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Internal Structure of the Earth and Plate Tectonics Lecture by Dr. Ken Galli, Boston College EESC116301 Environmental Issues and Resources July 28, 2015 Please do not distribute beyond the EESC116301 Class. Case History: Two Major CA Cities • San Andreas fault: a transform plate boundary between the North American and the Pacific plates • Two major cities on the opposite sides of the fault: Los Angeles and San Francisco •Many major earthquakes related to the fault system •Loss of many lives and billions of property damages due to earthquakes •New construction and retrofitting of infrastructures has become more expensive •When will be the next “big one” and what to do? How to deal with the potential consequence? Internal Structure of Earth • The Earth is layered and dynamic: Interior differentiation and concentric layers • Chemical model by composition and density (heavy or light): Crust, mantle, core, and Moho discontinuity between the crust and mantle • Physical property model (solid or liquid, weak or strong): Lithosphere (crust and upper rigid mantle), asthenosphere, mesosphere, liquid outer core, inner solid core Theory of Plate Tectonics • The upper mechanical layer of Earth (lithosphere) is divided into rigid plates that move away from, toward, and along each other • Most (!) deformation of Earth’s crust occurs at plate boundaries Earth’s Layers 3 main layers defined by composition: • Crust - Outer • Mantle - Middle • Core - Center http://www.physicalgeography.net/fundamentals/10h.html Study of Earth’s Interior Structure • Knowledge primarily through the study of seismology • Seismology: Study of earthquakes and seismic waves • Examining the paths and speeds of seismic waves through reflection and refraction • Magma likely generated in the asthenosphere • Slabs of lithosphere have apparently sunk deep into the mantle • Variability of lithosphere thickness reflects changes in its age and history Composition - How Do We Know? Best Guess! Whole Earth • Meteorites - Fe, Ni (same age as Earth) • Information from velocities of seismic waves indicate material Crust (5-40 Km) • Samples (mountain building helps!) Mantle (5/40 to 2885 Km) • Kimberlite pipes - intrusive igneous rock from the mantle • Lava / volcanic rock • Mountain building Core (2885 to 6371 Km) • Inference – Earth’s mean density = 5.5 g/cm3 – Crust 2.5 to 3 g/cm3; mantle 3.3 g/cm3 to 5.5 g/cm3 – Density of core at least 10 to 11 g/cm3 (iron and nickel) Seismic P Wave • Primary or push-pull wave, travels like sound wave • Direction of rock particle vibration parallel to that of wave propagation • Fastest rates of propagation, first arrival to the seismograph • Body wave travels through Earth interior and all media—solid and liquid Seismic S-Wave • Secondary or shear waves • The direction of particle vibration perpendicular to that of propagation • Propagates slower than P waves • Body wave, propagating through Earth’s interior, but not its liquid layers Seismic Waves and Internal Structures • Earth’s interior boundaries: Sudden changes in the speed of seismic waves • Different characteristics: Different rates and paths of wave propagation • Asthenosphere: Low velocity zone, major source of Earth magma • Outer Core: Liquid, no S wave transmits through it Model of Earth’s Interior Figure 2.2b Crust • Two types of crust: – Continental • 30% of crust • Granites and Diorites - rich in silicates and feldspars (lighter materials) • 35-40 Km thick • Oldest is 3.8 billion years (90% solar system age; missing ~700 m.y.) – Oceanic crust • Basalt - Mg, Fe (heavier materials) • 5-10 Km thick • 200 Ma oldest; 100 Ma average Our deepest hole: 9 Kilometers ….. we have a long way to go! Mantle • MOHO - Mohorovicic Discontinuity • Core mantle boundary - change in mineralogy • Density - getting heavier • 3.3 - 5.5 g/cm3 • Probably material such as peridotite (lots of heavy olivine - Fe, Mg) • Samples from kimberlites, xenoliths in volcanic eruptions, basalt composition; lab experiments Our deepest hole: 9 Kilometers ….. we have a long way to go! Core • Outer core – Molten, near solid point (does not transmit certain seismic waves) – Density of pure iron or nickel/iron – Includes ~ half of