The Geological Timescale and Absolute Dating (1.3.4)

Question 1

What is the geological column?

Answer 1

• Smith’s work led to define a universal geological column.
• Derived sub-divisions: Eons, Eras, Periods, Epochs.
• Refined into geological timescale today, with a few added rows like Ordovician.

Question 2

How are they dated - Herodotus of Halicarnasus

Answer 2

• Annual flooding of Nile provided sediment for crops, Nile Delta formed of thousands of layers, so must have taken thousands of years to build up by floods.

Many authors through C19th attempted calculations, but concept flawed because Earth’s sedimentary record is very incomplete.

Question 3

How are they dated - John Joly

Answer 3

• Sodium supplied to oceans by rivers, river water inflow rate known, Na content known in river, volume of ocean known, thus can calculate time required for ocean to acquire Na content, calculated Earth was 90 million years old.

Method flawed because of chemical exchange between seawater and rocks in sub-seafloor.

Question 4

How are they dated - Numerical Ages

Answer 4

• Discovery of radioactivity (1896) = precise dating of geological events.
• Relies upon radioactive decay of atoms in minerals.

Question 5

What is geochronology?

Answer 5

Science of numerical dating.

Question 6

How are they dated - Radioactive Decay

Answer 6

• Isotopes = elements with varying numbers of neutrons in nucleus but different mass numbers. Radioactive isotopes decay spontaneously.
• Radioactive decay proceeds along decay chain (decay may create new, unstable elements that also decay, proceeds to stable element end point).
• Decay occurs at fixed, unique rate for particular isotope = decay constant.
• Usually expressed as its half-life.

Question 7

What is Half-Life?

Answer 7

Time taken for half of unstable nuclei to decay. As parent (isotope decaying) disappears, daughter (product of decay) increases.

Question 8

Radiometric Dating - Age of Mineral

Answer 8

• Measuring ratio of parent-to-daughter isotopes.
• Calculating time elapsed using known half-life.
• Requires high analytical precision mass spec (£££).
• Minerals only dated if contain sufficient parent/daughter isotopes to be measurable.
• Igneous and metamorphic rocks are best for geochronology, but dates get reset if rock is reheated.

Question 9

Radiometric Dating - Establish a Numerical Timescale

Answer 9

Aim to find dateable tie points in sediment record.

Question 10

Other (Numerical) Dating Methods: Magnetostratigraphy

Answer 10

Earth’s magnetic field reverses periodically and repeatedly. Trace of magnetic field (normal/reverse) is recorded by rocks - potentially tied to Age.

Question 11

Other (Numerical) Dating Methods: Growth Rings

Answer 11

Trees, shells, corals.

Question 12

Other (Numerical) Dating Methods: Annual Layers in Sediments or Ice

Answer 12

Generally only suitable for dating short time intervals in geologically recent past.

Question 13

Dating the Age of the Earth

Answer 13

• Oldest rocks on Earth’s surface so far are 4.03Ga (billion years old), in North Canada - highly deformed metamorphosed gneisses.
• Oldest terrestrial materials are 4.4 billion years old (grains of zircon crystal in 3.0 billion years old sandstone in Western Australia - Jack Hills).
• Clair Patterson (1955) obtained age of 4.55 +/- 0.07Ga from meteorites, by U-Pb dating. Taken as age of Earth (4.543Ga).
• Best accepted age of solar systems - 4.571 +/- 0.001Ga.