Chapter 8: Plate Tectonics

Transcription

1 460:102 Historical Geology Chapter 8 Notes 1 Chapter 8: Plate Tectonics I. Continental Drift A. Alfred Wegener (1912) 1. German meteorologist - devoted much of his life to championing Drift. He pooled the existing pre-drift geologic data suggesting that the continuity of the older structure, formations and fossil faunas and floras (Glossopteris- Fig 8-3) across present continental shorelines was better understood on a pre-drift reconstruction. Glaciation (Tillites) in Southern Hemisphere continents; tropical conditions in Greenland and No. Europe (figs. 8-8,8-9). 2. Wegener postulated that all continents were joined into a single land mass termed Pangaea ("All the Earth"). 3. More appropriate to consider Pangaea as two super continents: Laurasia (Laurentia and Asia) - No. America, Greenland, Europe, and Asia and Gondwana (Land of the Gonds (India)) - S. America, Antarctica, Africa, Madagascar, India, Australia 4. Super continents were separated by Tethys Ocean 5. Resistance to Wegener was personal and trivial in many ways: 1) Wegener's CD rejected the static earth position (Scientific orthodoxy); 2) drew on several different lines of evidence which he was not an expert; 3) some details were not correct leading others to discard the whole hypothesis; 4) lack of reasonable mechanism which was the single most obstacle (Wegener postulated that centripetal forces acting on the higher standing continents caused them to drift.) B. A. Du Troit South African geologist - recognized sound geologic evidence and continued to advocate a drift scenario (see overview figure and Fig. 8-1 through 8-3) 1. Mesosaurus (Fig. 8-6) II. Paleomagnetism - Fossil Magnetism A. Rock have magnetic properties because some minerals have magnetic domains (particularly those with Fe). At the atom level, some elements have unpaired electrons. The orbits and spins of electrons create electrical currents and hence a magnetic field. Paired electrons cancel each other out. An unpaired electron therefore imparts a magnetic field on the mineral. Paramagnetic minerals are minerals that have weak magnetic fields that align with the external earth's field. However, once removed from that field, magnetic field in paramagnetic minerals revert back to their original direction. Ferromagnetic minerals have many unpaired electrons and therefore have a strong magnetic field. The individual magnetic fields when exposed to an external field align in the same direction of the external field creating a magnetic domain which is the sum of all of the individual fields. At high temperatures, the ferromagnetic minerals can realign with a changing magnetic field because the energy level is high enough to keep the molecules in motion. As the temperature decreases below the Curie Point or Temperature, the energy level is not high enough to allow the magnetic minerals to realign and hence has a fixed magnetic field. This is referred to as Remnant or permanent magnetism - preserved domain even after removal from the external field

2 460:102 Historical Geology Chapter 8 Notes 2 1. Natural Remnant Magnetism is Primary if it forms at the time of rock formation. a. Thermomagnetic Remnant Magnetism - igneous Cooling below the Curie Point b. Detrital Remnant Magnetism - settling of sedimentary particles which align with the prevailing field. In the water column they are aligned, on the bottom they flatten out in the unconsolidated layer, but realign as consolidation occurs. This depth is 10 to 20 cm and therefore is considered instantaneous. c. Chemical Remnant Magnetism - occurs in Fe-minerals that form from chemical precipitation. Less important for our purposes unless there is diagenetic alteration. 2. The Geomagnetic field is dynamic - resulting from convective circulation of electrical charges in the outer core. There are many thoughts on how this occurs. The geomagnetic spin axis is thought to be largely controlled by Coriolis force which is controlled by the spin of the earth. Therefore, the magnetic poles should be relatively constant as the earth's rotation shouldn't vary considerably. However secular variation do occur with change in core convection. 3. Paleomagnetic measurements include the Delination and Inclination (Dip) of the magnetic field recorded in the rock during formation (Fig Inclination of the paleomagnetic field tells us about the paleolatitude of the rock during formation. If the paleolatitude differs from the present latitude, then obviously the rock has moved in a N-S direction. The Inclination or paleolatitude also tells us how far away the pole was from that location. The declination as some refer to it, points in the direction of the magnetic pole. Therefore, with Inclination and declination, one can plot where the poles were in the past. If paleo pole positions differ from continent to continent (for rock of the same age) then there must have been relative movement between the two. Paleolongitudes are not possible because the magnetic field is symmetric about the dipole or magnetic axis. 4. To chart CD, one can use a fixed pole and plot the movement of continents from that pole or use a fixed continent and plot the apparent pole position relative to the continent. The latter is called Apparent Polar Wander and is used because it facilitates the display of more information. (Fig a. Observations that APW paths differ from continent to continent indicates that continents moved on surface relative to each other (Runcorn and Irving - N. America and Europe). Complete APW histories for continents can show Wilson cycle of Divergence nand Convergence of continents. Shapes of APW paths - similar shapes on continents imply that those continents moved as one. C. Seafloor Spreading 1. To understand Continental Drift it is necessary to understand the regions (ocean floors) that are now separating the once juxtaposed continents. Undertaking manned submersible, drilling, dredging etc. operations is expensive and provides only a very narrow view of the regional patterns. Geophysical tools (seismics, gravity, and magnetism) can be collected relatively inexpensively, and show properties over a large area. a. Harry Hess explored the Pacific ocean during WWII. 1. Key Observations from ocean exploration.

