GEOS 251 — Physical Geology

1. GEOS 251 — Physical Geology 24 April 2014 • Handouts • Lecture Summary 25: Energy Resources • Final • Thurs 15 May 2014, 10:30am-12:30pm • Class evaluation • Who will please take charge with returning the forms?

2. Last time: Landscapes • Overall form of continents • Landforms • Mountains, hills, mountain ranges • Plateaus, tablelands, mesas • Structurally controlled ridges and valleys • River valleys and bedrock erosion • Structurally controlled cliffs: Cuestas, hogbacks, fault scarps • Landscape evolution • Feedback between uplift and erosion • Feedback between climate and topography • Active research: Do regions of rapid uplift and coincide with areas of positive feedback between climate and topography? • Models of landscape evolution • Historical: Davis, Penck, and Hack • Modern view

3. Geomorphology and landforms • Geomorphology • The study of landscapes and their evolution • Faculty member in Geosciences: Jon Pelletier • Landforms • Mountains, hills, mountain ranges • Plateaus • Tablelands, mesas • Structurally controlled ridges and valleys • River valleys and bedrock erosion • Structurally controlled cliffs • Cuestas, hogbacks, fault scarps

4. Mountains, hills, mountain ranges • Hill • Large mass of rock that projects above its surroundings • Mountain • Large mass of rock that projects well above its surroundings • Mountain ranges • Clusters and alignments of mountains and hills Glacially sculpted mountains in southern Argentina; peaks are sharp arêtes

5. Plateaus • Large, broad, flat areas of appreciable elevation above surrounding terrain • Usually <3000 m elevation • But Bolivian Altiplano is ~3600m, and Tibetan Plateau is ~5000m Tibetan Plateau

6. Tablelands and mesas • Tablelands • Similar but smaller features • Mesa • Small, flat area with steep slopes on all sides • Results from differential weathering of bedrock of varying hardness, especially in deserts • Flat tops held up by erosion-resistant beds A mesa in Monument Valley, Arizona

7. Structurally controlled ridges and valleys • Young mountains • Topography may be consist of belts of upfolded (anticlines) and downfolded (synclines) rocks defined by the folds Young (Pliocene) folds in Zagros Mountains, Iran

8. Structurally controlled ridges and valleys • Older mountains • Where climate and weathering predominate over active tectonism • Topography may be inverted (e.g., valleys occurring in anticlines) in areas with rocks of sharply contrasting resistance to weathering

9. Structurally controlled ridges and valleys • Appalachian Valley and Ridge province • Tectonic controlled topography persists • Reflects resistant beds folded into anticlines and synclines in the Paleozoic

10. River valleys and bedrock erosion • Stream power (product of river slope and river discharge) determines how bedrock erodes • Generally higher stream power in steeper terrains • Erosion • Little occurs during low discharge • Most occurs during brief periods of high discharge

11. River valleys and bedrock erosion • Erosion of bedrock occurs principally by three processes • Abrasion by the suspended load and saltating particles • Plucking fragments from bedrock by drag forces on channel • Glacial erosion forming valleys that can be occupied by rivers • Review of profiles • River valleys form V-shaped profiles • Glacial valleys form U-shaped profiles

12. River valleys and bedrock erosion • Badland • Deeply gullied topography • Produced by rapid erosion of easily erodible lithologies (shales, clays) Badlands of South Dakota, Sage Creek Wilderness, Badlands National Park

13. Structurally controlled cliffs:Cuestas, hogbacks, fault scarps • Regions with a series of tilted and eroded beds • With strongly contrasting resistances to erosion • Cuestas • Asymmetrical ridges • Hogbacks • Landforms that have steeply dipping or vertical beds of resistant strata, with cliffs on both sides • Steep cliffs • Can be formed by fault scarps • These landforms can develop both in • Extensional tectonic settings (e.g., Rio Grande rift, Basin and Range) • Contractional tectonic settings (e.g., monoclines of Colorado Plateau and Rocky Mountains)

14. Structurally controlled cliffs:Cuestas, hogbacks, fault scarps • Cuestas • Asymmetrical ridges • Long slope of low to moderate dip, determined by dip of the resistant bed • Cliff formed at another side, where the edge of a resistant bed has been undercut by erosion of an underlying weaker bed Dinosaur National Monument, Colorado

15. Structurally controlled cliffs:Cuestas, hogbacks, fault scarps • Hogbacks • Landforms that have steeply dipping or vertical beds of resistant strata • Stand out as steep, narrow, more or less symmetrical ridges • Cliffs on both sides Hogback ridges in the Front Range of the Rocky Mountains near Denver, Colorado

16. Landscape evolution • Competition between tectonic forces and erosion • Landscape controlled by Earth’s interacting geosystems • Erosion • Responds to changes in base level and profile, as well as continental position

