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Earth’s Internal Heat, Isotopes &�Radioactive Decay

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�EARTH’S INTERNAL HEAT ENGINE AND CONVECTION��The diagram below shows a cross section of Earth to a depth of approximately 700 km (not to scale). Arrows show motion of Earth’s interior due to heat generated motion.�

1. Earth’s Interior

2. Asthenosphere

3. Convection

4. Divergence

6. Convergence

5. Convergence

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Rayleigh-Bernard Convection

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CROSS-SECTION VIEW OF EARTH’S INTERNAL STRUCTURE

Since its formation, Earth has been giving off heat through two processes:

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1. Decay of radioactive materials

2. Residual heat from Earth’s formation

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The phenomenon of radioactivity relates to our story about the age of the Earth in two ways:  

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  • As radioactive elements decay in the

Earth, they heat up the surrounding

rocks.

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(2) Radiometric Dating

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  • ATOM: the smallest particle that has the properties of an element.

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    • Nucleus (99% of atom’s mass):
      • uncharged Neutrons
      • positively charged Protons
      • Net charge of the nucleus is POSITIVE

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    • Electrons in constant motion create a “cloud” like a fan around the nucleus.
      • Charge of an electron is NEGATIVE

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The Atom

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ISOTOPES

  • Isotopes are atoms that have the same # of protons, but a different # of neutrons.

Number of Neutrons = Mass # - # of protons

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  • The difference in the number of neutrons cause isotopes to have different mass numbers

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Mass Number = protons + neutrons

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  • Isotopes are unstable and fall apart releasing atomic particles – they are radioactive, release energy

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Example of an Isotope

  • Example:

Carbon-12 (NORMAL) vs. Carbon-14 (ISOTOPE)

12C

Mass # = 12; Atomic # = 6

(6P, 6E, 6N)

How did we determine there were 6 neutrons?

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14C

Mass # = 14; Atomic # = 6

(6P, 6E, 8N)

How did we determine there were 8 neutrons?

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To estimate the age of a rock:

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D = amount of daughter product.

P = amount of parent.  

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For a particular radioactive element in a rock, determine the present ratio = D/P.

D

P

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  • Rate of decay (from theory and measurement)

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  • Make assumptions about original ratios (from theory of geochemistry).

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As time passes, the amount of parent decreases, and the amount of daughter product increases. This provides a way of estimating the amount of time since the "clock" got started (i.e., since the rock solidified).

D

P

D

P

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Half Life

Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb

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Half-Lives of Radioactive Isotpes