Lesson #4

1. Lesson #4 How did the Oceans get to be the way they are?

2. Origin of the oceans • We weren’t there when the earth was formed so this is all theory • Theory: guess, test, evaluate, repeat • Fact: observable • Origin theories are pretty good • Fit the observations • Predict what you’ll find in new places • But expect them to change as new facts are observed http://www.astro.psu.edu/~bmiller/astro11/pics/earth_from_space.jpg

3. Faith vs. Science • Different sets of rules: • Science: must be testable, subject to the “scientific method” • Question: theory, hypothesis • Test • Evaluate • Repeat, repeat, repeat… • Faith: must be un-testable • Problems happen when you mix these up www.britanica.com

4. The Origin of the Earth • This is the best theory we have • works very well • but expect it to change as we learn more • The universe is very large • nearest star is 27 million, billion miles away • 4.5 light years (light travels at 186,000 miles / second) • How do we measure distance in space? • triangulation • light intensity

5. A Light-Year is a Measure of Distance A light-year is equal to The distance light travels in one year exactly 9,460,730,472,580.8 kilometers about 5,878,630,000,000 miles Based on a year of 365.25 days; 86,400 seconds a day; 31,557,600 seconds a year A speed of light of 299,792,458 meters per second 670,616,629 miles per hour

6. The Origin of Earth • The universe was formed roughly 15 billion years ago • The solar system formed roughly 5 billion years ago • formed from a nebula, which is a revolving mass of particles • some collected into stars (such as our sun) • others collected into planets (such as the earth) which continued to revolve around it

7. The Origin of the Earth • The planets formed as the particles condensed • conditions on each planet are determined by: • distance from the sun • amount of mass

8. Earth’s Structure • Like other planets, the earth was formed through this process • As it cooled, the heavier elements (iron, nickel) settled to the bottom (center) and the lighter elements (aluminum, silicon) floated to the top • this resulted in the creation of concentric shells • Each with a distinct chemical composition • Layered on the basis of density

9. The Origin of the Oceans • As the earth continued to cool, water and gases were given off • these accumulated in the oceans and atmosphere respectively • originally all of the water was in gaseous state due to the hot earth • this processes took a long time and continues today • The composition of the original ocean and atmosphere was determined by this process

10. The Origin of the Oceans • Important Events in the Earth’s History • Each event paved the way for the next • Man is a relative newcomer

11. Earth’s Structure (Cont.) • The earth’s interior is made up of several layers. • The physical characteristics of each layer are determined by: • The chemical composition of the rocks • Temperature • Pressure • These layers are can be defined two ways: • By acoustic velocity • Crust, Mantle, Core • By physical characteristics (rigidity, viscosity) • lithosphere, asthenosphere, mesosphere

12. Earth’s Structure (Cont.)(according to velocity) • The crust is the outermost layer. • Relatively thin • Low density material: aluminum, silicon, oxygen

13. Geologic Differences Between Continents and Ocean Basins (Cont.) • Continental crust consists of granite • Igneous origin, solidified from molten material. • Light in color. • Mostly oxygen, silicon, aluminum. • Oceanic crust consists of basalt • Volcanic origin, solidified from material erupted onto Earth’s surface. • Dark in color. • Rich in oxygen, silicon, magnesium and iron.

14. Geologic Differences Between Continents and Ocean Basins (Cont.) • The density of granite is • 2.7 to 2.8 g/cm3 • The density of basalt is • 2.9 g/cm3 Granite Basalt

15. Oceanic Crust vs. Continental Crust • Continental crust is thicker than oceanic crust • Also more variable • Much thicker under mountains

16. Earth’s Structure (Cont.)(according to velocity) • The mantle lies below the crust. • Relatively thicker • Higher density material: magnesium, iron, silicon, oxygen

17. Earth’s Structure (Cont.)(according to velocity) • The core is below the mantle. • Thickest of all layers • Very dense material: iron, nickel • Outer: molten • Inner core: solid

18. Earth’s Structure (Cont.)(according to velocity) • These three layers are determined by the speed at which sound travels through them • Crust • Mantle • Core • But for plate tectonics, we need to consider their physical properties

19. Earth’s Structure According to Physical properties • Lithosphere • Upper layer • rigid • Asthenosphere • Just below the lithosphere • “plastic” • Mesosphere

20. Earth’s Structure According to Physical properties • The lithosphereconsists of the crust and uppermost mantle. • Hard, • Rigid, • Brittle Molten

21. Earth’s Structure According to Physical properties • The asthenosphere is below the lithosphere. • A portion of the mantle • Rocks are • semi-melted. • Behavior is plastic.

22. Earth’s Structure According to Physical properties • The mesosphere consists of the lower mantle. • Higher pressure causes mantle rocks to change structure. • More solid than the asthenosphere. • No plastic flow, but still some flow.

23. Earth’s Structure According to Physical properties • We know all of this through indirect evidence: • mass of earth • transmission of sound • samples • NO direct samples • too deep • too hot • too much pressure

24. Lithosphere • The crust “floats” on the mantle beneath it • Like an iceberg, there is more below the surface than above

25. This demo shows the effect of height and density on how an object floats This applies to icebergs and to the crust Isostasy Demo Isostasy

26. Isostasy explains the distribution of Earth’s topography • Continental crust is lighter than ocean crust, so • It floats higher than oceanic crust • Continental crust is thicker than ocean crust, so • It stands higher than ocean crust. • It has deeper roots than ocean crust.