17. Feedbacks between processes • One action produces an effect (the feedback) that tends either • To speed up the original action (positive feedback), or • To slow the original action, perhaps stabilizing the process at a slower rate (negative feedback) • Landscape evolution shows examples of both positive and negative feedbacks

18. Uplift stimulates erosion • Negative feedback between uplift and erosion

19. Isostatic mantle rebound raises mountain elevation • Positive feedback between climate and topography

20. Evaluating landscape evolution • Defining geologic surfaces, modern and ancient • Geologic mapping of surfaces and structures • Measuring active deformation of modern surface • GPS geodetic measurements (time scale of years to tens of years) • Constraining magnitude and timing of earlier deformation • Geologic mapping • Thermochronology (time scale of thousands to millions of years) • Dating absolute age of a weathered surfaces and landscapes • Radiogenic isotopic methods (e.g., C-14, U-Th) • Cosmogenic isotopic methods (e.g., Be-10: cosmic rays produce radioactive isotopes) • Tephrochronology (correlate and date volcanic ash beds) • Paleomagnetic (reversals and secular variations in strata) • Dendrochronology (tree rings)

21. Example of how topography determines local climate and, in turn, controls erosion and landscape development • Arabian Sea at Yemen-Oman border • Steep escarpment of the Qara Mountains • Wrings moisture from monsoons • Which allows vegetation to grow (green along mountain fronts and canyons) and soil to develop (dark brown)

22. Topic of active research • Do regions of rapid uplift and erosion (i.e., where deeper levels of crust exhumed) coincide with areas of positive feedback between climate and topography? • Being tested in Patagonia (Chile and Argentina)

23. Models of landscape evolution: Historical views • William Morris Davis • Cycle of uplift and erosion • Walther Penck • Erosion competes with uplift • John Hack • Landscapes achieve dynamic equilibrium

24. William Morris Davis • Cycle of uplift and erosion • Cycle initiated by strong, rapid uplift over geologically short periods of time • High, rugged, mountains of youth • Rounded forms of maturity • Plains of old age; mountains then stay tectonically fixed • Uplift and erosion largely separated in time • Early uplift and late erosion • Davis’ view dominated during his lifetime • Prestigious position (Harvard professor) • Effectively promoted ideas by traveling widely; was a prolific author

25. Walther Penck • Erosion competes with uplift • Uplift rate gradually increases then gradually decreases • Surface processes attack uplifting mountains throughout interval of uplift • Results in gradual decrease in both relief and mean elevation • Competition between uplift and erosion • Rather than temporal separation • Contemporary of Davis; challenged Davis’ view • Ideas not well recognized until after his death

26. John Hack • Landscapes achieve dynamic equilibrium • Moderate but constant uplift rate • Landscape undergoes minor adjustments during a period of equilibrium, but overall landscape will remain more or less the same • Elaboration on Penck’s model that erosion competes with uplift • That there will be periods of dynamic equilibrium

27. Implications of this historical overview • Science advances by questioning earlier hypotheses • Old views are overturned • New prevailing views or paradigms take over • Some ideas, later found to be incorrect, may persist for a long time before being overturned • Especially if they have forceful proponents • Unpopular, minority interpretations may take decades to be widely recognized as a better interpretation • Nonetheless, few unpopular ideas ever become the prevailing view--crackpots exist alongside the mavericks! • New ideas will also have to be tested and supported by evidence • In a decade or two from now • Some of what you have been taught in this class undoubtedly will be judged at that time to be incorrect Prof. Spence Titley, senior faculty member in Dept of Geosciences: I’ve been teaching here for more than 50 years; some of my exam questions stay the same, but the acceptable answers are what keep changing

28. Modern view of landscape evolution • Many combinations of tectonic forces and erosion are possible • Hence, there is no simple set of predictions • Relatively short intervals • Probably dominated by variations in climate • Much longer time intervals • Probably dominated by tectonically-driven uplift

29. Summary • Topography • Elevation and relief • Results from the interplay between tectonic forces (compression, extension), the amount of crust, and the removal of material by erosion • Continents consist of active and inactive parts • Areas can become passive or active with time • Landforms • Mountains, plateaus, structurally controlled ridges and valleys, river valleys, structurally controlled cliffs • Landscape evolution • Competition between tectonic forces and erosion • Type of feedback between uplift and erosion depends upon time scale • Type of feedback between climate and topography depends upon relative importance of isostasy • Modern view of landscape evolution • Many combinations of tectonic forces and erosion are possible; hence, there is no simple set of predictions • Relatively short intervals probably dominated by variations in climate; much longer time intervals probably dominated by tectonically-driven uplift Next: Energy Resources

30. Lecture 25: Energy Resources • Overview — Energy, population, and environment • Fossil fuels (sedimentary origin and distribution) • Petroleum and natural gas (mainly marine sources) • Coal (terrestrial sources) • Distribution reflects geology, climate, preservation • Other energy sources (origins and distribution) • Geothermal, nuclear (internal sources -- originally from??) • Solar, wind, hydropower (solar sources) • Environmental and economic consequences • Of production, utilization, distribution

