2000 January 27, SPS 1020 (Introduction to Space Sciences)

- Read TNSS Ch. 10 (The Earth's Moon) by Tuesday, February 1.

- Read TNSS Ch. 11 (Mars) by Thursday, February 3.

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Planet Earth (TNSS, Chapter 9):
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Recall the 4 primary geological processes:
- Impact cratering
- Tectonism
- Volcanism
- Gradation (incl. water and wind [or aeolian] erosion, but also
                micrometeorite erosion)

Earth is highly atypical, among planets, in that impact craters are so rare.
	=> Most geologically active of all terrestrial planets 
	  (not counting Io, which is a moon, anyway.)


Interior: probably less well understood than the interior of the Sun, and
most stars.  (We have only one Earth to study: can`t do comparisons.)

Internal heat comes from residual heat of formation and radioactive decay
of U, Th, Ru, K-40, other elements


Completely melted very early => differentiated, 
                                when heavier material sank to center

Only planet with detailed 3-D maps of interior (seismic tomography)

 	 density     thickness  state		composition
	 (g/cm^3) 	(km)			(primarily)
Average:   5.52		6438

Crust:       2.7	  60 	solid		granite (continental)
			  12			basalt (oceanic)
Mantle	     5		3000	plastic solid	olivine
Outer core   7		2400	liquid		metal
Inner core  12.5	1000	solid		metal (Fe)

How do we know? Seismology: P and S waves:

P = pressure (push: longitudinal)
S = shear (shake: transverse)











Other ways to observe Earth's interior: 
	- Gravity (to spot mass concentrations, or mascons)
	- Magnetometry (more later, when we cover space physics)
	- Heat flow measurements


Surface of Earth: 

Hydrosphere:  70% of Earth's surface covered by water; avg depth ~ 3.6 km;
only 2% fresh water.
	
Ocean floor, under sediment, is mainly basalts, most < 90 million years
old.  Nowhere is ocean floor > 200 million years old => Actively
resurfaced.

Oldest rocks on continents: 3.8 billion years (from solidification of
crust)

Isostasy ("roots of mountains"): crust thickest under mountains on
continents (since weight, pressure highest)


























Rocks: most are silicates (Si, O, & metals). General types:

_Igneous_ (formed by volcanism, solidify directly from lava): pumice,
granite, quartz, basalt (basalts, fine-grained metal-bearing silicates;
granites, coarse-grained)
	
_Sedimentary_ (formed by sedimentation, in water): limestone, clay, shale,
sandstone

_Metamorphic_ (igneous or sedimentary altered chemically by pressure and
heat in Earth's interior, then brought back to surface) e.g. slate, chalk,
marble


Gradation: very important in shaping most landforms.  Water erosion is the
dominant form of gradation on Earth, although some wind erosion occurs
(sandstone arches in Utah; Egyptian desert).


Tectonism: Earth's surface is divided into 12 - 16 plates that move about
over millions of years, riding on the convective currents of the mantle.  
Unifying principle of geology (like evolution for biology, Newtonian laws
of motion, heliocentric Solar System). Surprisingly, only became accepted
recently (proposed by Alfred Wegner in 1929; still hotly debated in
1960s).  Earth is only terrestrial planet with active plate tectonics,
oceans, and abundant life on its surface: interrelated?

=> Continental drift, seafloor spreading, mid-oceanic ridges, subduction
(one plate moving under another). See pp. 122 of TNSS















Evidence for continental drift:
        
- Coincidence of geographic features (Americas and Africa)

- Similar fossil species on separated continents

- Direct observation!  Can measure motion with satellite laser ranging
(cm/year)
        
Dominant form of mountain building on Earth: folding mountains, from
tectonism (Appalachians, Rockies, Andes near subduction zone, Mid-Oceanic
ridge in Atlantic, Himalayas where India crashed into Asia)

Faults, e.g. San Andreas fault in California =>

sudden sliding of plates along cracks (faults) due to pressure buildup

=> Earthquakes.  Richter scale: logarithmic scale, highest known 8.9; Kobe
6.8, Northridge 6.7.


Volcanism: heat venting through Earth's crust, in volcanic plumes, or
rising columns of hot mantle that break through crust. Volcanoes occur
mainly around rim of Pacific Ocean, the "Ring of Fire", where plates meet
and subduction often occurs.  Most volcanoes do not continuously erupt,
but are usually dormant; many, e.g. Mauna Kea, are extinct.  Volcanic
eruptions are less common than earthquakes, but can nonetheless be
devastating (Pompei).

Lava: melted rock, that comes out of volcanoes, along with ash and gas
Magma: melted rock, still beneath Earth's surface

3 types of  volcanoes:

1) Cinder cones: from thick, viscous lava, often erupt explosively
Cascades: whole mountain range of volcanoes, in Oregon/Washington state.

2) Shield volcanoes: from inviscid (runny) lava, so they slope gently:
Hawaii, La Palma

3) Composite volcanoes: intermediate cases, e.g. Mount St. Helens
(exploded 1980), Fujiyama, Krakatoa (exploded 1883)

Lava plateaus: lava can also ooze gradually out of fissures in crust, e.g.
lava fields in Idaho ("Craters of the Moon" National Park, a misnomer
since these are not impact craters), Siberian traps (size of Texas), ocean
floor (under sediments)


2 types of volcanic flows (both Hawaiian words, now used worldwide):

Pa-hoe-hoe (pronounced pa-hoy-hoy): goopy, obviously once liquid

A'a (pronounced like it's spelled): makes sharp, often impassable rocks.