Download Ch01 Earth in Context

Introduction
 
Cosmology: the scientific study of the Universe.
  Structure
  History
Earth 4 Part 1 Opener
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
What Is the Structure of the Universe?
 
Universe is made up of matter and energy.
  Matter—substance
of the universe; takes up space.
 Mass
 Density
 Weight
  Energy—the
ability to do work.
 Heat
 Light
 Pull of gravity
Fig. 1.2a
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Stars and Galaxies
 
Stars are immense balls of incandescent gas.
  Gravity
binds stars together into vast galaxies.
  Over 100 billion galaxies exist in the visible universe.
 
The Solar System is on an arm of the Milky Way galaxy.
  Our
sun is one of 300 billion stars in the Milky Way.
Fig. 1.2b, c
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
The Nature of Our Solar System
 
Our sun is a medium-sized star, orbited by 8 planets.
  The
sun accounts for 99.8% of our solar system mass.
  Planet—a planet:
 Is a large solid body orbiting a star (the Sun).
 Has a nearly spherical shape.
 Has cleared its neighborhood of other objects (by gravity).
  Moon—a
solid body locked in orbit around a planet
  Millions of asteroids, trillions of icy bodies orbit the sun.
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
The Nature of Our Solar System
 
Two groups of planets occur in the solar system.
  Terrestrial
Planets—small, dense, rocky planets
 Mercury, Venus, Earth, and Mars
  Giant
Planets—large, low-density, gas and ice giants
 Gas giants: Jupiter, Saturn (hydrogen and helium)
 Ice giants: Uranus, Neptune (frozen water, ammonia, methane)
  The
Solar System is held together by gravity.
Fig. 1.3a
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
The Solar System
The terrestrial planets are the four most interior.
  The giant planets occupy the four outermost orbits.
  All but two planets have moons (Jupiter has 63!).
  The asteroid belt lies between Mars and Jupiter.
  Clouds of icy bodies lie beyond Neptune’
’s orbit.
 
  Icy
fragments pulled into the inner solar system
become comets.
Fig. 1.3b
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Forming the Universe
 
The vastness of the Universe is staggering.
  Earth
is a planet orbiting a star on the arm of a galaxy.
  The sun and over 300 billion stars form the Milky Way.
  Over 100 billion galaxies exist in the visible universe.
  Where did all this “stuff”
” come from?
  The Big Bang initiated the expanding universe
 13.7 billion years ago.
Fig. 1.2a
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
The Doppler Effect
 
A moving star displays Doppler-shifted light.
  Approaching
starlight is compressed (higher frequency):
 Blue shift
  Receding
starlight is expanded (lower frequency):
 Red shift
This observer sees light waves
compressed—blue-shifted.
This observer sees light waves
”—red-shifted.
“spread out”
No Doppler shift
Fig. 1.4c
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
The Expanding Universe
 
Light from galaxies was observed to be “red-shifted.”
”
  Edwin
Hubble recognized the red shift as a Doppler effect.
 He concluded that galaxies were moving away at great speed.
 No galaxies were found heading toward Earth.
  Hubble
deduced that the whole Universe must
be expanding (analogous to raisin-bread dough).
 The expanding Universe theory.
 Did expansion start at some time in the past?
 If so, how far back?
 How small was the Universe before expansion?
Fig. 1.5a
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
The Big Bang
Researchers have developed a model of the Big Bang.
  During the first instant, only energy—no matter—was
present.
  Started as a rapid cascade of events.
 
  Hydrogen
atoms within a few seconds
  At 3 minutes, hydrogen atoms fused to form helium atoms.
  Light nuclei (atomic no. < 5) by Big Bang nucleosynthesis
 
The Universe expanded and cooled.
Fig. 1.5b
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
After the Big Bang
 
With expansion and cooling, atoms began to bond.
  Hydrogen
formed H2 molecules—the fuel of stars.
  Atoms and molecules coalesced into gaseous nebulae.
Gravity caused collapse of gaseous nebulae.
  Collapse resulted in increases in:
 
  Temperature.
  Density.
  Rate
of rotation.
Earth, 4th ed., Fig. 1.7
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
After the Big Bang
Mass in nebulae was not equally distributed.
  An initially more massive region began to pull in gas.
 
