INTRODUCTION

The Origin of the Earth

Origin of the solar system.

How and when did it begin? Any theory must answer the following:

  1. All planets revolve around the Sun in the same direction in elliptical orbits that lie in the same plane. Pluto is a slight exception, probably a captured comet.

  2. All planets except Uranus (which rolls) and perhaps Venus and Mercury which are in tidal lock with the Sun, rotate in the same direction (counterclockwise).

  3. The planets lie at regular geometric distances from the Sun. Each about 2X the distance of the previous.

  4. 99.9% of the angular momentum is in the planets.

  5. Separated into two distinct groups:

Hypotheses to explain the origin of the solar system

  1. Nebular Hypothesis - Proposed by Kant in 1755. Solar system began as a large rotating dust cloud. Explains 1 and 2 above. Cloud cools and contracts. Can use the analogy of a figure skater to explain how cloud spins faster as it contracts. Eventually spins off rings that agglomerate into planets. Big problem is the presence of all the angular momentum in the planets. Sun should be spinning faster.

  2. Collision Hypothesis - Begins in same manner as above with gas cloud cooling and contracting. Passing star yanks tongues of material from the proto-sun. Problem is that an expanding gas cloud should overcome the gravitational attraction of the Sun.

  3. Recent Theory - Revives nebular hypothesis. Sun condenses under the force of rotation and gravity. Compression causes temperature to exceed one million °C. Thermonuclear reaction occurs which synthesizes the various elements. Material is blown out into space. As tempereature falls condensation begins with higher temperature substances (heavier elements) first to condense. Hence, heavy planets are near Sun and lighter ones farther out.

Gross Structure of the Earth's Interior (Figure below)

1) Crust - 5-50 km thick. Density 2.85 gr/cm3

2) Mantle - 2900 km thick. Density 3.3 gr/cm3

3) Core - 3400 km thick. Density 15 gr/cm3

Knowledge of the interior structure of the Earth is based largely on the study of seismic waves as they travel through the Earth and studies of meteorites, which are thought to represent fragments of extraterrestrial planetary material.

Planetary Evolution

No recent hypothesis. Thought that planetesimals form by accretion of condensed clumps of silicon. oxygen, iron and magnesium. Accreted material attracted to other accreted material by the force of gravity. Compression provides heat. Further infalling chunks of material convert energy of motion to heat energy. Finally radioactivity generates additional heat. All three combine to heat Earth to the melting point. Iron and magnesium sink to the core (iron catastrophe). This differentiates the Earth into a core and mantle/crust.

Elemental Composition of the Earth

Element

Crust (wt.%)

Core (wt.%)

Oxygen

46.6

30

Silicon

27.7

15

Aluminum

8.1

1

Iron

5.0

35

Calcium

3.6

--

Sodium

2.8

--

Potassium

2.6

--

Magnesium

2.1

17

Looking at the Crust
  1. Bedrock - Solid outcropping bodies of rock. Example is the San Gabriel Mountains. Three distinct types of bedrock:

a) Igneous rock - Formed from the cooling of a magma.

b) Sedimentary rock - Rocks formed from fragments of pre-existing rocks at the Earth's surface in response to weathering.

c) Metamorphic rock - Rocks which are changed from their original nature in response to heat and pressure.

Geologic Time

Basic Concepts
  1. Principle of Uniformity - Proposed by James Hutton (aka Father of Geology). "The Present is the Key to the Past". Processes that operate on the Earth today probably operated in a similar manner in the past. Does not imply they operated at the same rate.
  2. Law of Superposition - Proposed by Nicholas Steno in 1669. In any sequence of layered rocks that have not been disturbed, the oldest layer is on the bottom and the youngest on the top.
  3. Law of Original Horizontality - (Steno) Sediments are deposited in layers parallel to the Earth's surface.
  4. Law of Cross Cutting Relationships - When one rock unit cross cuts another, the one that does the cross cutting is the younger.
  5. Law of Faunal Succession - W. Smith - Rocks with similar fossils are the same age.

Types of Geologic Time

  1. Absolute Time - The actual age of a rock or geologic event in years. Based on radioactive age dates.

  2. Relative Time - The relative of age of one rock compared to another. Is this rock younger or older than that rock?

Relative Time

 First attempts to establish ages based on relative age dating and the basic principles given above. Go through figures on relative age dating.


These attempts worked only over short distances (physical correlation). The problem is what happens over longer distances. Introduce the concept of differing sedimentary facies due to differences in the environments of deposition. Use analogy of So. Cal. (Figure) showing different environments of deposition. Another example is England and France (Figure).