diameter of Earth – 2x density of mantle • Inner core – Solid (higher pressure than outer core) – Density of pure iron or nickel/iron – ~ Size of moon Our deepest hole: 9 Kilometers ….. we have a long way to go! Earth’s Layers 3 layers defined by mechanical properties (strength): • Lithosphere • Asthenosphere • Mesosphere http://earthguide.ucsd.edu/mar/dec5.html • Lithosphere – – – – PLATES in Plate Tectonics Upper 100 km Crust and upper mantle Rigid • Asthenosphere – 100 km to ~350 km – Upper mantle – Near melting point; little strength; ductile - NOT A LIQUID! – Plates moving on this – Magma generation • Mesosphere – Extends to core – Also hot; strong due to pressure Internal Dynamics of Earth • Evidence – Earth’s landscape – Dynamic phenomena: earthquakes, volcanoes • Plate Tectonics: Hypothesis and Theory – Continental drift – Seafloor spreading – Plate tectonics – a unifying theory Dynamic Earth—Evidence • Mountain belts (continental mountain ranges and oceanic ridges) • Earthquake distribution: Concentrated zones • Earthquake occurrences over time • Volcanism in space: Concentrated zones • Volcanism over time Why Do the Plates Move? Got Heat? Earth - 3 Heat Sources: • Loss of original heat of formation (geothermal / core is cooling) • Radioactive decay of elements in Earth’s materials • The Sun - external; not important to plate tectonics Convection: Driving Force of Plate Tectonics • Interior of Earth has sluggish convection in some regions • Heat from core rises, creates convection cells in the mantle NOT LIQUID! http://pubs.usgs.gov/publications/text/unanswered.html • Rising hot material at mid-ocean ridges and midocean volcanic islands • Descending cooler material at trenches • Lithospheric plates “carried” with the convection cells Convection in the Mantle http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html and http://www.seismology.harvard.edu/Projects.html Blue blobs show where colder, denser material is sinking into the mantle. Near the surface, most of the colder material is in the ancient roots of continents. Subducting slabs of oceanic lithosphere appear, recycled into the mantle from oceanic trenches. Convection in the Mantle http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html and http://www.seismology.harvard.edu/Projects.html Red blobs are warmer plumes of less dense material, rising principally into the oceanridge spreading centers. A huge plume seems to be feeding spreading at the East Pacific Rise directly from the core. Most of the heat being released from the earth's interior emerges at the fast-spreading East Pacific Rise. More than Convection? • In addition to thermal convection cells, some geologists think that movement may be aided by – “slab-pull” - slab is cold and dense and pulls the plate – “ridge-push” rising magma pushes the ridges up and gravity pushes the ocean floor toward the trench Plate Tectonics as the Unifying Concept of Earth Science Accumulation of Observations Evidence Patterns of continents Paleontology Geology Patterns of sea floor ages Patterns of seafloor depth Patterns of volcanoes Patterns of earthquakes Alfred Wegener • 1912 Continental Drift • Observations – – – – • • Fit of Continents Geology Paleontology Climate belts Pangaea 200 Ma Breakup 180 Ma • Rigid bodies moving through yielding seafloor – could not provide mechanism of movement Continental Drift • Same fossils across both sides of the Atlantic Ocean Figure 2.18 Continental Drift • Matching Mountain Ranges (Rock distribution) Paleozoic Glaciations Figure 2.19 Seafloor Spreading • Lack of mechanism for continental drift • 1950s and early 1960s, ocean expedition increased knowledge of oceanography • In 1960s, Harry Hess proposed seafloor spreading – Seafloor not a single static piece – Mid-oceanic ridges, or spreading centers where new crust is formed and seafloor spreads Harry Hess and Seafloor Spreading • New ocean crust at midocean ridges • Ocean crust dragged down at trenches; mountains form here • Continental crust too light; remains at surface • Earthquakes occur where crust descends “It explains everything….” Seafloor Spreading • Paleomagnetic data – Dipolar magnetic field – Magnetic field recorded by iron-bearing igneous rocks – Striking symmetrical