3 460:102 Historical Geology Chapter 8 Notes 3 a. Guyots - Flat topped remnants of volcanoes - deepen w/ distance from ridge axes. b. Observed that the ocean sediment thickness indicated that the ocean (260 Ma) was much youger than the Earth (4.6 Ga). (average sediment thickness is 1.3 km; average sed rate is 1 cm per kyr; allow for compaction and the average ocean age = 260 myr.) now know that it is <200 Ma (average sed is ~2cm/kyr.) c. Observed that small number of underwater volcanoes (10,000) was inconsistent with old sea floor. 2. Key points about mid-ocean ridges a. High heat flow b. low seismic velocities c. high number of volcanoes 3. Convective Cells (fig 8-13) a. High heat flow - rises and cools to form new crust - rises or expands b. crust cools as it moves away from ridge - it shrinks / sinks - Guyots (fig. 8-15) c. Deep-sea trenches must exist to balance crustal formation (fig. 8-16) b. Magnetics - measure the strength of the earth's magnetic field continuously as the instrument is towed behind the ship. The measurement should reflect the earth regional magnetic field that originates in the core plus the addition (or subtraction) of the magnetic field in the rocks. Subtracting the present field strength from the measurement leaves an anomaly. This anomaly is positive or negative. If positive, then the underlying rocks were predominately magnetized under the same polarity as at present, if negative the magnetic field was acquired during a time of reversed polarity. 1. The first significant marine magnetic survey was conducted in 1961 off the northwest coast of the US by Raff and Mason. These results showed a pattern of stripes and steep gradients separating positive an negative anomalies. Lineations were remarkable persistence and could be for 100's of km. Basalt has a high proportion of ferromagnetic minerals. Lineations must arise when the crust of adjacent basaltic blocks are magnetized in opposite directions. b. Geomagnetic reversals - Brunhes (1909) and Matuyama (1928) first reported. c. Changes in the geomagnetic field is believed to originate by magnetohydro dynamics in the outer core. The magnetic field and the flow of the fluid outer core caused the solid inner core to always spin more rapidly than the mantle and crust - the result of magnetic coupling between the inner core and a jet-streamlike layer of rapidly rotating outer-core fluid at the inner-core boundary. Once in motion (rotation of the earth) this process become self perpetuating as long as there is an energy source to maintain convection. In explicably, the magnetic dipole switches causing the geodynamo to switch and polarity to switch. d. Vine & Matthews (1963) combined sea floor spreading with geomagnetic reversal. The magnetic lineations result from the lithification new oceanic crust - Curie point - ferromagnetic behavior. Measure total field then anomalies reflect the addition or subtraction of the rock magnetic field. (see fig 8-17)