  This
region gained mass and density.
  Mass compacted into a smaller region and began to rotate.
  Rotation rate increased, developing a disk shape.
  The central ball of the disk became hot enough to glow.
  A protostar is born.
Geology at a Glance
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Birth of the First Stars
 
The protostar continued to grow,
  pulling
in more mass and creating a denser core.
  Temperatures soared to 10 million degrees.
  At these temps, hydrogen nuclei fused to create helium.
  With the start of nuclear fusion, the protostar “ignited.”
”
Chapter 1 Opener
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Birth of the First Stars
Nebulae from which first-generation stars formed
consisted entirely of light elements.
  These first-generation stars exhausted H2 fuel rapidly.
  As the stars became H2-starved, they initiated:
 
  Collapse
and heating.
  Catastrophic supernova.
 
Where did heavy elements
(atomic no. > 5) come from?
Fig. 1.6a
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Where Do Elements Come From?
 
Big Bang nucleosynthesis formed the lightest elements.
  Atomic
 
#s 1, 2, 3, 4, and 5 (H, He, Li, Be, and B)
Heavier elements are from stellar nucleosynthesis.
  Atomic
#s 6–26 (C to Fe)
  Stars are “element factories.”
”
 
Elements with atomic #s >26 form during supernovae.
Fig. 1.6b
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Where Do Elements Come From?
First-generation stars left a legacy of heavier elements.
  Second-generation stars repeated heavy element
genesis.
  Succeeding generations contain more heavy elements.
  The sun may be a third-, fourth-, or fifth-generation star.
 
  The
mix of elements found on Earth include:
 Primordial gas from the Big Bang.
 The disgorged contents of exploded stars.
 
We really ARE all made out of stardust!
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Nebular Theory of the Solar System
The nebular theory of Solar System formation
  A third-, fourth-, or nth-generation nebula forms 4.56 Ga.
 
  Hydrogen
and helium are left over from the Big Bang.
  Heavier elements are produced via:
 Stellar nucleosynthesis.
 Supernovae.
 
The nebula condenses into a protoplanetary disk.
Geology at a Glance
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Solar System Formation
The ball at the center grows dense and hot.
  Fusion reactions begin; the sun is born.
  Dust in the rings condenses into particles.
  Particles coalesce to form planetesimals.
 
Fig. 1.7
Geology at a Glance
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Differentiation of Earth
Planetesimals clump into a lumpy protoplanet.
  The interior heats, softens, and forms a sphere.
  The interior differentiates into:
 
  A
central iron-rich core, and
  A stony outer shell—a mantle.
Geology at a Glance
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Formation of the Moon
~4.53 Ga, a Mars-sized protoplanet collides with Earth.
  The planet and a part of Earth’
’s mantle are disintegrated.
  Collision debris forms a ring around Earth.
  The debris coalesces and forms the moon.
 
  The
moon has a composition similar to Earth’
’s mantle.
Geology at a Glance
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
The Atmosphere and Oceans
The atmosphere develops from volcanic gases.
  When Earth becomes cool enough:
 
  Moisture
condenses and accumulates.
  The oceans come into existence.
Geology at a Glance
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Magnetic Field
Space visitors would notice Earth’
’s magnetic field.
  Earth’
’s magnetic field is like a giant dipole bar magnet.
 
  The
field has north and south ends.
  The field grows weaker with distance.
  The magnetic force is directional.
 It flows from S pole to N pole along the bar magnet.
 It flows from N to S along field lines outside the bar.
Fig. 1.9a
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Magnetic Field
Earth’’s magnetic field is like a giant dipole bar magnet.
  The N pole of the bar is near Earth’
’s geographic S pole.
 
  A
compass needle aligns with the field lines.
  The N compass arrow points to the bar magnet S pole.
 Opposites attract.
 
Magnetic field lines:
  Extend
into space.
  Weaken with distance.
  Form a shield around
Earth (magnetosphere).
Fig. 1.9b
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
Magnetic Field
 
The solar wind distorts the magnetosphere.
  Shaped
like a teardrop
  Deflects most of the solar wind, protecting Earth
 
The strong magnetic field of the Van Allen belts
intercepts dangerous cosmic radiation.
Fig. 1.9c
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
What is Earth Made Of?
 