27. Plate Tectonics • The plate Tectonic theory revolutionized geology. • It explains: • earthquakes • distribution of volcanoes • character of the seafloor • age of the seafloor • shape of the ocean basins • just about everything http://nzphoto.tripod.com/volcano/PacificPlate.gif

28. Plate Tectonics • Plate tectonics theory is based on two types of data • 1) Continental Drift • Data about the continents, their shape, and what’s on them • All based on observations above sea level • 2) Seafloor spreading • Data about the sea floor, its shape, and what’s on it • All based on observations below sea level

29. Continental Drift • The theory was proposed in 1915 by Alfred Wegener (a meteorologist) to explain Earth’s history. • The theory states that a single continent, Pangaea, existed more than 200 million years ago. • It broke up and the continents drifted apart

30. Evidence for Continental drift • There are several lines of evidence for continental drift, including: • (1) “map fit” • when you reposition the continents on a globe the all fit together • the fit is stunning!

31. (2) Glacial scarring Glaciers gouge the rocks Regions of glacial scour “match” on opposing continents Must have happened before breakup of original continent Continental drift (cont.) Glacial scarring Glacial striations Glacial pot holes Glacial striations

32. (3) Geologic evidence (rock type). Age of rock. Mineralogy. Structure. Continental drift (cont.)

33. (4) Fossils Identical fossils found on continents which are now separate Must have been connected at some point in the distant past Can determine age of separation by dating fossils Continental drift (cont.)

34. Continental Drift (cont.) • But Wegener’s theory lacked a mechanism • What made the continents move? • Most scientists scoffed at the theory • It was not accepted for more than 50 years! http://www.ucmp.berkeley.edu/history/wegener.gif

35. Breakup of Pangaea Drift 1 Drift 2

36. Sea-Floor Spreading • Sea-floor spreading explains how ocean basins form • But it also provides the mechanism that Wegener was missing • It is based upon various pieces of scientific evidence • Deep sea soundings showing the existence and structure of midocean ridgesystems • Magnetometer readings of ridge systems • Pattern of earthquakes • Distribution of volcanoes • Chemistry of the rocks • Lots of other evidence

37. Sea-Floor Spreading (Cont.) • The midocean ridge was discovered in the mid-20th century from soundings of the deep sea-floor. • It forms a continuous mountain range girdling the earth • ~60,000 km long • ~3 to 4 km high • Geologically active • Volcanism, • Earthquakes • Faulting

38. Force Force Force Force Sea-Floor Spreading • Although ridge systems resemble mountain chains, they are formed in different ways. • Mountain chains form by compression • Ridge systems form by tension

39. Sea-Floor Spreading • the crust is pulling apart at the ridge crest (tension) • a valley forms where this happens

40. Sea-Floor Spreading (Cont.) • When a ridge comes on land, it rifts (tears) apart the continent! Mediterranean Sea Rifts Sinai Red Sea

41. Sea-Floor Spreading • The ridge is visible where it goes ashore as “mirrored” mountains. • It looks just like this under water Force Force Normal faults Scarp

42. Sea-Floor Spreading(Cont.) • As the sea floor spreads. • It gets older (doesn’t everything?). • It cools and thickens. • It accumulates sediment.

43. Global Plate Tectonics • Plate Tectonics = Sea Floor Spreading + Continental drift • These two concepts came from separate sets of observations • The Plate Tectonic theory: • Combined these ideas. • Answered almost all of our questions. • Provided incredible predictive POWER! http://www.enchantedlearning.com/egifs/Earthsplates.GIF

44. Global Plate Tectonics (Cont.) • Surface divided into 12 major plates.

45. Global Plate Tectonics (Cont.) • These plates move! • As they do, they collide with each other • Some portions are destroyed by subduction • Some are crushed into mountains • The collisions create earthquakes http://images.google.com/imgres?imgurl=http://vulcan.wr.usgs.gov/Imgs/Gif/PlateTectonics/Maps/map_plate_tectonics_world_bw.gif&imgrefurl=http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/map_plate_tectonics_world.html&h=540&w=859&sz=23&hl=en&start=1&tbnid=15vAfuTbbcud9M:&tbnh=91&tbnw=145&prev=/images%3Fq%3Dplate%2Btectonics%26hl%3Den

46. Plastic flow Limited motion Mesosphere Global Plate Tectonics(Cont.) • Plates are slabs of lithosphere • Lithosphere = rigid • Crust and uppermost mantle • “floats” on the asthosphere (remember isostacy?) • Asthenosphere = plastic, flows

47. Nearly all seismic and volcanic activity is associated with plate boundaries. Plate boundaries and seismic activity: Co-occur. Exist on continuous lines. Global Plate Tectonics(Cont.)

48. Global Plate Tectonics (Cont.) • Three flavors of plate boundaries: • Midocean ridges • Plates move apart • “spreading centers” • Addition of new crust to trailing plate edge • Subduction zones • Plates collide • Destruction of old crust • Transform faults • Plates move past one another • Crust is neither created or destroyed Let’s take a look at these in some detail:

50. Volcanism and faulting Hydrothermal circulation 1) Midocean Ridges Basalt pillows, fresh from an eruption in the rift valley. Copper Zinc Lead etc. Rift valley of a midocean ridge. Normal faults Fault at axial fissure caused by extensional force in the rift valley. Hydrothermal vent