magnetic anomaly stripes • Age of seafloor rocks: Progressively younger toward the mid-oceanic ridge • Thickness of seafloor sediments: Progressively thinner toward the ridge Seafloor Spreading Figure 2.15 Seafloor Spreading - Observations • Fit of continents - new material pushes them apart • Topography of ocean floors - hot ridges, trenches • Volcanism at ridge axes - hot mantle material • Seismic zones near margins - descending plates Magnetism – The Final Piece • Earth has magnetic field • Similar to a giant dipole magnet – magnetic poles essentially coincide with the geographic poles – may result from different rotation of outer core and mantle Magnetic Reversals • Earth’s present magnetic field is called normal – magnetic north near the north geographic pole – magnetic south near the south geographic pole • At various times in the past, Earth’s magnetic field has completely reversed – magnetic south near the north geographic pole – magnetic north near the south geographic pole When magma cools, takes on signature of Earth’s prevailing magnetic field magnetic iron-bearing minerals align with Earth’s magnetic field Vine and Matthews (1960’s) “MOR” Evidence for Hess’s seafloor spreading idea What does this have to do with seafloor spreading? Mid-Atlantic Ridge Oceanic Crust Is Young • Seafloor spreading theory indicates that – oceanic crust is geologically young – forms during spreading – destroyed during subduction • Radiometric dating confirms young age – youngest oceanic crust occurs at mid-ocean ridges – oldest oceanic crust is less than 200 million years old – oldest continental crust is 4.3 billion yeas old Theory of Plate Tectonics Supporting Evidence: Fit of continents Patterns of heat flow Ocean floor topography Age patterns of seafloor Volcanism at ridge axes / hot spots Magnetic stripes Seismic zones Patterns of mountains Plate Tectonics • A unified theory: Study the dynamic creation, movement, and destruction processes of plates • Plates: Fragments of lithosphere • Plates move in relation to each other at varied rates • No major tectonic movements within plates • Dynamic actions concentrated along plate boundaries • Plate boundaries: Defined by the areas of concentrated seismic and volcanic activities, rifts, faults, and mountain ridges Plate Tectonics • Three major types of plate boundaries • Divergent: plates moving apart and new lithosphere produced in mid-oceanic ridge • Convergent: plates collide, subduction and mountain building • Transform: two plates slide past one another Plate Tectonics Figure 2.4a Plate Tectonics Table 2.1 (1 of 2) Plate Tectonics (3) Table 2.1 (2 of 2) Plate Tectonics • Divergent plate boundary – Plates move away from each other – Mid-oceanic ridges – Continental rift valleys – Creates new seafloors – Extensional stress and shallow earthquakes – Basaltic volcanism Plate Tectonics • Convergent plate boundary Plates collide with each other and three subtypes – C-C boundary: Major young mountain belts and shallow earthquakes – C-O boundary: Major volcanic mountain belts, subduction zone and oceanic trench, earthquakes – O-O boundary: Subduction zone, deep oceanic trench, volcanic island arc, wide earthquake zones Plate Tectonics • Transform plate boundary – Locations where the edges of two plates slide past one another – Spreading zone is not a single, continuous rift; offset by transform faults – Most transform plate boundaries are within oceanic crust, some occur within continents – Famous transform plate boundary on land is the San Andreas fault Plate Boundaries Plate Motion • Plates move a few centimeters per year: about the growth rate of human fingernails • The rates of movement changes over time • North American plate along the San Andreas fault about 3.5 cm (1.4 in.) per year • When rough edges along the plate move quickly, an earthquake may be produced • Often slow creeping movement Hot Spots • Places on Earth: Volcanic centers with magma source from deep mantle, perhaps near the core-mantle boundary • Hot spots can be on continents and oceans, Yellowstone and Hawaii • Forming a chain of volcanoes over a stationary hot spot: Example, the Hawaiian–Emperor Chain in the Pacific Ocean • The bend of a seamount chain over a hot spot representing