Terrestrial Planetary Geology: Basic Processes & Earth

1. Terrestrial Planetary Geology:Basic Processes & Earth

2. What are terrestrial planets like on the inside? Planets (inc. Moon) to scale, with typical surface features

3. Earth’s Interior • Core: Highest density; nickel and iron • Mantle: Moderate density; silicon, oxygen, etc. • Crust: Lowest density; granite, basalt, etc. • Other terrestrial planets have similar layers

4. Terrestrial Planet Interiors • Applying what we have learned about Earth’s interior to other planets tells us what their interiors are probably like

5. Differentiation • Gravity pulls high-density material to center • Lower-density material rises to surface • Material ends up separated by density • This generates heat inside planet

6. Lithosphere • A planet’s outer layer of cool, rigid rock is called the lithosphere: crust + outer mantle • It “floats” on the warmer, softer rock that lies beneath: most of the mantle is “plastic”-- the rock slowly deforms

7. Strength of Rock • Rock stretches when pulled slowly but breaks when pulled rapidly--like Silly Putty but MUCH slower • The gravity of a large world pulls slowly on its rocky content, shaping the world into a sphere if bigger than about 300 km in diameter • Rapid shear, like an earthquake or impact breaks rock

8. Seismic Waves Let UsKnow What’s Inside a Planet • P (primary) waves push matter back and forth (longitudinal like sound waves) • S (secondary) waves shake matter side to side (transverse, like water waves)

9. SEISMIC WAVES REVEAL INTERIOR SEISMOGRAPHS detect EARTHQUAKES, VOLCANOS, and EXPLOSIONS at varied distances. • Long surface (L) waves travel fastest, but are not very useful as they don't probe the interior. • P-waves, PRIMARY, (push-pull waves) are COMPRESSIONAL, LONGITUDINAL waves. Propagate through liquids as well as solids. VP = function of (compressibility; composition, T, P) • S-waves, SECONDARY, (side-slip) are SHEAR, TRANSVERSE waves. CANNOT propagate through liquids (OUTER core). VS = (a new) function of (rigidity; composition, T, P) • We have some seismic measurements for the Moon too: seismometers left by Apollo astronauts

10. Seismic Wave Paths Both S and P waves can be detected from earthquakes on the same side of the earth, • ONLY P waves are detected on the opposite side of the earth “shadow zone”: region where no S (absorbed) & only weak P (refracted) waves are detected. Tells us the outer core of the earth is LIQUID

11. Seismographic measurements have found for Earth • Boundaries between: inner/outer core; outer core/mantle; composition changes in mantle (600 km); oil and natural gas deposits in crust. • Seismic measurements can even detect excess rotation of earth’s inner core, upwellings and sideways motions in the mantle. • MODERN SEISMOLOGY CAN GIVE A 3-D PICTURE, CHANGING IN TIME, OF THE EARTH'S INTERIOR! --- Tomography • There is a good analogy with 3-D images of people's interiors from MRI or CAT scans.

12. Thought Question What is necessary for differentiation to occur in a planet? a) It must have metal androck in it b) It must be a mix of materials of different density c) Material inside must be able to flow d) All of the above e) b and c

13. Thought Question What is necessary for differentiation to occur in a planet? a) It must have metal androck in it b) It must be a mix of materials of different density c) Material inside must be able to flow d) All of the above e) b and c

14. Causes of Geologic Activity • Heating of Interior • Accretion and differentiation when planets were young • Radioactive decay is most important heat source today

15. Cooling of Interior • Convection transports heat as hot material rises and cool material falls (outer core; inner mantle) • Conduction transfers heat from hot material to cool material (lithosphere) • Radiation sends energy into space (surface & atmosphere)

16. Role of Size is Dominant • Smaller worlds cool off faster and harden earlier • Moon and Mercury are now geologically “dead” • Mars lasted longer; Venus may still be active • Earth is VERY active

17. Surface Area to Volume Ratio Gives Cooling Time • Heat content depends on volume • Loss of heat through radiation depends on surface area • Time to cool depends on surface area divided by volume • Larger objects have smaller ratio and cool more slowly

18. Why do some planetary interiors create magnetic fields? Iron filings follow the magnetic field lines of a bar magnet

19. Sources of Magnetic Fields • Motions of charged particles create magnetic fields • Electromagnets via currents in coil of wire (usually amplified by magnetic material, like iron) • Permanent magnets: electron spins act as currents in iron or nickel

20. Sources of Magnetic Fields in Planets • A world can have a magnetic field if charged particles are moving inside • 3 requirements: • Molten interior • Convection • Moderately rapid rotation Earth has molten iron outer core Outer planets have metallic hydrogen Stars have ionized H

21. What processes shape planetary surfaces? Their surfaces are amazingly different, yet same forces act!

22. Key Processes that Shape Surfaces • Impact cratering • Impacts by asteroids or comets • Volcanism • Eruption of molten rock onto surface • Tectonics • Disruption of a planet’s surface by internal stresses • Erosion • Surface changes made by wind, water, or ice

23. Impact Cratering • Most cratering happened soon after solar system formed: the heavy bombardment era • Craters are about 10 times wider than object that made them • Small craters greatly outnumber large ones • Areas with many craters are old; those with few were “repaved”

24. Impact Craters: Classical Tycho (Moon) Barringer Meteor Crater (Arizona) Rim at edge of shock; rebound makes peak in center

25. Impact Craters on Mars: Evidence for Liquid Water in the Past “standard” crater impact into icy ground eroded crater Cratering History Movie

26. Volcanism • Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface • Molten rock is called lava after it reaches the surface • It solidifies to create volcanoes

27. Lava Properties & Volcano Types Runny lava makes flat lava plains Slightly thicker lava makes broad shield volcanoes Thickest lava makes steep stratovolcanoes

28. Outgassing • Volcanism also releases gases from Earth’s interior into atmosphere: in the past for other terrestrial planets • We’ll talk more about this later

29. Tectonics • Convection of the mantle creates stresses in the crust called tectonic forces • Compression forces make mountain ranges (Appalachian Mts on Earth on left) • Valley can form where crust is pulled apart (Ceraunius Valleys on Mars on right)

30. Plate Tectonics on Earth • Earth’s continents slide around on separate plates of crust • Plate Tectonics Applet • Tectonics: Mantle Convection

31. Erosion • Erosion is a general term for weather-driven processes that break down or transport rock • Processes that cause erosion include • Glaciers • Rivers • Wind (which demands an atmosphere!)

