Building a con,nent; plate tectonics. GEOL115 Alexander Lusk

Transcription

1 Building a con,nent; plate tectonics GEOL115 Alexander Lusk

2 Lecture goals: 1. Earth structure (con,nued) 2. Plate boundaries Divergent plate boundaries Convergent plate boundaries Transform boundaries 3. Evidence for plate tectonics Con,nental drik Distribu,on of earthquakes and volcanoes Rigidity of the plates Sea-floor magne,c anomalies Current plate mo,ons GPS measurements of crustal mo,ons 4. Plate tectonics of western North America

3 Recall Earth s internal structure: Crust Mantle Liquid iron outer core Radius of Earth: 6370 km Solid iron inner core Radius of core: 3480 km Thickness of mantle: 2890 km Thickness of crust: 6-70 km The crust (and lithosphere) is extremely thin compared to the rest of the Earth

4 Layering based on mechanical strength Lithosphere Asthenosphere Mesosphere (lower mantle) Inner and outer core Lithosphere High strength outer, rocky layer; km depth Asthenosphere Weak layer; km Mesosphere Lower mantle; km Outer core Liquid core; km Inner core Solid core; km

5 Flavors of earth s crust The crust is a composi,onal layer lying above the mantle separated by the Mohorovic Discon6nuity (MOHO) Oceanic crust is thin and dense (richer in Fe and Mg) Con6nental crust is thick, and has lower density (richer in SiO2) Both oceanic and con,nental crustal rocks are less dense than mantle rocks, and thus float

6 Crust, mantle, lithosphere, and asthenosphere The crust is the upper part of the lithosphere Mantle The Moho lies within the lithosphere The cold, uppermost mantle is also part of the lithosphere

7 Distribu,on and movement of plates Plates are pieces of Earth s lithosphere that fit together like a jigsaw puzzle, covering the en,re earth. Take note: some plates are only oceanic crust, some are oceanic and con,nental, but none are only con,nental

8 Recall: Hea,ng of the earth Primary sources of heat: Accre6onary (primordial or residual) heat. Kine,c energy Gravita,onal compression Heat from the decay of radioac6ve elements present within the earth. Uranium Thorium Potassium Addi,onal heat sources: Latent heat of crystalliza,on Tidal forces

9 Heat transport through the earth Rocks are poor conductors of heat Convec'on is a more efficient process for removing heat Flow in the earth s mantle allows convec,on, even though the mantle is solid! Mantle convec'on is the earth s heat engine, driving plate tectonics On the stove In the earth

10 Geothermal gradient Temperature Lithosphere Cool, high geothermal gradient Depth Asthenosphere warm, slightly lower geothermal gradient Earth used to be much warmer, and as a result, mantle convec,on was more rapid and probably plate tectonics as well.

11 Principles of Plate Tectonics Earth s surface is broken into a mosaic of moving lithospheric plates Plates are generally rigid except near their boundaries, where rela,ve mo,ons between plates occur Total plate area is conserved (i.e. Earth is not expanding) Plates can be created at divergent boundaries and destroyed at convergent boundaries

12 Plate boundaries Direc,on of rela,ve plate mo,on dictate the type of plate boundary: Plates move apart (divergence) Plates move towards each other (convergence) Plate slide past one another (transform mo,on)

13 Divergent boundary Mid-ocean ridges and rises: Mid-Atlan,c Ridge, East Pacific Rise

14 Divergent boundary Baja California: Con,nental RiKing

15 Transform and strike-slip boundaries On the ocean floor, connec,ng spreading centers: On the con,nents: E.g. San Andreas fault

16 San Andreas Fault The SAF cuts through the Carrizo Plain and Temblor Mountains, causing an offset in Wallace Creek. Right-lateral or lek-lateral?

