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Transcript
October 29, 2003
I. The planet Earth
Beginning of class – watching the movie “Earth Revealed” and answering the following
questions:
1. What is the chemical composition and the location of the asteroids?
- Silicon, Aluminum, Iron, Nickel
- Between Mars and Jupiter
2. Are the compositions and densities of the Earth and those of the asteroids and
meteorites similar?
- Yes, the meteorites represent the same kind of raw material that accreted
to form the Earth
3. How did the Earth form?
- By accretion (due to gravity) of dust particles into planetesimals, these
planetesimals stuck together and accreted to larger (irregularly shaped)
structures; also through accretion of small rocks from the asteroid belt
that were never incorporated into planets. Due to accretion of meteorites
the Earth grew larger and more spherical in shape.
4. Describe the early Earth shortly (less than ½ billion years) after it was formed.
- Early Earth – irregularly shaped ball of rocks
- Later - homogeneous sphere of partly molten rock (silicon compounds,
iron, magnesium oxide). The material was free to move within this sphere,
which slowly triggered the process of differentiation. After ½ billion years
– layers inside the Earth.
5. Where does the heat inside the Earth come from?
- Radioactive decay
- Some of the heat came from the impacts with rocks coming from the
asteroid belt
6. How was the Earth transformed from a homogeneous mass to a planet with
distinct layers?
- sphere of partly molten rocks
- material was free to move inside
- the dense iron-rich material sank to the center and the less-dense siliconrich material floated to the surface – process of differentiation
- layers with different densities were developed
7. What can you say about the atmosphere of the early Earth? How was the early
atmosphere created?
- Carbon dioxide rich atmosphere; also sulfur, carbon oxide, water vapor
-
Built by volcanic activities
8. Describe the young Earth (after ½ billion years after its formation).
- Hotter than today
- A lot of volcanic activity
- A lot or liquid rocks
- Atmosphere rich in carbon dioxide, no oxygen
- Probably no water
9. How was the atmosphere changed 2 billion years after the earth’s formation?
- The tiny blue-green algae (a life-form very important in the evolution of
life on Earth) takes in the carbon dioxide from the environment and
releases oxygen as a waste product
- Free oxygen began accumulating in the atmosphere about 1.8 - 2 billion
years after the formation of the Earth and the increased amount of oxygen
led to the formation of the Earth’s ozone layer
10. Why is the inner core of the Earth solid?
- Superheated metal (iron, nickel) under enormous pressure from the other
layers - can’t expand to liquidize
11. Where did the water come from?
- From volcanic activities
- In comets
12. Why do we say that the Earth is geologically alive?
- Has an internal heating - the tectonic plates are moving and interacting
with each other, producing a lot of geological activity (volcanoes,
earthquakes, mountains)
13. Why is the radioactive decay inside the Earth important?
- Provides a large part of the internal heating (takes place in the core of the
earth)
- This process creates a temperature difference inside the Earth
- This temperature difference is sufficiently large to drive convection
- Convection is responsible for the motion of the tectonic plates
- the fact that the Earth is geologically active is largely due to the
radioactivity
II. More about dating radioactive rocks
1. A method to measure the age of the individual rocks (the time that has elapsed
since these rocks were formed).
2. Uses the property of natural radioactivity
3. Structure of the atom – nucleus with protons and neutrons and electrons orbiting
the nucleus. Atomic number. Isotopes.
4. Some atomic nuclei are not stable but spontaneously split apart (decay) into
smaller nuclei
5. For a very large number of radioactive atoms it is possible to say after how long
time half of them will decay – this time is called half-life time
6. Example – if we have 1 gram of pure radioactive material whose half-life time is
100 years, after 100 years we will have ½ gram of the material + ½ gram of the
decay material
7. The radioactive element Uranum-238 will decay into Lead – 206 with a half-life
time rate 4.5 billion years. It takes 4.5 billion years for ½ of any amount of
Uranium-238 to change into Lead-206.
Uranium 235 – Lead 207 --- 713 million years
C 14 – C12/13
--- 5730 years
Uranium-238 was one of the components of the Earth’s rocks at the time
Earth was formed, and Lead – 206 was not.
8. Now, measuring the amount of Uranium – 238 and Lead-206 in the meteorites
that reach the Earth, we find that the amount of Lead-206 is slightly more than
that of Uranium- 238 – this can happen for a time interval slightly larger that 1
half-time
III. Age and origin of the Solar system - Review
1.
2.
3.
4.
Origin of the Solar system – the solar nebula
Estimating the age of the Sun – 4.6 billion years
Measuring the age of meteorites (radioactive dating) - 4.6 billion years
Dating surfaces of planets and satellites
a. The age of the surface is not necessarily the age of the planet as a whole usually the solid surfaces are formed after the formation of the planets
b. Counting the numbers of impact craters – the technique works because the
rate of creation of these craters has been constant for several billions years
c. Measuring the ages of the individual rocks using the properties of natural
radioactivity