Earth as a Planet
February 24, 2005: Chapter 7, Pages 152–173
Tuesday: Chapter 8, Pages 174–186
When we look at the Earth from space or the Sun, planets, other Solar System objects, and stars from
Earth, we see the history of the Universe as a snapshot in time. By studying the cycles and patterns
around us, both short and long, we can begin to understand the nature of our World and our Universe.
How has the Earth and the Solar System evolved? How did they form and how did they get to what we
see today?
EARTH’S CYCLES AGE, INTERIOR, PLATE TECTONICS
I. What makes the surface of the Earth look like it does? What makes mountain ranges, the Grand Canyon?
A. By 1800, the concept of “uniformitarianism”  shaped by the processes acting today, just over long
periods of time, was being developed.
B. Cutting Grand Canyon through erosion, but wouldn't that take a long time?
C. Mountains that have fossil seashells on top? What process going on today?
II. How old is the Earth? Consider what was thought at the turn of the century.
A. Short (relatively young): Bible; cooling rate of the Sun (burning fuel, coal).
B. Long: salt in the ocean (current rates, 300 My); sedimentation rates; time to cut Grand Canyon;
Darwinian evolution requires enormous amounts of time.
C. How can you find out? (need process with constant rates).
1. Radioactivity; nuclear physics: 14C, 40K; constant decay.
2. If know how much decay product have, and how much “parent” you started with, know how
long it took. Ages of rocks: Earth04By. Moon3.84.5By. Meteoritesmostly 4.56By.
III. Now we know how old the Earth is, but what about mountains?
A. Idea of continental drift (plate tectonics).
B. Problem  why do continents move (chunks of rock)? Need source of power.
C. Confirmed: Mid-Atlantic Ridge; stripes in paleomagnetism; motion measured by radio telescopes,
satellite laser-ranging and radio signals (cm per year).
D. Implications and theory of plate tectonics: plates riding on liquid, liquid moves; plates separate, stuff
comes up (divergent, Mid Atlantic Ridge); plates slide, earthquakes (transform, San Andreas); plates hit:
Subduction (volcanoes), buckling (Himalayas, Alps); hot spots, island chains (Hawaii).
IV. But is there liquid in the interior?
A. How to tell structure of interior: seismic studies; chemical analysis of deep volcanoes for chemistry
(Hawaii). Know they are deep from seismic studies.
B. Physical structure: layers determined by temperature, pressure, melting point; why does temperature
increase as you go down?
C. Chemical structure: why do you have different layers (Differentiation); Heavier stuff sinks in a liquid
(it all had to be liquid once).
V. What is the energy source for making liquids, driving plates?
A. Radioactivity (again).
B. Energy of accretion.
C. Note: Most processes on the surface get their energy externally: weather driven by sunlight;
photosynthesis also driven by sunlight; erosion results from weather (wind and rain), biological
activity; tides from lunar gravity.
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HISTORICAL GEOLOGY
I. Rock Types
A. Igneous Rocks: volcanic rocks (extrusive rocks), lava (basalt); Intrusive rocks (granite)
B. Sedimentary rocks: conglomerate; sandstone, siltstone, mudstone, and shale; limestone (calcium
carbonate)
C. Metamorphic rocks (marble, slate)
MAJOR DATING TECHNIQUES
PARENT
DAUGHTER
HALF-LIFE (Ga)
238
4
Uranium
Helium, 206Lead
4.47
235
4
Uranium
Helium, 207Lead
0.704
232
4
Thorium
Helium, 208Lead
14.0
187
187
Rhenium
Osmium
41
147
143
Samarium
Neodymium
106
87
87
Rubidium
Strontium
48
40
40
Potassium
Argon
1.28
14
14
Carbon
Nitrogen
0.0000057
(5730 years)
I. Atmosphere  composition: present  N2 76%, O2 23%, Ar 1%; past  H2, NH3, CH4, H2O  i.e., H-rich.
II. Atmospheric evolution  past.
A. H2O to ocean
B. H2 photodissociates, escapes (takes most things with it).
C. Volcanoes  H2O (to ocean), N2 (sits there), CO2 (dissolves in ocean, forms carbonate rocks).
D. Plants breathe out enough O2 to get O3  ozone: Absorbs ultraviolet, makes it possible to live on
land.
III. Atmospheric evolution  present
A. Chewing up ozone: CFCs increase UV
IV. Greenhouse effect: Explain (car, greenhouse); is it real?
HISTORY OF LIFE
I. Our knowledge largely limited to fossil record (paleontology).
A. Types of fossils.
1. Petrification  turning into rock.
2. Carbonization  soft tissues buried and compressed
3. Molds and casts  the organism leaves an imprint in surrounding rock
B. Fossil record is incomplete
1. Fossil: has to get buried quickly, reappears recently (or get dug up).
2. Mud that was lithified (turned to rock), uplifted.
3. Most complete  fossils of hard-bodied animals living in water.
4. Least complete  soft parts of land animals.
II. So what do we know? History of Earth.
A. First 3 billion years. No evidence of anything on land, or anything big; no oxygen. Stromatolites
(algae)  live in oxygen-poor environments, exhale oxygen
B. Within last 1 to 2 billion years, oxygen atmosphere, so things can breathe oxygen: ozone
C. Land animals come from amphibians (tidepools? Swamps?): reptiles, dinosaurs, birds/mammals
D. Humans: come from primates; why did they evolve?
E. Civilization begins 10,00015,000 yrs ago (near end of last Ice Age): Cultivation of plants,
domestication of animals. Permanent settlements.
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