the change of plate motion Hot Spots Figure 2.16a Plate Tectonics and Environmental Geology • Significance of tectonic cycle – Global zones of resources (oil, gas, and mineral ores) – Global belts of earthquakes and volcanic activities – Impacts on the landscape and global climates – Geologic knowledge on plate tectonics: Foundation for urban development and hazard mitigation Tectonics and Environmental Geology Figure 2.4b Isthmus of Panama • Formed 3 Ma from collision between Pacific and Carribbean plates • Shut down the flow of water between the Atlantic and Pacific • Atlantic currents were forced northward, forming the Gulf Stream • With warm Caribbean waters flowing toward the northeast Atlantic, the climate of northwestern Europe grew warmer (winters would be as much as 10 °C colder without the Gulf Stream) http://www.nationsonline.org/oneworld/panama.htm Isthmus of Panama • The Atlantic, no longer mingling with the Pacific, grew saltier • In short, the Isthmus of Panama directly and indirectly influenced ocean and atmospheric circulation patterns, which regulated patterns of rainfall, which in turn sculpted landscapes • Subsequent warm, wet weather over northern Europe may have resulted in the formation of an Arctic ice cap and contributed to the ice age during the Pleistocene Epoch Gulf Stream http://www.nc-climate.ncsu.edu/education/ccms/2003/ben_jon/Pictures/Gulf-Stream2.gif Gulf Stream http://oceancurrents.rsmas.miami.edu/atlantic/img_mgsva/gulf-stream-YYY.gif Effect of Andes Mountains • Continued uplift raised the Andes to an altitude of >4000 m by the end of the Neogene • Barrier for southeast trade winds in the subtropics and for westerly winds in the midlatitude regions of South America • Today, the blocking of the westerly winds results in enhanced precipitation on the western side of the mountain range (Chilean and Patagonian glaciers) and causes a strong rain shadow on the eastern side (Patagonian desert) Terra.rice.edu/plateboundary Supercontinents and Climate • During periods of supercontinent formation, an ice age often results • Sediments increase as continents collide • Removal of carbon dioxide out of the atmosphere by sediments leads to a decrease in global temperatures • Mountain building at equator upsets temperature balance • Causes decrease in temperature at the equator, resulting in a lower global temperature Supercontinents and Climate • Moisture – The interior of a supercontinent is a significant distance away from a moisture source (i.e. large bodies of water) – Climates were extremely arid in the interior reaches of supercontinents http://vishnu.glg.nau.edu/rcb/globaltext.html Climate History and Tectonics • We can determine the past climate of the Earth by mapping the past positions of the continents • Plot distribution of ancient coals, desert deposits, tropical soils, salt deposits, glacial material, as well as the distribution of plants and animals that are sensitive to climate, such as alligators, palm trees & mangrove swamps • Certain types of rocks, such as coals, which need abundant rainfall, form under certain climatic conditions (for example, coal forms in tropical rainforests or temperate forests) • By mapping the past distribution of thousands of these rock types, we have begun to map the distribution of the ancient climatic belts A Few Questions... • Assume the Pangaea never broke up, how might today’s environments be different? • What are the major differences in plate tectonic settings between the U.S. eastern and western coasts? • Will the tectonic cycle ever stop? Why or why not? • Why is most seismic and volcanic energy released along the Pacific rim? • Does plate tectonics play a role in shaping your local environment? Why do the plates move? Convection! Fig. 3. The continent of Africa is thought to have been split by a series of rift valleys in various states of development. Those in East Africa are still in thick crust. Those in West Africa are associated with thick oil-bearing sediments. In the Red Sea area the rifting has gone so far as to form a narrow ocean. In the south-east Madagascar has been completely separated from Africa by rifting. Source: http://www.le.ac.uk/geology/art/gl209/lecture3/lecture3.html accessed 7/22/14.