91.2% of Earth’’s mass comprises just four elements:
  Iron
(Fe)—32.1%
  Oxygen (O)—30.1%
  Silicon (Si)—15.1%
  Magnesium (Mg)—13.9%
 
The remaining 8.8% of Earth’
’s mass consists of the
remaining 88 elements.
Fig. 1.12
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
A Layered Earth
 
The first key to understanding Earth’
’s interior: density.
  When
scientists first determined Earth’
’s mass they realized:
 Average density of Earth >> average density of surface rocks.
 Deduced that metal must be concentrated in Earth’
’s center.
  These
ideas led to a layered model:
 Earth is like an egg.
 Thin, light crust (eggshell)
 Thicker, more dense mantle (eggwhite)
 Innermost, very dense core (yolk)
Fig. 1.13
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
A Layered Earth
 
Earthquakes: seismic energy from fault motion
  Seismic
waves provide insight into Earth’
’s interior.
 Seismic wave velocities change with density.
 We can determine the depth of seismic velocity changes.
 Hence, we can tell where densities change in Earth’
’s interior.
Fig. 1.14a, b
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
A Layered Earth
 
Changes with depth
  Pressure (P)
 The weight of overlying
rock increases with depth.
 
Temperature (T)
 Heat is generated in
Earth’’s interior.
  T increases with depth.
 
Geothermal gradient
  The rate of T changes with depth.
  The geothermal gradient varies.
 ~ 20-30°C per km in crust
 < 10°C per km at greater depths
 Earth’’s center may reach 4,700°C!
Earth, 4th ed., Fig. 2.13
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
The Crust
 
The outermost “skin”
” of our planet is highly variable.
  Thickest
under mountain ranges (70 km or 40 miles)
  Thinnest under mid-ocean ridges (7 km or 4 miles)
Relatively as thick as the membrane of a toy balloon
  The Mohorovičić discontinuity (Moho) is the base.
 
  Seismic
velocity change between crust and upper mantle
  The crust is the upper part of a tectonic plate.
Fig. 1.15a
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
The Crust
 
There are two kinds of crust: continental and oceanic.
  Continental
crust underlies the continents.
 Average thickness 35–40 km
 Felsic (granite) to intermediate in composition
  Oceanic
crust underlies the ocean basins.
 Average thickness 7–10 km
 Mafic (basalt and gabbro) in composition
 More dense than continental crust
Fig. 1.15a
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
The Mantle
Solid rock, 2,885 km thick, 82% of Earth’
’s volume
  The mantle is entirely the ultra-mafic rock peridotite.
  Convection below ~ 100 km mixes the mantle.
 
  Like
oatmeal on a stove: hot rises, cold sinks.
  Convection aids tectonic plate motion.
 
Divided into two sub-layers:
  Upper
Mantle
  Transitional zone
  Lower Mantle
Fig. 1.15b
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context
The Core
An iron-rich sphere with a radius of 3,471 km
  Seismic waves segregate two radically different parts.
 
  The
outer core is liquid; inner core solid.
  Outer core
 Liquid iron alloy
 2,255 km thick
 Liquid flows
  Inner
core
 Solid iron-nickel alloy
 Radius of 1,220 km
 Greater pressure keeps solid
 
Outer core flow generates
Earth’’s magnetic field.
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Fig. 1.15b
Chapter 1: The Earth in Context
Lithosphere-Asthenosphere
 
We can also regard layering based on rock strength.
  Lithosphere—the
outermost 100–150 km of Earth
 Behaves rigidly, as a nonflowing material
 Composed of two components: crust and upper mantle
 This is the material that makes up tectonic plates.
  Asthenosphere—upper
mantle below the lithosphere
 Shallow under oceanic lithosphere; deeper under continental
 Flows as a soft solid.
Fig. 1.17
Essentials of Geology, 4th edition, by Stephen Marshak
© 2013, W. W. Norton Chapter 1: The Earth in Context