Great Ideas in Science:
Lecture 9 – Earth as a Planet
Professor Robert Hazen
UNIV 301
Great Idea: Earth, one of the planets that
orbits the Sun, formed 4.5 billion years
ago from a great cloud of dust and gas.
Today: Key Ideas About Earth
1. Earth, one of the planets that orbits the
Sun, formed 4.5 billion years ago from a
great cloud of dust and gas.
2. The surface of our planet changes
constantly; no feature is permanent.
3. The entire Earth is still changing, due to
the slow convection of soft, hot rocks
deep within the planet.
4. Earth materials move in cycles; a change
in one cycle affects others.
Clues to the Origin of the Solar System
Solar System = Objects
gravitationally bound to the Sun
Clue #1: Planetary Orbits
Features of solar system
– All planets orbit in the same direction
– All planets orbit in the same plane
– Most planets rotate in the direction of orbit
Clue #2: Distribution of Mass
• Almost all mass is in the Sun (99.9%)
• Two types of planets
– Terrestrial planets
– Jovian planets
• Other objects
– Moons, asteroids, comets
The Nebular Hypothesis
Terrestrial (Inner) Planets
• Mercury, Venus, Earth, Mars
– Rocky and relatively small
– Mercury and Venus too hot for life
– Mars may have had life long ago
Terrestrial (Inner) Planets
• Mars Exploration
– Multiple missions
– Found evidence of water
Jovian (Outer) Planets
Jupiter, Saturn, Uranus, Neptune
Jovian (Outer) Planets
Jupiter, Saturn,
Uranus, Neptune
– Layered structure
– No solid surface
Pluto and the Plutoids
– Only 0.3% of Earth’s mass
– Three moons
– Captured comet or asteroid?
– Is it a planet?
Asteroids, Comets,
and Meteors
Asteroids
Small rocky bodies
Orbit sun
Asteroid belt
Comets
Dirty snowballs
Highly elongated orbits
Stardust and Deep Impact missions
Meteors and Meteorites
Meteor showers
Original solar system material
The Formation of Earth
The Great
Bombardment
Differentiation
Heat produced from collisions
Dense material sank to center
Lighter material rose to surface
Layered structure
– Core
– Mantle
– Crust
The Formation of the Moon
“Big Splash”
Large object impacted Earth
Parts of mantle blown into orbit
Moon formed from this material
The Formation of the Moon
“Big Splash”
The Evolution of
Earth’s Atmosphere
1. Volcanoes released N2, CO2, H2, & H2O
2. Large impacts blew off most atmosphere
3. Hydrogen lost by gravitational escape
4. Living organisms introduced oxygen
Volcanoes and Earthquakes:
Evidence of Earth’s Inner Forces
• Volcanoes: Magma breaks through surface
Volcanoes and Earthquakes:
Evidence of Earth’s Inner Forces
Earthquakes
– Rocks breaks along fault
– Energy transmitted as wave
– Tsunamis
The Case of the
Disappearing Mountains
Old
Young
Deep Time
James Hutton at
Jedburgh Scotland
ca.1790
Deep Time
1. Sediments
2. Burial
3. Uplift
4. Sediments
Plate Tectonics
Great Idea: Earth is constantly changing,
due to the slow convection of soft, hot
rocks deep within the planet.
Plate tectonics – 6 lines of evidence
1. Physiology: Similar shapes of continents
Plate tectonics – 6 lines of evidence
1.
Physiology: Shape of continents
2. Seismology: Earth’s inner structure
Plate tectonics – 6 lines of evidence
1.
2.
Physiology: Shape of continents
Seismology: Earth’s inner structure
3. Geology: Distribution of rocks and fossils
Plate tectonics – 6 lines of evidence
1.
2.
3.
Physiology: Shape of continents
Seismology: Earth’s inner structure
Geology: Distribution of rocks and fossils
4. Oceanography:
Sea floor topography
Bruce Heezen and Marie Tharp
Plate tectonics – 6 lines of evidence
1.
2.
3.
4.
