Earth Systems Science Chapter 7 I. Structure of the Earth II. Plate Tectonics The solid part of the earth system includes processes, just like the atmosphere and oceans. However, the time scales for processes in the solid earth are much longer. As a result, solid earth processes are important for climate over much longer time scales (e.g. millions of years). Earth Systems Science Chapter 7 I. Structure of the Earth 1. Seizmology, structure of the earth 2. The crust and lithosphere 3. The mantle 4. The core Seismology: study of earthquakes and related phenomena Earthquake: sudden release of energy resulting from the rapid movement between two blocks of rock Surface Waves: like ocean surface Seismology: study of earthquakes and related phenomena Body Waves: P-Waves Primary waves Body Waves S-Waves Secondary waves 1
Seismology: study of earthquakes and related phenomena Surface Waves (similar to ocean waves): Structure of the Earth using seismology, the general structure of the earth can be determined Moho: the boundary between the crust and the mantle Moho Upper mantle Direction of wave movement Lower mantle Metallic fluid Metallic Solid Direction of particle movement Structure of the Earth The Crust and Lithosphere Continental crust: thicker, less dense, older 20 km (weathered areas) to 75 km (younger areas) thick Oceanic crust: thinner, more dense, more recent ~7km thick Lithosphere: the crust and uppermost (rigid) mantle This system is dynamic, not static! http://www.enchantedlearning.com/subjects/astronomy/planets/earth/inside.shtml http://www.enchantedlearning.com/subjects/astronomy/planets/earth/inside.shtml 2
The Crust and Lithosphere The Mantle asthenosphere: The relatively plastic layer of the upper mantle of the Earth on which the tectonic plates of the lithosphere move; small fraction of molten material Upper mantle, lower mantle: upper differs from lower in seismic wave velocity, but not clear what causes the difference Composition of mantle is similar to carbonaceous chondrites, the original nebular composition, except depleted in volatiles and iron Where did the volatiles and the iron go? The Core Mostly iron, ~6% nickel ~8-10% other materials More dense than the mantle Source of the earth s magnetic field When a metal (conductor of electricity) undergoes convection, a magnetic field is created Over geologic time the polarity of the magnetic field shifts back and forth (North becomes South, and vice versa) II. Plate Tectonics 1. Sea Floor Spreading 2. Continental Drift 3. Plate Tectonics 4. Deep Earth Processes 5. The Rock Cycle 3
Sea floor spreading: ocean topography Sea floor spreading: magnetic polarity and age Sea floor spreading: magnetic polarity and age Sea floor spreading: magnetic polarity and age 4
Continental Drift and Pangea Pangea Periods of statis (stress builds up between plates) Periods of movement (stress is released, earthquakes) Periods of statis (stress builds up between plates) Periods of movement (stress is released, earthquakes) Periods of statis (stress builds up between plates) Periods of movement (stress is released, earthquakes) 5
Types of boundaries, or margins: a) Divergent: plates moving away from each other b) Convergent: plates moving towards each other c) Transform: plates slipping past each other Periods of statis (stress builds up between plates) Periods of movement (stress is released, earthquakes) convergent margin passive margin transform margin divergent margin http://neic.usgs.gov/neis/plate_tectonics/plates.html 6
Deep Earth Processes 1. Heat Deep Earth Processes 2. Mantle convection Radioactive decay Gravitational energy released during accretion and formation of earth Not sure exactly how the convection occurs http://www.enchantedlearning.com/subjects/astronomy/planets/earth/inside.shtml Deep Earth Processes 3. How they affect plate movement: Gravitational forces associated with density differences, due to hot less-dense magma from mantle more dense less dense The Rock Cycle: igneous rocks Rock Types: Igneous, sedimentary, metamorphic Igneous: cooling and solidification of magma that upwells from the mantle in mid-ocean ridges; all rocks originate from igneous Ocean lithosphere: average age 80 M yrs; max age 200 M yrs Continental lithosphere: max age 4 B yrs; parts of the continent that became tectonically dormant and now reside in the middle of continents, called cratons Why is the maximum age of the ocean crust so much less than the maximum age of the continental crust? 7
The Rock Cycle: cratons The Rock Cycle: weathering, erosion, & sediments Weathering: decomposition of exposed rocks into finer material called sediments and dissolved into water Weathering caused by physical, biological, and chemical forces Erosion: transport of sediments and dissolved material by wind, landslides, and streams to basins in the ocean or on land Erosion forms landscapes: more resistant material is not eroded, leaving landforms such as peaks and ridges The Rock Cycle: sediments and sedimentary rocks Sediments overly much of the oceanic and continental crust. Ocean sediments: sediments that were washed in through rivers, as well as shells from plankton near the ocean surface, that settle through the water column and settle on the ocean floor. Continental sediments: from weathering on mountains, sediments accumulate in low lying basins; also, largely from former ocean floors that were transported, exposed, and uplifted due to tectonic activity Sediments are deposited in layers, accumulating through time; as the depth increases, temperature and pressure increases Lithification: Sediments are compacted into sedimentary rocks 8
http://neic.usgs.gov/neis/plate_tectonics/plates.html The Rock Cycle: uplift, subduction, and metamorphism uplift: due to continental collisions, former low-lying surfaces are forced up creating mountains and plateaus As soon as higher areas are formed, weathering and erosion begins In general, higher mountain ranges are younger, having experienced less erosion Subduction: sediments deposited in subduction zones are transported into the earth s mantle; igneous rocks that were never eroded also enter subduction zones Metamorphism: subducted material undergo intense heat and pressure, are transformed into magma in the upper mantle, eventually to re-surface through mid-ocean ridges The Rock Cycle The Rock Cycle Kump et al. system diagram http://www.enchantedlearning.com/subjects/astronomy/planets/earth/inside.shtml Mountains 5 (all types) uplift Sea floor (igneous) 4 extrusion, volcanism 1 Sedimentary erosion, Basins sedimentation Frei system diagram Aesthenosphere (metamorphic) 2 subduction Subduction Zone (all types) metamorphism 3 9