Lecture 2

The Earth as a Planet

History, Age, Continental Drift, Interior

Why Study The Earth

• It’s our home!
• Where did life come from, where is it going
• To understand the other planets
• Study of other planets will, in turn, help us understand the Earth.
• We will concentrate on large-scale characteristics of the interior and atmosphere for comparison with other terrestrial planets.

The Age of the Earth

• Greek philosopheres (déjà vu all over again)
  • Time is cyclic - no beginning, no end
  • "Age" of earth is meaningless
  • No room for evolution or change in world
• Mayan peoples of Mesoamerica:
  • Very elaborate and accurate calendar
  • Time had beginning but millions of years ago.

The Age of the Earth - 2

• Western religions replace cyclic time by linear time with definite start and stop dates
  • Bishop James Ussher (1654, Ireland): "the beginning of time fell on the beginning of the night which preceded the 23rd day of October in the year… 4004 B.C.E.
  • Kepler and Newton recalculated and got ~3996 B.C.E.
  • acceptance of beliefs vs. enquiry into Nature

Two World Views - 

• 
  From “The Age of the Earth”, by G. Brent Dalrymple

  
  The Age of the Earth - 3

• The start of a true picture of Earth’s age:
  • Industrial revolution, digging, and the similarity in order of layers of rocks at different places
  • Fossils found ® extinction happens
  • Microscope ® variety of living things
  • Biology and Geology ® revolution in thinking about the Earth
  • Earth may not be center of Universe, but it was all there because of man - the perfect Chain of Being; no missing links

Two Opposing Views of How the Earth Got to be as it was:

• Catastrophism - cataclysmic upheavals
  • gradual change in earth’s appearance impossible because age only ~ 5500 years
• Uniformitarianism - gradual change
  • biological and geological change due to ordinary processes that have always operated.
  • Extinction caused by these processes

Uniformitarianism

• Charles Lyell: Scottish geologist (1797-1875) - an observer and a traveler
• • Principles of Geology
  • Earth needed millions of years to evolve
  • an open, inquiring science, not dogma
• Charles Darwin: Voyage of the Beagle (1831-1835); Lyell’s influence.
  • Two concepts: great age and need for change.
  • Biological evolution alone needs vast time

Age: A Dilemma and a Solution

• Biology and geology needed vast spans of time but had no proof of its existence.
• The physicists enter the picture:
  • Internal heat of earth - known from mining
  • Lord Kelvin (late 19th cent): age of sun can not be more than a few 100,000,000 years (gravitational energy)
• Kelvin & Darwin allowed that something important could be missing from their age estimates

Radioactivity and Earth’s Age

• Becquerel (1885) - radioactive decay as source of internal heat for Earth.
• Half life of radioactive elements:
  • Age of Earth is ~3-4 Byr from Earth rocks
  • Lunar rocks, Meteorites ® age of 4.6 Byr.
  • Oldest rocks on earth destroyed by geological activity.
• Radioactive Decay & Half-life - important

Main Parent and Daughter Isotopes Used to Determine Ages of Rocks

Parent         Daughter        Half life
Isotope         Isotope         (Byr)
⁴⁰K               ⁴⁰Ar              1.25
⁸⁷Rb             ⁸⁷Sr             48.8
²³²Th         ²⁰⁸Pb             14.0
²³⁵U           ²⁰⁷Pb              0.704
²³⁸U           ²⁰⁶Pb              4.47

The Earth as a Planet

• Key point: Interior of Earth is hot due to radioactivity. This supplies all energy for geological activity.
• Study interior with earthquakes: natural and man made.