17 Ocean floor topography Ocean ridges, rises, transform faults, and fracture zones

18 Convergent boundary Oceanic-oceanic forms an island arc. Examples: Mariana Islands, Japan

19 Convergent boundary Oceanic-con,nental denser and thinner oceanic plate subducts underneath the less dense con,nental plate and forms a con'nental arc. Examples: Andes, Cascades, Sierra Nevada are an ex5nct arc

20 Convergent boundary Con,nental-con,nental Neither plate wants to subduct, generally resul,ng in massive mountain ranges. Examples: Himalaya, Appalachians/Caledonides

21 Lava lake tectonics hops://

22 Evidence for plate tectonics: Con,nental drik - large-scale mo,ons of the con,nents. Distribu,on of earthquakes and volcanoes Rigidity of the plates Magne,c anomaly symmetry on the sea floor Present-day surface plate mo,ons (GPS)

23 Alfred Wegener: 1912 Hypothesis of Con,nental DriK Con,nental drik

24 Con,nental DriK The hypothesis was based on: the fit of con,nents, specifically around the Atlan,c geologic match-ups between the con,nents Similar fossils on now widely separate con,nents Similar climates in the past on con,nents that are now at very different la,tudes

25 Fossil evidence for con,nental drik Early Triassic ( Ma 220) Permian (250Ma) Permian-Pennsylvanian ( Ma 270)

26 Evidence from past climates Glacial features of the same age restore to a welldefined polar area.

27 Plate Tectonics and Con,nental DriK The original hypothesis of con6nental dria did not include a mechanism. In contrast, the theory of plate tectonics provides an explana,on for con,nental drik, as well as explaining how oceans form and are destroyed.

28 Plate mo,ons over the past 250Ma

29 In the future: Plate mo,ons

30 Evidence for plate tectonics: Con,nental drik - large-scale mo,ons of the con,nents. Distribu,on of earthquakes and volcanoes Rigidity of the plates Magne,c anomaly symmetry on the sea floor Present-day surface plate mo,ons (GPS)

31 Distribu,on of earthquakes and volcanoes Distribu,on of volcanoes and earthquakes around the Pacific Ocean. Called the Pacific Ring of Fire outlines the plate boundaries.

32 Global earthquake distribu,on outlines plate boundaries

33 Earthquake depth in subduc,on zone

34 Evidence for plate tectonics: Con,nental drik - large-scale mo,ons of the con,nents. Distribu,on of earthquakes and volcanoes Rigidity of the plates Magne,c anomaly symmetry on the sea floor Present-day surface plate mo,ons (GPS)

35 The good fit of the con,nents shows that they did not change shape significantly as a result of drik.

36 Evidence for plate tectonics: Con,nental drik - large-scale mo,ons of the con,nents. Distribu,on of earthquakes and volcanoes Rigidity of the plates Magne,c anomaly symmetry on the sea floor Present-day surface plate mo,ons (GPS)

37 Earth s magne,c field At the surface, Earth s field resembles that produced by a bar magnet. Earth s magne,c north does not coincide exactly with the North Pole. The magne,c poles move around on the Earth s surface. The magne,c field reverses polarity at irregular intervals - magne5c reversals Magnetic field lines

38 Marine magne,c anomalies

39 Magne,c anomaly Isochrons Magne,c anomalies on the mid- Atlan,c ridge south of Iceland Magne,c anomalies on the ocean floor off Oregon and Washington These observa,ons in the 1960 s led to the theory of plate tectonics

40 Age of seafloor Where is the oldest seafloow? Where is seafloor spreading the fastest?

41 Evidence for plate tectonics: Con,nental drik - large-scale mo,ons of the con,nents. Distribu,on of earthquakes and volcanoes Rigidity of the plates Magne,c anomaly symmetry on the sea floor Present-day surface plate mo,ons (GPS)

42 Global Posi6oning System (GPS)

43 Global Posi,oning System The global posi,oning system uses signals from a system of satellites to allow receivers to determine posi,on accurately

44 How GPS works: Satellites are constantly transmimng data: 1) 5me the message was transmi8ed and 2) satellite posi5on at 5me of message transmission. Based on this data, the GPS receiver can calculate its posi,on.

45 The Southern California GPS network

46 Pacific - North America plate mo,on

47 Plate tectonic driving forces

48 Hot spots Sta,onary, narrow streams of hot mantle material Can be used to determine past plate veloci,es and direc,ons Examples: Hawaiian/Emperor Seamount chain, Yellowstone/Snake River Plain

49 Yellowstone NP/Snake River Plain

50 END LECTURE