Objectives

1. Distinguish between physical and historical geology.
2. Contrast the concept of catastrophism and uniformitarianism.
3. Describe the nature of scientific inquiry.
4. Have a basic understanding of and be able to explain the theory for the origin of the solar system and earth.
5. Describe the earth's basic internal structure.
6. Describe the three types of plate tectonic motion and give examples of which type occurs where on the Earth.
7. Understand the rock cycle and how the formation of various types of rocks can be interrelated.

Outline

Definition of Physical Geology

Age of the Earth

Scientific Method

Origin of the Universe

Origin of the Earth and Solar System

Early History of the Earth

Earth's Surface

Geology is divided into two broad areas:

Physical geology

(examines materials Earth is made of and processes occurring on and beneath the surface of the earth.)

Historical geology

(examines origin of the Earth, origin of life, and changes in Earth and life through time)

Deals with the layered rock record and fossils

Physical Geology:

1. Earth materials
   atoms, chemicals, metals, oil, gas, rocks, minerals, salt, gems, coal, sand, gravel, clay, soil, atmosphere, hydrosphere, organisms, etc.

   How do Earth materials affect people?
   building materials, fuels, soil to grow food, salt for food, jewelry, we are made of Earth materials

2. Earth processes
   earthquakes, volcanic eruptions, floods, continental drift, weather, climate, landslides, subsidence and collapse, tides, geysers, erosion, etc.

   Any geologic processes recently in the news?
   How do these processes affect people?

Determined through radiometric dating (Uranium, Thorium).
Using an instrument called a mass spectrometer.

Early ideas of the age of the Earth:

1. 1654 Archbishop Usher (Ireland), genealogy in Bible
   Earth was created October 26, 4004 BC, 9:00 am
   Earth was 6000 years old.

   Led to the Doctrine of Catastrophism:
   Earth was shaped by series of giant disasters.
   Had to fit many processes into a short time scale.

   ---

2. 1770's, 1780's "Revolution"
   James Hutton, Father of Geology (Scotland) 1726-1797.
   Published Theory of the Earth in 1785.

   Hadrian's Wall built by Romans, after 1500 years no change. Suspected that Earth was much older.

   Slow processes shape earth.
   Mountains arise continuously as a balance against erosion and weathering
   Doctrine of Uniformitarianism: "Present is key to the past".
   The physical and chemical laws that govern nature are uniform
   Unconformity at Siccar Point, Scotland
   "No vestige of a beginning, no prospect of an end"

Quantitative scientific methods.

1. In 1897, Lord Kelvin assumed that the Earth was originally molten and calculated a date based on cooling through conduction and radiation.
   Age of Earth was calculated to be about 24-40 million years.

   Problem: Earth has an internal heat source (radioactive decay)

2. In 1899 - 1901, John Joly (Irish) calculated the rate of delivery of salt to the ocean. River water has only a small concentration of salts. Rivers flow to the sea. Evaporative concentration of salts.

   Age of Ocean = Total salt in oceans (in grams) divided by rate of salt added (grams per year)

   Age of Earth was calculated to be 90-100 million years.

   Problems: no way to account for recycled salt, salt incorporated into clay minerals, salt deposits.

3. Thickness of total sedimentary record divided by average sedimentation rates (in mm/yr). In 1860, calculated to be about 3 million years old. In 1910, calculated to be about 1.6 billion years old.

   Early measurements of maximum thickness of sediment ranged from 25,000 m to 112,000 m. With more recent mapping, thickness of fossiliferous rocks is at least 150,000 m.

   Sedimentation rates average about 0.3 m/1000 years.

   At this rate, the age of the first fossiliferous rocks is about 500 million years.

   Problems: did not account for past erosion or differences in sedimentation rates; also ancient sedimentary rocks are metamorphosed or melted.

4. Charles Lyell 1800's compared amount of evolution shown by marine mollusks in the various series of the Tertiary System with the amount that had occurred since the beginning of the Pleistocene. Estimated 80 million years for the Cenozoic alone.

