called plates • Plates “float” on the asthenosphere • Plates move around and interact with each other Isostasy: plates “float” at an elevation depending on thickness and density • Continental crust is thicker and has a lower density • Oceanic crust is thinner and his a higher density • Continents stand at a higher elevation because they are composed of thick masses of lower density materials Evidence for continental drift: • Fit of the continents: part of eastern South America can fit into south-western Africa • Matching rock units: found common group of rocks on eastern part of South America and South Western Africa • Matching mountain belts: If the Atlantic didn’t exist, Paleozoic mountain belts on both coasts would be adjacent • Fossils: occur on multiple continents; hard to explain if continents were always separated by oceans • Matching paleoclimate belts: distribution of late Paleozoic coal, sand-dune deposits and salt deposits could define climate belts on Pangaea • Glaciations: studying age of glacial till deposits → determined that a variety of continents had glaciers at the same time Evidence for Sea-Floor Spreading Topography • Mid-ocean ridges – bathymetry on one side of the axis is nearly a mirror image of bathymetry on the other side • Deep-ocean trenches – border volcanic arcs, curving chains of active volcanoes • Fracture zones – ocean floor is diced up by narrow bands of vertical cracks and broken-up rock • Layer of sediment becomes progressively thicker away from the mid-ocean ridge axis Heat Flow • Heat rising up from the Earth’s interior up through the crust → not the same everywhere in the oceans o More heat rises beneath mid-ocean ridges o Magma might be rising into the crust below mid-ocean ridges Volcanoes • Seamount chains – volcanoes that no longer erupt; one island at the end of a seamount chain remains capable of erupting today Earthquakes • Occur in distinct belts; some follow mid-ocean ridge axes, trenches or fracture zones Formation of Magnetic Stripes on Sea Floor • Magnetic reversals: there is an alternating normal and reversed polarity in rock layers • Reversed: south magnetic pole lies near the north geographic pole and vice versa o May be result of changes in circulation patterns in the outer core • Positive anomalies form when sea-floor rock has the same polarity as the present magnetic field • Negative anomalies form when sea-floor rock has polarity that is opposite to the present field Evolution of Sea Floor • Ridge: rocks = young, sediment = thin • Away from ridge: rocks = older, sediment = thicker • Youngest rocks (at the crest) have present-day, normal magnetism • Stripes of rocks parallel to the crest alternate in magnetism (normal-reversed- normal, etc ...) Earthquakes • Continental crust can’t be subducted because it is less dense than oceanic crust Lecture 4 Plate Boundaries 1. Divergent: moving away from each other, two oceanic plate move apart by the process of sea-floor spreading, new oceanic lithosphere forms, takes place at mid-ocean ridges. 2. Convergent: moving towards each other, two plates move toward one another, one plate sinks down while the other one rises. Oceanic lithosphere has to sink down, and continent or oceanic lithosphere can rise up. 3. Transform: moving past each other Evolution of a divergent boundary • Rifting causes the continent to crack and break apart; new mid-ocean ridge forms and sea-floor spreading begins • As continents pull apart, magma rises from the mantle into a magma chamber • Some magma along the sides of the magma chamber cools to form gabbro o Gabbro moves away from the magma chamber o New magma rises to keep volume of magma constant • Some magma rises to fill vertical cracks above the magma chamber, solidifying to form dikes o Dikes break in half as sea-floor spreading continues and move to the side o New magma rises to fill the crack and create new dikes • Some magma makes it to the surface and extrudes as basalt pillows → forms the top of the sea-floor crust Divergent can be ocean-ocean (mid Atlantic ridge), this is very common because of the mechanism of convection or continent-continent (African rift valley) this is uncommon because it will become ocean-ocean soon after. Divergent ocean-ocean boundaries are not linear because as rifting progresses spreading ridges form perpendicular and transform faults form parallel to plate motion. Example of Continental Rift • Mid atlantic ridge → separates North American and Eurasian plate • Symmetrical, young along the