Interior Structure of the Earth

• Earth has differentiated:
• Hot Iron Core (5000 K)
  • inner solid core
  • outer liquid core
• Convective Mantle - silicate rocks
• Crust - Oceanic and Continental

Stages of Planetary Evolution

• Differentiation (rocky planets)
• • Radioactivity heats the interior soon after formation
  • Heating causes iron (+other metals) to melt
  • Iron & other metals sink to center
  • Interior is differentiated by density

• Cratering (rocky planets)
• • Crust modification, persisted until about 3.3 billion years ago
  • Realization of importance of cratering only in last 20 to 30 years.
  • Persistence of impact craters depends on "age" of surface

• Surface evolution - depends on several factors that vary from planet to planet:
• • Interior convection like Earth - may be unique in solar system
  • Atmospheric erosion
  • Water with no atmosphere
  • Volcanism
  • Impacts

The Interior of the Earth

• Observations of the interior structure depend on propagation of seismic waves.
• Two types of Seismic waves:
  • S-waves: "Shear waves"
    • Matter is displaced perpendicular to direction of propagation.
  • P-waves: "Pressure waves"
    • Compressional waves, like sound waves.

Seismic Waves

• Differences in speed, etc. of two wave types means multiple stations can thus pinpoint the epicenter
• Have one seismograph for each of the two types of wave motion.

Continental Drift (1960s)

Changing Face of the Earth

• The appearance of the earth’s surface in 60 million year intervals from the Cambrian to the present with computer generated maps.
• The assembly and break-up of the super-continent Pangaea, the two chief episodes of continental drift leading up to the present arrangement of the earth’s land masses.
• 

Convection in the Earth

• Analogous to the Sun although physical conditions vastly different.
• Earth’s surface is made up of plates.
• Convection acts as a conveyor belt moving the crustal plates around.

Earth’s Interior: Continental Drift

• Key things to know:
  • Interior of Earth is hot
  • Upper and lower mantle are convective
  • The Lithosphere is on top of mantle and is broken into plates due to convective cells
  • Volcanoes bring up magma from deep in mantle
  • Plates have oceanic and continental crust on them
  • Oceanic crust denser (heavier) than continental.
  • Plates move around - continental drift or plate tectonics.
  • Earthquakes usually occur on plate boundaries

Continental Drift:
A Revolution in Geology

• Proposed by Wegner (early 1900s)
• Not accepted until 1960s when critical evidence was found.
• Wegner’s main arguments based on:
  • edges of continents fit together like a jig-saw puzzle; even better if use continental shelves.
  • similarity of plants, fossils, rock formations on continents separated by oceans.
• Critical evidence:
  • Mid-Ocean ridges and sea-floor spreading
  • magma comes up at mid ocean ridges making new oceanic crust.
  • Magnetic field reversal arguments
  • Theory of convection in earth’s interior
  • Old oceanic crust gets subducted back into mantle at trenches.
  • Oceanic crust completely renewed in a few 100 million years.

Activity Near Plate Boundaries

• Plates moving apart
  • in ocean: - mid-ocean ridges, (Mid-Atlantic)
  • on land: rift valleys (East Africa)
• Plate collisions:
  • Mountains if continental plates (Himalayas)
  • Subduction zones if oceanic - continental.
• Earthquakes most common at boundaries
• Volcanoes found in 3 places: rift valleys, plate collisions, hot spots
• 

"Plates" of the Earth’s Surface

Lithosphere: broken into ~ a dozen plates that move quasi-rigidly, floating on a partially molten upper mantle.

The Surface of the Earth

• Rift zone: where plates spread apart. Young surface: East Africa, Iceland
• Mid-Atlantic ridge is an example of a rift zone.
• North American and Eurasian plates are pulling apart here.
• The Mid-Atlantic Ridge runs right through Iceland, and we can actually see the two plates pulling apart.
• 
• Subduction zone: Collision of plates, results in mountain building.
• Oceanic crust denser than Continental Crust so it usually gets subducted.

Volcanoes On a Subduction Zone

The Andes Mountains, Southern Chile

Mount St. Helens, Washington State, 1980 May

Himalayan Mountains

• Collision of Continental Plates - generally no subduction because both are light & equal density

Hawaii: Drifting over a Hot Spot

• The Hawaiian Island chain is formed by a plate drifting over a permanent hot spot in the Earth’s mantle.

• Mauna Loa on Hawaii is the world’s largest shield volcano.

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