Lecture 7

Question 1

Igneous rock formation

Answer 1

Cooling and crystallization of magma

Question 2

Metamorphic rock formation

Answer 2

Partial melting and recrystallization of minerals in rocks

Question 3

Sedimentary rock formation

Answer 3

Burial and diagenesis of sediments

Question 4

Radioactive isotopes allow ______

Answer 4

The ages of Earth material to be determined

Question 5

Absolute dating

Answer 5

Sometimes referred to as direct dating

Can directly update the rock using the minerals or material it contains

Question 6

Absolute dating is mainly used for which type of rock

Answer 6

Igneous because it contains radioactive isotopes

Question 7

Relative dating

Answer 7

Indirect dating

Can not directly date the rock, therefore must rely on other methods that can put it in “relative” age order

Used commonly for sedimentary rocks

Question 8

Do stable or unstable isotopes decay

Answer 8

Unstable-they emit particles or energy (radioactivity)

Each decay is random but rate is predictable!

Question 9

Radioactive decay enables measurement of

Answer 9

Geological time

Question 10

Requirements for age determination

Answer 10

1. Half life or decay constant of the isotope
2. original amounts of parent isotope in the sample and daughter isotope in the sample
3. final amount of parent or daughter isotope in the sample

Typically we find parent isotope in the original sample but no daughter

Question 11

Uranium-lead Zircon dating

Answer 11

ZnSiO4

Amounts measured by mass-spectrometer which zaps grains to show decay areas

Question 12

Limitations to radioactive dating

Answer 12

Mineral grains must have formed at the same time as the rock

Need to use another method for sedimentary rock because of this

Question 13

Stratigraphy

Answer 13

The order and relative position of rock strata (generally sedimentary and layered volcanic rock)

Question 14

How is indirect dating for sedimentary rock achieved?

Answer 14

1. Order of strata
2. Cross cutting relationships
3. Correlation (lithology, fossils, Chemistry, paleo magnetism)

Question 15

Order of strata

Answer 15

Principle of horizontality
-sedimentary rock is initially horizontal

Principle of superposition
-in an undisturbed succession, rocks get younger as you move up the section

Question 16

Correlation by lithostratigraphy

Answer 16

Lithology-classifying rocks through their general physical characteristics

E.g. grain size, mineral composition, colour

Question 17

Correlation by fossils

Answer 17

Bio stratigraphy-using fossils or fossil assemblages to correlate and get relative ages

Question 18

Correlation by Chemistry

Answer 18

Chemostratigraphy-using chemical signatures of rocks to correlate and relatively date rocks

E.g. stable isotopes, iridium spikes

Question 19

Correlation by magnetism

Answer 19

Paleomagnetism-measuring orientation of magnetic minerals in sediments or layered volcanic rock to correlated or relatively date rocks

Question 20

Cross cutting relationship

Answer 20

Also show relative ages if features

• igneous intrusive relationships
• faults
• uncomformities

Question 21

Igneous intrusive relationships

Answer 21

Intrusion must be younger than host cell

Question 22

Faults

Answer 22

Faults must be younger than surrounding rock