Lecture Twenty One - Concepts of time and the age of the Earth Flashcards Preview

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Flashcards in Lecture Twenty One - Concepts of time and the age of the Earth Deck (7)
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1
Q

What relative geological time scale was established in the 19th century?

A

Based on: - Superposition relationships between different rock successions with different fossil assemblages. - Evolutionary succession of life forms. - Major extinction events.

2
Q

Explain radioactive decay.

A

Involves breakdown of an atom:

  • Release of energy (gamma decay), or
  • Release of energy and an electron (beta decay), or
  • Release of energy and part of nucleus of an atom (equal to an alpha particle = 2 protons and 2 neutrons).

Transformation of an unstable parent atom to a stable daughter atom and energy and/or beta, alpha particles.

3
Q

What is a radioactive half life?

A

For each radioactive isotope, decay occurs at a constant rate, calle the half-life.

This is the time taken for half of the parent atoms to decay to daughter atoms.

4
Q

What is radiometric dating?

A

When a mineral crystalises, all atoms and ions are locked into the crystal at a temerature called the blocking temperature (a closed system).

It is possible to measure the numbers of parent and daughter atoms using mass spec.

Using this experiemntally calculated half life, the age of the mineral can be determined.

5
Q

How can igneous rocks be dated?

A

Igneous rocks - rocks that cool and crystallise from molten magma.

Two types:

Volcanic - errupted from vents on Earth’s surface -> lavas and explosive pyroclastic deposits.

Plutonic - magma bodies that cool and crystalise below the Earth’s surface (intrusive bodies).

Dating - the age that the rock crystallised from magma.

6
Q

How can igneous rocks be dated?

A

Aggregates of mineral and rock fragments eroded from old exposed source rocks.

Deposited in sedimentary environments.

Buried and compacted to make a rock.

Age that original grain formed is not the age of the sedimentary rock, as the rock is made up of a number of different aged minerals.

7
Q

How can metamorphic rocks be dated?

A

Metamorphic rocks = Igneous and sedimentary rocks that are heated and subject to pressure during major tectonic deformational events.

  • Chemical reactions cause.
  • Growth of new metamorphic minerals.

Age of new mineral formation = age of change in rock, not age of original rock.

I.e. the age is calculated from when the rock cooled enough to form a crystal.