ridge and gets older moving away • Slips of faults can produce earthquakes and volcanoes Features of Convergent Zones Ocean-ocean: Australia-Pacific near New Zealand • Shallow waters around land mass; deeper depths further away from land mass • Dense activity of earthquakes • Thin belt of volcanic activity → presence of hydrothermal vent and hot springs Ocean-continent: Nazca-South America • High elevation on land and low depth in waters • Dense earthquake activity • Thing belt of volcanic activity • Young oceanic crust Continent-continent: India-Eurasia • Wide belt of high elevations along plate, some instances of low sea levels • Randomly sparsed pattern of earthquake activity • Low instances of volcanic activity Continental Transform Fault: North America-Pacific in the US • Actively slipping segment of a fracture zone between two ridge segments o One plate slides sideways past another o No new plate forms and no plate is consumed • Earthquakes occur on segment of a fracture zone that lies between the two ridge segments • Very young oceanic crust • High elevation; wide and decreases moving away from the ridge Triple Junctions: 3 plate boundaries come together, Hot Spots: Hawaiian Islands • Volcanoes that exist as isolated points and are not a consequence of movement at a plate boundary o Heat source for hot spots = mantle plume – very hot rock rising up through the mantle to the base of the lithosphere o Hot-spot track forms when overlying plate moves over a fixed plume o Movement of plates slowly carries the volcano off the top of the plume → becomes extinct Driving Mechanisms of Plate Tectonics • Mantle convection: When mantle rocks near the radioactive core are heated, they become less dense than the cooler, upper mantle rocks. These warmer rocks rise while the cooler rocks sink, creating slow, vertical currents within the mantle (these convection currents move mantle rocks only a few centimeters a year). This movement of warmer and cooler mantle rocks, in turn, creates pockets of circulation within the mantle called convection cells. The circulation of these convection cells could very well be the driving force behind the movement of tectonic plates over the asthenosphere. Certain areas in mantle are hotter less dense, so material in mantle moves up and cools down, and eventually sinks, only to get heated up again, this process will carry other things with it, it can drag the lithosphere, slow moving convection currents, take lithosphere with it. • Ridge- push force: Ridge push is a gravitation force that causes a plate to move away from the crest of an ocean ridge, and into a subduction zone. • Slab-pull force: is the result of a plate subducting at a steep angle through the mantle; this downward motion tends to pull the other side of the plate away from the ridge crest Lecture 5 Epicentre: the point on the surface of the Earth that lies directly above the hypocentre Hypocentre: the place within the Earth where rock ruptures and slips, or the place where an explosion occurs, focus of the earthquake. Faults along which Earthquakes occur 1. Divergent – Normal Fault: result of stretching of the crust o Hanging wall moves down 2. Convergent – Thrust Fault: result of shortening of the crust o Hanging wall moves up and the fault’s slope is gentle 3. Transform – Strike-slip Fault: one block of crust slides laterally past another o No vertical displacement takes place Relationship between Plate Tectonics and Earthquakes • Earthquakes occur in fairly narrow seismic belts that correspond with plate boundaries o Shallow = top 20 km of earth; intermediate = 20-300 km; deep = 660 km • Happen at faults along plate boundaries → relative motion between plates is accommodated primarily by slip on these faults 1. Divergent plate boundary: o Seismicity along mid-ocean ridges take place at shallow depths, earthquakes here are shallow 2. Convergent plate boundary: o Have intermediate and deep earthquakes → downgoing slab as it sinks into the mantle Wadati-Benioff: defines intermediate and deep earthquake at convergent boundaries >660km = no earthquakes. 3. Transform plate boundary: o Have a shallow focus; larger ones on land can cause disaster Seismograph • There are two types: horizontal and vertical-motion seismographs • Horizontal revolving paper cylinder connects to a frame • Heavy weight with an attached pen hangs from the frame o Weight and pen remain fixed in space because of inertia o When earthquake oc
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