4 460:102 Historical Geology Chapter 8 Notes 4 D.Plates 1. Definition of a. Lithosphere 1. Crust plus upper Mantle 2. thicker continental plates b. Asthenosphere 1. Mohorocivic Discontinuity 2. Plastic layer with particial melting 2. Plate Boundaries a. Divergent - Ridge Axes b. Convergent - Trenches c. Transform - Strike Slip II. Divisions of Inner Space (NOT IN BOOK) A. Tools - Seismic Waves 1. Generated when rocks are suddenly disturbed they break or rupture 2.Vibrations spread out in all directions from the source of the disturbance they move outward in waves that travel at different speeds through materials that differ in chemical composition or physical properties a. Body (penetrating) waves - primary secondary or surface waves 1. Primary -p-waves a. are the speediest of the three b. travel through the upper crust of the Earth at speeds of 4-5 km/sec c. near the base of the crust they speed along at 6-7 km/sec d. will echo or reflect off rock masses of differing physical properties 2. Secondary - also termed s-waves or transverse waves a. travel 1-2 km/sec slower than p-waves b. able to penetrate deep into the interior or body of the planet 3. Factors that influence the behavior of body waves a. s-waves cannot propagate through fluids b. p-waves are markedly slowed through fluids

5 460:102 Historical Geology Chapter 8 Notes 5 Compressional or P waves in which particle oscillate in the direction parallel to the direction of the wave propagation. P-waves are the fastest and most abundant therefore easiest to detect. Typical values for V P are: water = 1.5 km/s; Sediments = 1.8 km/s; Basalt = 3.4 to 6.2 km/s Shear or S-Waves oscillate in a direction perpendicular to the propagation direction. Cannot be transmitted through fluid. Reflection of seismic waves. These studies use the principle that as P-waves encounter internal boundaries in the earth some of the energy is reflected back to the surface. The energy source is usually man-made (air or water guns, dynamite) and is detected by geophones or hydrophones. The amount of energy returned is a function of the change in physical properties at that layer (acoustic impedance). Acoustic impedance is the contrast of the density x velocity. This determines how much energy will be reflected or returned to the surface. R = A r A i = r 2 V P2 - r 1 V P1 r 2 V P2 + r 1 V P1 Therefore the greater the density contrast the greater the reflected energy. One of the largest occurs at the seafloor where the velocity in water is r 1 = ~1 andv P1 = 1500 m/s and while the sediments are r 2 = 1.8 and V P2 = 1800 m/s. (1.6 to 2.5 km/s). Another is at the boundary between sediments and the crust r 1 = 1.8 and V P1 = 1.8 km/s. (1.6 to 2.5 km/s) and r 2 = 2.8 and V P2 = 3.4 km/s. (3.4 to 6.2 km/s).. Refraction - as the seismic waves propagate through the earth the wave energy not reflected by at a boundary is refracted or bent. In general, where the wave velocity increases with depth, the waves are bent upwards. The travel times of the rays from the source to various receivers are plotted against the distance of the receivers. The ray or wave velocity is a function of the velocity of the layer in which that wave traveled. Because P and S-waves have different velocities information about the earth's interior can be learned. This information includes densities and elastic properties and the velocity changes indicate where the changes occurred. As the wave encounters a change in velocity, the refracted angle (r) is calculated by sini sinr = V 1 V 2

6 460:102 Historical Geology Chapter 8 Notes 6 Normal to Boundary i r V1 V2 sin i = V1 sin r = V2 Low Velocity Layer i' r' V2 V3 sin i' = V1 sin r' = V2 V2 Seismic Ray Note that the shallow slope for the deeper layers reflects increasing velocities. Travel time of Crustal Pwaves Slope = 1/V 1 Travel Time of Pwaves T Explosion Pwave Paths Seismometer Travel time of Mantle Pwaves Slope = 1/V 2 Distance Crust: Pvelocity= V 1 Mantle: Pvelocity= V 2 B. Main Discontinuities in Earth 1. Mohorovicic ( km) - boundary between crust and upper mantle lies at about km below the surface of the continents (as deep as 100 km under mountains) and lesser depths beneath the ocean floors discovered using seismic refraction 2. Gutenberg (2891 km) - located ~ halfway to the center of the Earth at depth of 2900 km Location marked by abrupt decrease in p-wave velocities and disappearance of s-waves marks the outer boundary of the Earth' s core C. Earth's Core 1. Detected by P and S waves shadow zones Inferences from Body Waves