32. Erosion by Water • Colorado River continues to carve Grand Canyon • Land has been uplifted, but river manages to wear it down

33. Erosion by Ice • Glaciers carved the Yosemite Valley • They covered most of the Northern US and Europe in recent ice ages

34. Erosion by Wind • Wind wears away rock and builds up sand dunes • Frequent on Earth,also seen on Mars

35. Erosional Debris • Erosion can create new features by depositing debris • Again, frequent on Earth,but remnants seen on Mars too

36. The Earth as a Planet You gotta love it, it's our pretty home! You gotta know something about it too! (At least to pass this class.)

37. The Earth is (nearly) a Sphere and it Rotates • Proofs (other than Greek): Masts of ships visible before their decks. Cicumnavigation in 1522! Distances corresponding to degree of latitude change; bulges near equator. • Newton realized that a rotating object has extra support perpendicular to its axis: • Re = 6.378136 x 106 m • Rp = 6.356753 x 106 m • ELLIPTICITY = (Re - Rp )/Re = 1/298.3 = 0.0033528. If the earth were “mushier”, the same rotation speed would yield a higher ellipticity.

38. More Proofs of Earth’s Rotation • FOUCAULT PENDULUM --- fixed plane of oscillation, with the Earth rotating underneath it. • Satellites in POLAR ORBIT see different sections every 90 minutes as the Earth rotates under the satellite's FIXED path.

39. Tides are due to the DIFFERENTIAL force of GRAVITY. M and D are the mass and diameter of object on which tides are raised; m the mass of the object raising tides and r the distance between their centers. TWO HIGH TIDES a day because of Earth rotating through BOTH the FRONT and BACK bulges.

40. Lunar Tides Raised on Earth

41. Lunar + Solar Tides • SPRING (strong) TIDES: new moon and full moon; the Sun's weaker tide and the Moon's stronger one add up • NEAP (weak) TIDES: first and third quarter moon the Sun's weaker tide partially cancels the Moon's stronger one

42. Tides Affect the Earth’s Rotation Energy is dissipated by oceans rubbing against crust: the Earth's spin slows down (but only 1.5 ms/century!) About 500 Myr ago, the day was 22 hours long and the year had 397 days!

43. Conservation of Angular Momentum • as the Earth's rotation slows, decreasing its angular momentum (AM), the Moon's orbital AM increases--the Moon moves slightly further away! • Eventually no more total solar eclipses!

44. Earth’s Differentiated Interior

45. CRUST • Mostly made of ROCKS. • The main rock classes are: Igneous (of volcanic origin); Sedimentary (compressed @ ocean, lake bottoms); Metamorphic (either igneous or sedimentary placed under so much pressure that their crystalline structure changes) • Composition: mostly SiO2 (quartz-like), Al2O3 (granite-like) and other elements combined w/ Silicon, Aluminum and Oxygen • Oceanic Crust: Covers 3/5 of earth, averages 7 km thick,  = 3.0 g cm-3 ; all < 200,000,000 yr old • Continental Crust: Ave. 36 km thick;  = 2.8 g cm-3 some very old, up to 4 Gyr;

46. MANTLE • ‘Plastic’ rocks --solid but will yield; mostly Fe, Mg, Si, O (basalt-like) About 2900 km thick, contains most of earth's mass;  = 5 g cm-3 ; T = 1800 K Heated mainly from below, convection currents are present. • RECALL MODES OF HEAT TRANSPORT:Radiation: microscopic (photon) mode works best in vacuum, well in low density gases Conduction: microscopic (molecules bumping) mode works best in solids, then liquids, then gases Convection: macroscopic (large-scale) motions of blobs of matter: works in liquids, gases and (very slowly) in plastics

47. Interior Density and Temperature

48. CORE Metallic; mostly Fe (iron), Ni (nickel), (little S/O) In toto, about 3500 km radius or 1/6 of Earth's volume. OUTER CORE: liquid, about 2200 km thick;  = 11 g cm-3 ; T = 4000 K ; Temperature “wins” INNER CORE: solid, about 1300 km radius; c = 13 g cm-3 ; Tc = 5200 K; Pressure “wins” over temp.

49. DIFFERENTIATED via MELTING • Separation into metallic core and rocky mantle could be explained either if: • Core formed first, mantle and crust added later, OR • Entire Earth formed together but was molten early on; the denser (metallic) liquid sinks to the center. • This 2nd hypothesis fits the evidence better. COLLISIONAL HEATING (formation) plus RADIOACTIVE HEATING (fission) plus continued FRICTIONAL HEASTING (differentiation) can nicely explain current temperatures. Mostly RADIOACTIVE HEAT now.

50. Idea Test Recall the formula for tidal forces: Now, imagine the Earth had a second Moon, called Nemoon at twice the distance of the real Moon. Also imagine that the mass of Nemoon is 4 times that of the Moon. You would then conclude that the tides due to Nemoon would be about _____ times as high as our actual tides. A. 1/8 • 1/2 • 1 D. 2 E. 4