Download Crust - UNLV Geoscience

Origin and Structure
of the Earth
Marshak – Chapter 1
(plus an introduction to Chapter 2)
The Earth is part of the solar
system and thus most likely
formed at the same time…
So, what do we know about the solar
system and it’s structure?
These are the observations which are
needed to come up with an idea
(hypothesis) for how the solar system
(and Earth) formed.
Formation of the solar system
and differentiation of Earth
• Hypotheses must satisfy observations: planets orbit sun in
one direction, axes of rotation nearly perpendicular to orbit,
most planets rotate in same direction as orbit about sun,
>99% solar system mass in sun, ~99% solar system
angular momentum in planets
• Inner Terrestrial - Mercury, Venus, Earth, Mars
• Outer Jovian - Jupiter, Saturn, Uranus, Neptune, Pluto?
• Terrestrial – dense, rocky, >3 g cm3, Mg, Fe, Si, K, Ca,
metals combined with O
• Jovian - “gassy” <~1.5 g cm3, ice, H, He, CH4 methane
CO2
• Asteroid belt between Mars and Jupiter, source of
meteorites
Origin of our Solar System: The Nebular Hypothesis
The Sun is ~99% of the mass of the solar system
~99% of the angular momentum is in the planets
Inner planets are rocky and dense – terrestrial planets
Outer planets are gassy – gas giant planets
We know the Earth is composed of layers – Why?
Planetary Differentiation
• Why?
– There is a motive
• Layers of different chemical composition can have
different density, and gravity provides a driving force
whereby planets can lower their potential energy by
sorting the denser material towards the center.
– There is a means
• Solids are hard to sort mechanically, but liquids are
easily separated gravitationally. Partial or complete
melting allows large-scale differentiation.
– There was an opportunity
• Heating beyond the melting point of most components of
undifferentiated solar material during planet formation is
inevitable for bodies above a certain size (> approx.
1,000 km radius) that formed early enough or fast
enough.
Chemical Differentiation of the Earth
Early Earth
Earth Today
Early Earth likely entirely molten – gravitational segregation
of dense metals (mostly Fe) to the center is the result.
Origin of the moon by planetary impact on Earth
This occurred ~4.5 billion years ago (4.5 Ga)
(very early in Earth history as age is only ~4.6 Ga)
Whole Earth has significant Fe - due to the core
However, outer layers of Earth are much different
Earth’s crust (thin outer layer) mostly Si and O
Earth’s mantle (between core and crust) is
similar to the crust, but with lower Si, and
higher Fe and Mg
Introduction to
Plate Tectonics
• Plate Tectonics: Structure of Earth’s surface is
largely caused by the formation, movement, and
destruction of large rigid plates…
• Major conclusions of Plate Tectonics:
– The lithosphere (outermost shell of Earth) is
composed of 13 or more large rigid plates and
numerous smaller ones
– The plates move with respect to one another and
thus continents are mobile (imbedded in plates)
– Continents are relatively old, ocean basins
relatively young
– Geologic activity (earthquakes, volcanoes) is
concentrated along the boundaries between plates
January 20, 2011 – Earthquakes in the past 5 years
from www.iris.edu
Earthquakes mark outline of Earth’s tectonic plates.
Known volcanoes of the world – do the locations look familiar?
from the Smithsonian Global Volcanism Project
Note that earthquakes
and volcanoes generally
occur in the same
locations.
Where are Earths large
mountains found?
Are all of these generally
found in the same places?
Earth’s outermost layer comprises plates which move relative to each other.
These movements are now measured by GPS and VLBA techniques.
The Theory of Plate Tectonics
Earth’s outer layer broken up into 13 major tectonic plates
which are made of the crust and uppermost mantle beneath.
Plates may contain oceanic or
continental crust or both
Others are mainly
oceanic crust
contain both
continental
Some plates
and oceanic crust
Earth is Composed of Multiple Layers from Core to Crust.
Crust and Upper Mantle (Lithosphere) = Locked Together as Rigid Plate.
In terms of overall radius of Earth the plates are only 1-2%.
The lithosphere is cold, rigid and solid.
What about the asthenospheric mantle beneath?
Oceanic crust (mostly basalt)
Continental crust
(mostly granite)
Mantle
(mostly olivine)
Cold, rigid
Lithosphere
Asthenosphere
Hot, ductile
The rigid lithosphere slides on the ductile
asthenosphere, which is partially molten.
Crust, mantle, and core refer to composition
(what is it made of?).
Crust:
mostly granite on continents
mostly basalt on oceans
Mantle: made mostly of the mineral olivine
Core: mostly iron and some nickel
(we will talk about
these later…)
Lithosphere and asthenosphere refer to the
strength (Is it hot, or is it cold? Is it rigid, or does it
flow like toothpaste?)
Lithosphere : Strong, rigid, cold outer shell of rock which includes the
crust and part of the upper mantle.
Asthenosphere: The hotter, weak, ductile layer of solid rock below
the lithosphere that flows plastically. Analogy – cold toothpaste.
3 Types of Plate Boundaries
• divergent
• convergent
• transform (strike-slip)
transform
divergent
convergent
Divergent plate boundary
• plates move apart
• new lithosphere created (oceanic)
• volcanism and earthquakes
convergent plate boundary
• plates move towards one another
• lithosphere destroyed (oceanic)
• volcanism, earthquakes,
mountain belts
Convergent Plate Margins
Ocean-Ocean
Ocean-Continent
Continent-Continent
Lithosphere created at divergent plate boundaries
is destroyed at convergent plate boundaries.
Motion at Plate Boundaries
Hotspot volcanoes are created where a
plume of bouyant, hot mantle rises.
Hot Spot Volcano Tracks
What Forces Drive Plate Tectonics?
Early Earth was
mostly molten
due to:
1) Impact events
2) Gravity
3) Radioactivity
Earth’s internal heat is still escaping
today and is most obviously expressed
in volcanic eruptions.
What role does Earth’s internal heat
play in the operation of plate tectonics?
Three modes of heat transfer.
Only convection causes motion.
How does convection work?
• Within Earth’s interior:
- Cold dense rock
sinks in subduction
zones.
- Hot, ductile mantle
inside rises and
convection occurs.
Fig 1.15c
Plate tectonics is caused by convection in the mantle.
In detail there are other driving forces, we will
discuss these later in the semester…..
Plate Tectonics provides a comprehensive explanation for all of
the major features of the Earth that we can observe.
Island chain from hot spots
deep ocean trenches
earthquakes
volcanoes
Fig 1.10
Plate velocities measured with GPS
Confirms plate tectonic motions beyond reasonable doubt!
Mantle tomography – provides images similar to ultrasound.
Mantle tomography – hot
material in red (slower
seismic wave velocity),
cold material in blue
(faster seismic wave
velocity).
Earthquake locations
shown by white dots.
Clearly shows the
subducting oceanic
lithosphere (cold) beneath
the Japan volcanic arc
system (hot).
More detailed image of subduction zone beneath Japan.