7 460:102 Historical Geology Chapter 8 Notes 7 a. The precise boundary of the core was determined by the study of earthquake waves b. The outer core barrier to s-waves results in an s-wave shadow zone on the side of the Earth opposite the earthquake - passed through a liquid medium c. Radius of the core is about 3500 km d. inner core is solid with a radius of ahout 1220 km e. evidence for the existence of a solid inner core is derived from the study of hundreds of seismograms a transition zone approximately 500 km thick surrounds the inner core with the same composition as the outer core - Important for Latent Heat of Fusion 2. Average density 10.7 g/cm 3 a. The Earth had an overall density of 5.5 g/cm3 the average density of rocks at the surface is <3.0 g/ cm 3 rocks of the mantle have a density of about 4.5 g/cm 3 the average density of the core is about 10.7 g/cm 3 3. Composed mainly of Fe and Ni a. Composed of 85% iron with lesser amounts of nickel as determined from the study of meteorites consist of metallic iron allowed with a small percentage of nickel may be fragments from the core of a shattered planet abundance in the solar system suggests the existence of an iron-nickel core additional evidence trom the existence of a magnetic tield produced bv an electric current tlowin through a wire 4. Radius: 3500 km 5. Inner core (solid) and outer core (liquid) Layers of the Mantle C. Earth's Mantle Materials of the Mantle Average density is about 4.5 g/cm3 1. Stony composition (4.5 g cm~3) a. oxygen and silicon predominate accompanied by iron and magnesium b. the mineral peridotite approximates the kind of material inferred for the mantle appropriate for the mantle's density similar in composition to stony meteorites 2. Layers: Upper, transition (250 km thick), lower (above core) Detected by studying earthquake data Three zones of increasing wave velocity a. upper mantle: 400 km b. transition zone: extends down to about 650 km c. Lower mantle: lies beneath the transition zone and above the core Consists mostly of silicates and oxides of magnesium and iron d. rearranged into denser and more compact crystals 3. Upper mantle features The upper mantle is of particular importance because its evolution and internal movements affect the geology of the crust a. low velocity zone

8 460:102 Historical Geology Chapter 8 Notes 8 Low velocity zone: a region in which there is a decrease in the speed of s- and p-waves a region that is in the state of a crystalline-liquid mixture 1-10% molten (most believe 3 or 4%) capable of considerable motion and flow serve as a slippery mobile layer on which overlying lithospheric plates could move b. asthenosphere (plastic layer) D. Earth's Crust Seismically defined as all of the solid Earth above the Mohorovicic discontinuity The thin rocky veneer that constitutes the continents and the floors of the oceans 1. Oceanic crust (basaltic; 3.0 g cm~3) a. Approximately 5-12 km thick b. Average density of 3.0 g/cm c. The upper mantle is the ultimate source for the lavas that formed the oceanic crust 2. Continental crust (granitic; 2.7 g cm 3 ) a. thickest crust (average 35 km; 20 to 100 km) b. floats due to isostasy continents float higher on the denser mantle than the adjacent oceanic crustal segments isostacy Composed of a variety of rocks that approximate yranite in composition rich in silicon and potassium poor in iron magnesium. and calcium extensive regions are hlanketed hy sedimentary rocks c. Origin of the Continental Crust billion years ago: upwellings of lava derived from the partially molten upper mantle subjected to repeated episodes of remelting where the lighter components were separated out and distributed near the Earth s surface Lavas were subjected to weathering and erosion whose products were the Earth s earliest sediments altered by rising hot gases and silica-rich solutions Recycling and melting Lead ultimately to the rocks of granitic character formed the nuclei of continents provided a source for additional sediment which was also metamorphosed and melted during orogenic events Heat Heat is the measure of internal energy of the atoms and molecules - translational and rotational motions. Temperature is an arbitrary numeric scale proportional to the average translation kinetic energy. Kinetic energy is the energy associated with motion. The heat flux is the transfer of energy from high to low temperature. Understanding the earth's heat flux is important to determine:

9 460:102 Historical Geology Chapter 8 Notes 9 1) How is heat transferred within the earth; 2) Where does it come from; 3 What processes produce or release heat; 4) Is the earth heating up or cooling off? The downward gradient in the earth's temperature was first discovered first by measuring the vertical temperature in caves. Typical gradient within the earth is 20 to 30 C/km. Sources of heat within the earth are considered to be: 1) primordial heat which is heat left over from the original accretion of the Earth from planetary nebula. 2) radioactive decay - less obvious but more significant. Radioactive decay of Uranium, Thorium, and Potassium. Because the earth's crust must have originated during partial melt, the radioactive elements are highly concentrated in the crust relative to the mantle. Granite>basaltic (order of magnitude); Basalt> mantle rocks (4x). 3) crystallization of inner core. The released latent heat of crystallization my be the energy source that is driving convection in the core and providing heat to the base of the mantle. Latent heat is invisible heat. It is energy stored in the molecules that is only released when there is a change of state. In this case when the Iron Nickel core changes from liquid to solid, energy (heat) must be released because the molecules are now at a lower state. Types of heat transfer: 1) Conduction is the heat that is transferred through molecular collision. Molecules with higher vibrational energy collide with molecules with lower vibrational energy causing a transfer of energy. Heat again is the measure of this vibrational energy. So the first molecule loses energy (cools) and the second molecule gains energy (warms). The best example is that of immersing a cold spoon into a pot of boiling water. Over time, the handle of the spoon will heat up as heat is conducted up the handle. This flux of heat follows Fourier's Law: Heat flux (q x ) = -k T x where T is the change in temperature measure over the change in distance x. K is the thermal conductivity of the material. Some material conduct heat better than others (e.g., aluminum). 2) Radiative Heat Transfer - occurs as the internal energy at one place is converted into electromagnetic radiation which radiates out and is absorbed by material at another location. The electromagnetic energy is converted by into internal energy (Heat). These electromagnetic waves are longer than visible light spectrum - Infrared. Examples would be the radiative warming for the sun. Electromagnetic waves produced in the sun are absorbed by your skin and converted into heat. Microwave ovens cook by this principle. While this is a major source of heat for the earth's surface, radiative heating is small scale with respect the interior earth.

10 460:102 Historical Geology Chapter 8 Notes 10 3) Convective Heat Transfer is the heat transferred by the motion of the material itself. Movement of material within the earth occurs by density differences. As lower mantle heats upper by conduction heat transfer across the core mantle boundary, increase temperature causes decreased density. Eventually, the lower mantle material with rise or convect because of the gravitational forces to the top of the mantle bringing heat to the base of the lithosphere. Once at the base of the lithosphere, heat is lost or transferred to the lithosphere through conduction and the material increases in density and sinks back into the lower mantle. This form of heat transfer is the most efficient and is considered the driving mechanism for Plate tectonics, orogeny, volcanism, earthquakes. Rheologic Model of Earth Traditional subdivisions of the earth interior are based on compositional changes. Crust, Mantle, Core. Each of the layers have been further subdivided based on changes in geophysical properties that represent compositonal changes. The Rheological model of the earth is based on the flow properties-rigid vs plastic. The large-scale features of the outer part of the earth show a rigid layer in isostatic equilibrium underlain by a weaker layer that deforms by flow. The Lithosphere is the styrong outer layer that deforms elastically. This layer includes the crust and upper mantle. Under continents, the lithosphere is ~100 to 150 km thick. In oceanic settings, the lithosphere is km thick. However, the oceanic lithosphere varies as a function of age. The high heat flow under the ridge spreading centers allows the asthenosphere to approach the surface. As the ridge moves away and cools, the lithosphere thickens. The lithosphere- Asthenosphere boundary is more like a transition zone, not distinct.

11 460:102 Historical Geology Chapter 8 Notes 11 Asthenosphere is weaker and reacts to stress in a fluid manner. The Asthenosphere extends from the base of the lithosphere to ~700 km. Temperature is believed to be the main phenomena that controls the strength of the subsurface. Although temperature increases with depth, the hydrostatic pressure (pressure of surrounding material exerted equally in all directions), also increases with depth. This causes the melting point of rocks to increase. Think of a pressure cooker. Food cooks at a higher temperature because as the pressure increases (with temperature), the boiling point of water increases. Consider also the altitude effects on water. With increasing altitude, water boils at a lower temperature. Melting can only occurs when the temperature intersect the melting curve. The melting curve is dependent on the thermal properties of the rocks and the pressure. (See below).

12 460:102 Historical Geology Chapter 8 Notes 12 The Asthenosphere represents the location in the mantle where the melting point is closely approached. The rocks in the asthnosphere are not molten. S-waves penetrate this layer but the velocity drops considerably. This is partial melting. It is estimated that only 1% melting occurred to explain the observed decrease in Seismic wave velocity. The Mesosphere underlays the Asthenosphere its compositional equivalent is the lower mantle. This is considered to be a zone of high strength.