Physiology: Shape of continents
Seismology: Earth’s inner structure
Geology: Distribution of rocks and fossils
Oceanography: Sea floor topography
<5
5. Volcanology: Ages
of Atlantic volcanoes
95
75
55
125
Plate tectonics – 6 lines of evidence
1.
2.
3.
4.
5.
Physiology: Shape of continents
Seismology: Earth’s inner structure
Geology: Distribution of rocks and fossils
Oceanography: Sea floor topography
Volcanology: Ages of Atlantic volcanoes
6. Paleomagnetismof
sea floor rocks
New Support for Plate Tectonics
• Measurements of the distance
between continents
• North America and Europe are
separating at ~5 cm per year
Earth’s Tectonic Plates
Plate Boundaries
Three Main Boundary Types
– Divergent
– Convergent
– Transform
Divergent Boundaries
Convergent Plate Boundaries
Three Types:
1. Ocean-ocean
Convergent Plate Boundaries
Three Types:
1. Ocean-ocean
2. Continent-continent
Convergent Plate Boundaries
Three Types:
1. Ocean-ocean
2. Continent-continent
3. Ocean-continent
Transform Plate Boundary
Another Look at Volcanoes
and Earthquakes
Volcanoes Occur:
1. At Divergent Plate Boundaries
2. Close to Convergent Plate
Boundaries
3. Above Hotspots
Another Look at Volcanoes
and Earthquakes
Earthquakes Occur at:
1. Transform Plate Boundaries
2. Convergent boundaries
Earth’s Geochemical Cycles
Earth materials move in cycles;
a change in one cycle affects the others.
1. Atoms recycle; they’re used over and over.
Reservoirs (Aluminum can)
Hydrologic cycle
Atmospheric cycle
Rock cycle
2. Energy flows through the system.
Geochemical Cycles
• Account for all the repositories
of that substance.
• Document processes by which
the substance moves from
repository to another.
• Gold
ROCK CYCLE: Igneous Rocks
• Igneous Rocks
– Solidify from hot liquid
• Types
– Extrusive rocks solidify
at the surface
– Intrusive rocks solidify
below surface
ROCK CYCLE: Sedimentary Rocks
Rocks deposited layer by layer
from weathered particles (sand,
silt, etc.) or chemicals (i.e., coral).
ROCK CYCLE: Metamorphic Rocks
• Rocks formed by
pressure and heat
• Examples
– Slate
– Schist
– Gneiss
– Quartzite
– Marble
Atmospheric Cycle
Air mass: Uniform temperature and moisture
Weather: State of the atmosphere
Climate: Long-term average of weather
The General Circulation
of the Atmosphere
• Circulation powered by Sun
– Air heated and rises
– Cools and sinks
• Prevailing winds
– Caused by earth’s rotation
Weather
Five variables define
state of atmosphere
–Temperature
–Air pressure
–Humidity
–Cloudiness
–Prevailing winds
Climate
• Gradual change over long periods
• Influences on climate
– Large bodies of water
– Ocean currents
– Mountain ranges
– Position of tectonic plates
– Solar output
– Greenhouse gases
The Water (Hydrologic) Cycle
• The total amount of Earth’s water is constant;
the same atoms cycle from one reservoir to
another
• Total volume ~ 1.4 billion km3
Oceans 97.3%
Lakes and Rivers 0.01%
Groundwater 0.6%
Ice Caps & Glaciers 2.1%
Atmosphere 0.001%
All life 0.00004%
How does water move
among reservoirs?
Ocean Currents
• Redistribute heat across planet
Ice Ages
As the ice caps and
glaciers grow, the
sea level drops.
Cycle tied to Earth’s
tilt and orbit
Two facts about water use
• A human requires about 2 liters
of water per day to survive
• The average American uses
about 6,000 liters of water per day
The Water Table
• The water table will drop when discharge
exceeds recharge (like a bank)
• In the US we “mine” about 100,000,000 gallons
of water every day (more than the recharge)
• Artificial recharge helps
• Urbanization and pollution hurt