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Earth Formation, Earth Layers,
& Plate Tectonics
Earth’s beginning:
• Most accepted scientific theory dates the Earth at _4.6_ billion
years old.
• Scientists determined the age of Earth by dating Precambrian
zircon crystals in rocks at _4.4 _ billion years old, moon rocks
at __4.45_ billion years old, and meteorites which range from
_4.5_ – _4.7_billion years old
The early Earth was a hot, molten ball
Sources of internal heat:
1. Gravitational Contraction: as Earth’s mass grew, gravity
caused the mechanical energy of contraction to be converted
to thermal energy.
2. Radioactive Isotopes released (and still do release)
thermal energy as the isotopes go through radioactive decay.
3. Bombardment by Asteroids and Meteors also caused
mechanical energy to be converted into thermal energy .
http://channel.nationalgeographic.com/videos/the-birth-of-earth/ (2 ½ minute video on Earth’s birth)
• As Earth cooled, crust formed as its upper layer solidified.
• These early crust pieces returned to the mantle carrying water
which when recycled produced a less dense, granitic crust.
• Differentiation: Layering of the Eearth as it cooled due to
differences in density, (iron sank) while Earth was still in a
molten phase.
First water & atmosphere:
• As earth cooled the raw materials for water and an atmosphere
emerged due to geologic activities (i.e. volcanoes, geysers,
rifts, trenches, etc.) + comets & meteors.
•
https://www.youtube.com/watch?v=YrruvHNwGq4 (10½ minute video: Earth's formation and history)
Earth’s Layers
• Two things increase the closer to the center of the Earth (core)
we go:
1. Density: mass per unit volume.
(how closely packed matter is in a space)
2. Temperature: (It gets Hotter!!!)
Main layers of the Earth:
1.Crust
2.Mantle
3. Outer Core
4. Inner Core
The Crust (Lithosphere)
• Solid: Made up of Silicates.
(very light material)
• Oceanic crust is more dense
(Basaltic).
• Continental crust is less dense
(Granitic).
• 8-40 km thick.
• Temperature increases at
1ºC per 40 m after 1st 20 m
The Mantle
• Middle, semi-solid hot rock layer; composed of
mostly silicates, but is also rich in iron and
magnesium which makes it more dense than
the crust.
• It has Plasticity; which is the ability of a solid to flow.
• 2900 km thick
• Asthenosphere – upper layer
that the lithosphere floats on.
• Mesosphere – the lower layer
• Temperature ranges from
870 ºC - 2200 ºC
https://www.youtube.com/watch?v=Kpoko_l34ZE
(1 minute convection video)
The Core: center of Earth
•
•
•
•
Outer Core – liquid
2250 km thick
2200 ºC – 3200 ºC
Inner Core – solid due
to extreme pressure
• 1300 km thick
• 3200 ºC – 5500 ºC
Plate Tectonics
• If you look at a map of the world, you may notice that some
of the continents could fit together like pieces of a puzzle.
• In 1910, Alfred Wegener’s proposed Continental Drift: the
hypothesis that Earth’s plates are floating on the mantle in a
slow & steady motion caused by convection currents.
• Pangaea– the name given to the single land mass that split
and moved apart about 200 mya to form today’s continents.
(means all lands)
https://www.youtube.com/watch?v=5ppyGg3vRs8
(Continental Drift video: 1 ½ min.)
https://www.youtube.com/watch?v=2It3ETk2MGA
(future drift video: 1 minute)
Evidence for continental drift and Pangaea
1. Landforms – shape of continental coastlines, mountain
ranges, coal fields, etc.
2. Fossils – same plant & animal fossils on continents now
separated by water.
3. Climate – tropical plant fossils in cold climates today
{Spitsbergen Norway & Antarctica} & glacier tracks in rocks
in warm climates today {Africa, S.A., Australia & India}
4. Magnetic reversal of polarity
stripes in rocks in the ocean
floor crust.
5. Satellite &
Sonar data
Plate Tectonics: The earth’s crust is divided into
tectonic plates which move in various directions.
3 Types of Plate Boundaries:
1. Divergent Boundary: plates move apart creating
new crust (rift valley & mid-ocean ridge) ← →
2. Convergent Boundary: plates collide and destroy or
buckle crust (move together) → ←
- Oceanic-Oceanic: subduction of one
plate under another creating trenches
& volcanic islands
- Oceanic-Continental: oceanic (more dense)
subducts under continental (less dense)
creating trenches & volcanic mountains.
- Continental-Continental: compression
and buckling forms tall folded mountains.
3. Transform Boundary: plates sliding side-to-side
(creates faults & earthquakes) ↑↓
https://www.classzone.com/books/earth_science/terc/content/visualizations/es0804/es0804page01.cfm?chapter_no=visualization
(plate boundaries animation)