Second figure shows how the correlation using fossils in England and France was much more successful (Figure). With fossil assemblages it is possible to create a chronologic sequence of rocks based on relative ages. This sequence is termed the Geologic Column.


Geologic Time Scale

  1. Precambrian - Thought to represent all rocks deposited prior to the evolution of life. Now recognized to be period of time prior to the evolution of complex organisms. (4/5 ths of earth's history)

  2. Paleozoic - Generally period of time when life was confined to the seas. (about 1/2 of the remaining 1/5th)

  3. Mesozoic - Age of reptiles and other primitive land animals. Advanced land plants (most of the remaining span of time)

  4. Cenozoic - Age of mammals. (about 1% of the earth's history)

Absolute Time

While relative time provides useful information regarding the timing of one event relative to another, it does nothing to answer the fundamental question about the age of the Earth.

Early Attempts at Absolute Age Dating

  1. Bible - Archbishop Ussher - 9 AM, Oct. 26, 4004 B.C.

  2. Rate of Cooling of the Earth - Lord Kelvin estimates the Earth is 70 MY old. Unfortunately, he was unaware of radioactive decay and neglected its contribution to the total heat being lost by the Earth.

  3. Salt in the Oceans - Estimates of about 90 MY based on this method. It neglects all the salt in trapped in sedimentary rocks.

  4. Rate of Sediment Accumulation - 3 MY to 1584 MY. Very inaccurate due to problems determining the total thickness of all sediments accumulated through geologic time.

Radioactive Decay

Matter - Anything that occupies space and can be seen by the human eye. Definition serves well until the first microscope is built (17th century). Scientists realize there are particles smaller than the eye can see.

Atom - The smallest particle of which matter is composed.

Scientists could not see atoms, but over centuries they devised a model of the atom based on simple logic. They reasoned the atom had to be composed of at least two sub-atomic particles, which they termed the proton (+) and the electron (-). The latter revolved in orbitals around the former, analogous to the revolution of the planets around the Sun. Mass deficiencies lead them to propose a third sub-atomic particle, the neutron (no charge) which they place in the nucleus with the protons. From this was born the concept of the element.

Element - A unique combination of protons, electrons and neutrons. Each element differs in the number of protons in the nucleus.

Atomic Number - The number of protons in an atom of a particular element.

Atomic Weight - The number of protons and neutrons. Electrons have less than 1% of the total mass of an atom and can be ignored.

Late in the 19th century the theory of radioactivity was first proposed. It explained why certain elements were unstable, that is, they were observed to spontaneously decay to other differing elements. Central to this theory was the concept of the isotope.

Isotope - An atom of an element that differs from another atom of the same element only by the number of neutrons in the nucleus. Example 12C 13C 14C

Certain isotopes are radiogenic (unstable) and with time will decay to another element. There are several decay schemes.

Decay Schemes

  1. alpha decay - emission of a helium nuclei (2 protons and 2 neutrons). The result is a loss of 2 in atomic number and 4 atomic weight. The decay of uranium to lead follows this scheme.

  2. beta decay - a neutron decays to a proton with the emission of a beta particle (electron). The result is a gain in 1 in atomic number and no change in mass. An example is the decay of rubidium to strontium.

  3. electron capture - an electron falls into the nucleus and combines with a proton to form a neutron. The result is a loss of 1 in atomic number and no change in mass. An example is the decay of potassium to argon. (Figure)


The usefulness of radioactive decay to geology comes from the fact that the rate of decay is a constant that is unaffected by any physical process. Figure shows how the decay of parent atoms and growth of daughter atoms with time.

How does it work in nature?

  1. Magma is generated and begins to cool. Radioactive decay occurs throughout this process, but since the magma is a liquid the parent atoms quickly separate from the daughter atoms.

  2. When the rock solidifies that separation can no longer occur and the newly formed daughter atoms are trapped.

  3. Scientists sample the rock and in the lab release both the parent and daughter atoms, counting each.

  4. The decay rates have been experimentally determined and from the decay equation the age of the rock can measured.

Using this method we can date:

Since we cannot age date most sedimentary rocks and the geologic column was complied on the basis of sedimentary rocks we have a slight problem assigning ages to rocks in the geologic column.  Figure shows how we reconciled this problem.


The age of the Earth based on

    1. meteorites - 4.5 BY

    2. moon rocks - 4.5 BY

    3. minerals - 4.0 BY

From these the age of the Earth is at least 4.5 BY and generally agreed to be 4.6 BY.