31. Renewable vs. non-renewable resources • Renewable resources • Continuously regenerate on a human time scale • Fuel examples: Animal, wood, wind, hydroelectric • Non-renewable resources • May continue to form, but only on geologic time scales • Fuel examples: Coal, petroleum, natural gas, uranium, geothermal; also virtually all mineral commodities

32. World energy consumption • Increased exponentially in the last 200 years • Even faster than the exponential increase in population • Reflects • Industrialization • Mechanization of many processes • Ubiquity of powered transportation • During same time, sources have changed • From renewable resources • To a mix that is dominated by non-renewable resources

33. World energy demand and production

34. U. S. consumption history by type of energy • Virtually all from fossil fuels • Similar to global pattern • Resource base and sustainability

35. Important distinctions • Resources • Economic vs. sub-economic • Discovered vs. hypothetical • Underlie much debate about • Future supplies • Land use • Geopolitical concerns

37. World energy resources

38. Fossil fuels • Product of trapping a small fraction of the sun’s energy • In biological (organic) material • Preserved in the sedimentary record • Hence, fossil fuels are • Fossilized products of photosynthesis • Burning • Releases carbon dioxide and water from which they were made

39. Fossil fuels (hydrocarbons) • Oil and coal • Complex organic molecules with C, H, N, and S • In general, their combustion releases more CO2 (greenhouse gas) and more pollutants, such as S (acid rain) and metal-bearing ash, than does the burning of natural gas per unit of energy • Natural gas • Mostly methane (CH4) • Cleaner burning than coal and oil • CH4 + 2O2  CO2 + 2H2O

40. Formation of oil and natural gas:A complicated process • Deposition of marine and lake sediments (“source rocks”) • Contain abundant organic debris (generally plankton) • More produced than destroyed by scavengers and decay (high productivity areas with low oxygen) • Sedimentary environments: Deltas, reefs, deeper basin fills, etc. • Preservation of organic matter in sediments • With modification by biological activity near the surface • Maturation during burial and diagenesis (heat, pressure, time) • Oil and natural gas form, generally between 75-150˚C • Trapped organic material breaks down • Analogy: During cooking of a burger (of decayed plankton), the lighter, more hydrogen-rich, components given off are oil and gas • Leaving behind the less reactive, C-rich material, which becomes closer to graphite, in the source rock (e.g., black shale), • Fluids generated during diagenesis (water, oils, gases) migrate hydraulically • To be trapped in a “reservoir” or to be destroyed

41. Hydrology and global distribution of oil and natural gas • Source  Migration  Trap • Types of traps • Structural (anticlines, faults) • Stratigraphic • Salt domes • Reservoir — What are the major ones (rock types)? • Porosity (aquifers) • Seals (aquacludes) • Global distribution in space and time • Reflects climate, geography, preservation… (i.e., geology!)

42. Traps — Structural • Anticlines, faults, etc. • Why in the sequence gas / oil / water? • Oil reservoirs (aquifers): e.g., permeable sandstones, carbonate rocks • Seals (aquacludes): e.g., shales

43. Traps — Stratigraphic and salt domes • Stratigraphic pinch outs in transgressive sequences • Channel and sand bar deposits • Salt domes • Many geological challenges in exploration and production

44. World oil reserves • Mainly in rocks <200 m.y. old (why?) • Localized in space by tectonics and paleoclimate (why?)

45. Global oil production(past-present-future) • Demand accelerates • Geologic limits • US example

46. Can't we just discover/produce more? • Yes, but the opportunities (prospective basins) are limited and many (most) have been explored

47. Basins have been thoroughly tested • All of these downwarps produce significant oil and/or gas • Each has been explored to the crystalline basement • Historical context?

48. Can't we just discover/produce more? • Then what about getting more out of existing fields? • Production is lengthy and technology continually improves • Commonly less than half the oil present is produced • This helps, but at best may double/triple supply • The best technology is widely used

49. Alternative hydrocarbonsOil (tar) sands and oil shales • Very large resource • Comparable to all petroleum (1-3 trillion bbl) • Major resources in Alberta, Venezuela, Colorado-Utah • Produced by mining, not pumping • Higher cost and energy-intensive to produce • Bigger impact on emissions than oil (similar to coal) • Large production from Athabasca oil sands in Alberta • Each particle coated with a layer of water and a thin film of bitumen (“tar”) • Supplying much energy to US

50. Formation of coal:A comparatively simple process • Woody material (terrestrial plants) • Accumulates in estuaries, swamps, and bogs • Burial by continued sedimentation • Leads to diagenetic and metamorphic changes • These increase “rank” (or “grade”) of the coal • Residual part of the coal is preserved • Analogy: Coal is the cooked burger (made from decayed plants), after loosing the oils (grease) and vapors • Composition of coal • Much higher in C but lower in H than oil and gas