Maybe the most straightforward method is measuring the thickness of sedimentary layers. Note that the date for the first fossiliferous rocks is about right. The base of the Cambrian, with the first macrofossils of animals with skeletons is about 540 million years (through radiometric dating). The thickness method gave a date of 500 million years, not accounting for erosional events, which would have made the time longer. Adding in the thicknesses of older rocks, the age was extended back into the billions of years.

5. Discovery of radioactivity by Henri Becquerel in 1896. In 1905, Rutherford and Boltwood used radioactive decay to measure the age of rocks and minerals. Uranium decay produces He, leading to a date of 500 million years.

   In 1907, Boltwood suspected that lead was the stable end product of the decay of uranium. Published the age of a sample of urananite based on Uranium-Lead dating. Date was 1.64 billion years.

   So far, oldest dated Earth rocks are 3.96 billion years.
   Older rocks include meteorites and moon rocks with dates on the order of 4.6 billion years.
   Moon rocks, highland ~ 4.5 by, mare basalt ~ 3.2 - 3.8 by
   Meteorites - older than 4.5 by

   Mass spectrograph was used after WWI (1918). Led to the discovery of over 200 isotopes.

   Many radioactive elements can be used as geologic clocks. Each element decays at its own nearly constant rate. Once this decay rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent and the amount of stable daughter elements.
   Example: Potassium-Argon dating.

B. Why is the Earth younger than the moon and meteorites?

• Earth is geologically active.
• Has a hot, molten interior.
• Rocks are remelted and their internal clocks are reset.
  
• Also, rocks on Earth's surface are acted on by erosion and weathering. Rocks on Earth surface are not as old as the Earth, they are "recycled" rock materials

Rocks broken down into sediment (gravel, sand, silt, clay).
Sediment will turn into sedimentary rock over time. Older rocks are buried deeply under younger rocks.

C. Where do we find the oldest rocks on Earth?

Before this, oldest rocks known were from Isukasia region of Greenland (3.8 by).

Glaciers 2 miles thick scraped off young recycled rocks.
Land rose 250 ft since ice was removed => more erosion.
Isostasy

Very old rocks are at the surface in the Canadian Shield area.
Up to about 3.8 or 3.96 billion years old.

Multicellular life did not appear until about 1 billion years ago.
Before this, 3 billion years ago single celled life only.

Hard parts like shells don't appear until 600 million years ago. (Trilobites)

Return to Outline

1. Observations of natural phenomena (collect data)

2. Hypothesis (ideas to explain observations)
   Looking for a unifying principle. Must be verifiable; must be able to deduce and test consequences from a hypothesis.

   Method of multiple working hypotheses

3. Test the hypotheses by experimenting
   • a. accept
   • b. reject
   • c. modify

4. When a hypothesis is accepted and others are rejected, it becomes a theory. A theory is a hypothesis with considerable experimental support. Many consequences of a theory may be verified, but a theory can never be proved, only disproved. Theories may have limits in certain situations.

5. Laws are narrower in scope and expressed mathematically

Scientific Method lecture notes from Marin Community College, California, which also contains links to additional sites on the Scientific Method. Interesting graphics and ideas. Check them out.

Return to Outline

A. Origin of the Universe

1. the galaxies are rapidly moving apart (Hubble's Law); indicates that galaxies were closer together in the past
   (This was discovered in 1929 by Edwin P. Hubble.)
   
2. observed temperature of the universe today (background microwave radiation) 3 degrees above absolute zero
   
3. present abundances of hydrogen and helium
   interpretation: the universe is expanding; everything began together at a point, and a big explosion occurred, causing things to move apart rapidly

How old is the Universe?

Calculations of the age of the universe depend on the calculation of the Hubble Constant, a number which refers to the rate of expansion of the universe.

Controversy arose in Fall 1994
Age of the universe has been calculated to be about 10-15 billion years; calculations show that the age must be less than 20 billion years.
BUT 1994 data from Hubble Space Telescope was interpreted to indicate a high rate of expansion, resulting in an age of only about 8 billion years.
This stirred up a lot of excitement in Astronomy. For details, see the March 6, 1995 issue of TIME Magazine, p. 76-84.

