Ch01 Earth in Context - San Francisco State...

Essentials of Geology, 4th edition, by Stephen Marshak © 2013, W. W. Norton Chapter 1: The Earth in Context

Introduction

  Cosmology: the scientific study of the Universe.  Structure  History

Earth 4 Part 1 Opener


  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

What Is the Structure of the Universe?

Fig. 1.2a


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


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.


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


  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.

The Solar System

Fig. 1.3b


  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.

Forming the Universe

Fig. 1.2a


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

No Doppler shift

This observer sees light waves ““spread out””—red-shifted.

This observer sees light waves compressed—blue-shifted.

Fig. 1.4c


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


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


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


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


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


Fig. 1.6a

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?


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


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!


  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.

Nebular Theory of the Solar System

Geology at a Glance


  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.

Solar System Formation

Geology at a Glance

Fig. 1.7


  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.

Differentiation of Earth

Geology at a Glance


  ~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.

Formation of the Moon

Geology at a Glance


  The atmosphere develops from volcanic gases.   When Earth becomes cool enough:

 Moisture condenses and accumulates.  The oceans come into existence.

The Atmosphere and Oceans

Geology at a Glance


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


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


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


  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.

What is Earth Made Of?

Fig. 1.12


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


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


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


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


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


  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

The Mantle

Fig. 1.15b


  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.

The Core

Fig. 1.15b


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

Ch01 Earth in Context - San Francisco State...

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