Update: April 7, 1997, front page, New York Times. National Academy of Sciences held a colloquium on the age of the universe in March 1997. Many cosmologists now think that the age of the universe is likely to be between 12 and 14 billion years. Later observations from the Hubble Space Telescope observations of Cephids (pulsating stars) have given somewhat lower expansion rates. Lower expansion rates have been suggested by other recent studies as well. This seems to mean that the age of the universe is most likely 15-20 billion years.

---

How do we know the galaxies are moving apart?
Red shift.
Light is shifted toward the red end of the spectrum (long wavelengths).
Explain by the Doppler Effect. (Sound analogy.)

If object is moving toward you, the wavelengths are compressed -> high pitched sound or shorter wavelengths.

If object is moving away, the wavelengths are elongated -> low pitch or shorter wavelengths.

Same thing works with light as sound. Both travel in waves.

Light reaching us from distant receding galaxies has shifted toward the red end of the spectrum. (look at the wavelength position of absorption lines).

---

Light in spectrum ROY G BIV

Red = long Blue = short
Microwaves and radio waves are longer than visible

Ultraviolet and X-rays are shorter than visible.

B. Formation of stars, galaxies, solar systems, planets, etc.

"Nebular hypothesis". Nebular means cloud.

See description and illustrations in textbook.

The planets

• Terrestrial Planets (Inner planets, small, dense, and rocky. No rings. Few or no moons.)
  • Mercury
  • Venus
  • Earth
  • Mars
• Asteroid belt lies mainly between orbits of Mars and Jupiter.
• Jovian Planets (Outer planets, large, low density, and gaseous(H, He, CH₄). All have rings. All have many moons)
  • Jupiter - largest planet
  • Saturn
  • Uranus
  • Neptune
• Other planet that does not fit into the two previous categories:
  • Pluto (the outermost planet - small, icy, low density. one moon)

Know the names of the nine planets
Know which are Terrestrial and Jovian (and Pluto)

What is in the center of the solar system?

The sun is in the center of the solar system. Composed primarily of H, He.
Our sun is a star.

9 planets orbit or revolve around the sun.

Composition of the Sun?
Dominantly H and He
All originally formed from a cloud of gas and dust

Abundances of some elements in the sun:


H 78.5% percent by mass

He 19.7%

O 0.86%

C 0.40%

Fe 0.14%

Si 0.10%

N 0.09% etc.

(from p. 392 in Flower, Understanding the Universe, first edition)

These are the proportions of the raw materials in the solar nebula

Evidence:


All planets orbit sun in same direction (CCW)
Rotation of Sun and most planets is also CCW
Orbits are basically co-planar (flat)
Orbits nearly circular
Jovian moons orbit in same direction as parent planet and in planet's equatorial plane, resembling miniature solar systems
Planets' chemical composition and size change with distance from sun

Hypothesis (Nebular Hypothesis):


Cloud spins, contracts under gravity -> flattened disk


Concentration of matter in center -> proto sun

(most of mass, 99.85%)


Dense concentration of H and he at center -> fusion


Hydrogen burning, He produced


Source of sun's energy


Collisions of particles of dust and gas in disk -> protoplanets

Continued impacts enlarge planets

Heat of sun drove off H and He from inner planets (weak gravity)

When?

4.6 - 5 bya

Before this?

Hypothesis:

A supernova occurred near the solar nebula at time of its collapse (tremendous stellar explosion ejecting most of a star into space).


Our sun is a second generation star. There was an earlier star/sun that exploded into a supernova, creating heavier elements.

Evidence:

Iron is the heaviest element that can be created in a star through nuclear reactions.


Gold, silver, titanium, uranium (all heavier elements than iron in sun and on Earth) were created by neutron capture during a violent explosion marking the end of a star's life.
(See Flower, Understanding the Universe, p, 572, 1st edition).