Topic 1-Dynamic Earth. Outcomes: By the end of this chapter you should be able to: Describe evidence that the Australian continental landmass began developing 4.1 billion years ago ( byo ). Explain how radioisotopes can be used to date rocks
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By the end of this chapter you should be able to:
Describe evidence that the Australian continental landmass began developing 4.1 billion years ago (byo).
Explain how radioisotopes can be used to date rocks
Outline geological evidence that provides information about patterns of continental movement
Explain how convection currents in the asthenosphere can cause the movements of plates
Explain the interaction of paltes during subduction, collision and break-up
Outline the movements of Australia through geological history
There are 3 parts to this topic
There is evidence to suggest that the Australian continental landmass began developing 4.1 billion years ago. In this first part of Dynamic Earth, you will be looking at the sort of evidence that leads scientists to this conclusion. The discovery of radioactivity around 1900 was a huge step forward in this understanding.
To understand why some elements are radioactive, you must understand the structure of an atom.
Structure of an Atom:
The Nucleus is the central part, or core of an atom and contains protons and neutrons.
Protons: positively charges
Neutrons: No charge, neutral
As a result, the nucleus has an overall positive charge.
Protons and neutrons have approximately the same mass.
Electrons are quite different in a number of ways. They:
In a neutral atom, the number of electrons orbiting the nucleus equals the number of protons.
Elements are characterised by the number of protons present in the nucleus. We call this the atomic number.
It is this atomic number that is used to arrange the elements on the periodic table.
We already know the majority of the mass of an atom is in the nucleus. The atomic mass of an atom is obtained by adding the number of protons and neutrons together.
This is a diagram representing Helium. This atom has 2 protons, 2 neutrons and 2 electrons.
What is the atomic mass of helium?
The number of protons in the nucleus of an atom never varies. However the atomic mass of an atom can vary.
Remember: atomic mass = protons + neutrons.
If the number of protons cannot vary, what do you think causes the variation in mass?
Isotopes are varying types of atoms for the one element. The variation is in the number of neutrons and therefore the mass of the atom.
Isotopes of an element have the same number of protons and are therefore placed in the same position in the Periodic Table.
Lets look at an Example:
All lead atoms contain 82 protons in their nucleus and therefore have an atomic number of 82.
However, using a mass spectrometer , it has been found that lead has four different variations, each having a different mass. This variation in mass is caused by a variation in the number of neutrons in their nucleus.
The four lead isotopes are:
The number refers to the atomic mass of each atom
Complete the table
Dynamic Earth Vocab List
As stated in the previous lesson, the number of protons in the nucleus of a stable atom never varies. However, all atoms are not stable, particularly the larger heavier atoms such as uranium and thorium located at the bottom of the Periodic Table.
Unstable atoms can emit protons and neutrons from their nuclei and break down to form different elements.
An unstable atom is said to be radioactive.
To understand what an unstable or radioactive atom is we must look again at the structure of the nucleus.
Remember the nucleus has an overall positive charge because it is made up of protons (+) and neutrons (= no charge)
It would be expected that the protons in a nucleus would be repelling against each other because they have the same charge.
Why these protons are held together in the nucleus is still not fully understood but it is thought that an enormous amount of energy is used to hold these protons together and that the neutrons have something to do with it.
If the nucleus breaks apart, some of this energy is released in the form of radiation. This is what happens in an unstable atom.
The nuclei of unstable atoms can emit (give off) or radiate protons and neutrons in two forms:
Question: What would the mass of an particle be?
Every time an unstable or radioactive atom emits an particle, its atomic mass decreases by 4 and its atomic number decreases by 2.
Not only can large and heavy atomic nuclei result in an unstable atom, nuclei that have to many neutrons for the number of protons can also be unstable.
If a nuclei has to many neutrons, these excess neutrons change into protons and as a result electrons are then emitted.
These emitted particles are called particles.
Atoms that emit radiation increase the number of protons in their nucleus at the expense of neutrons.
How will this effect the atom’s:
Atomic Mass: (Stay the same) nothing is lost, only changed.
Atomic Number: (goes up by 1) neutrons are changed into protons
Half-lives and Dating rocks
In the previous chart, uranium-238 decays to the more stable lead-206. Below is a list of other examples of atoms decaying to become more stable :
In these examples, the unstable atoms on the left are called the parent material and the more stable atoms on the right are called the daughter product or remnant isotope.
The time taken for each of the parent atoms to decay to their daughter products varies from millions of years to minutes.
The time taken for half of the parent material to decay into its daughter product is know as the Half Life of that parent material.
The half-life for each radioactive element remains the constant. For example it takes 5370 years for half a sample of carbon-14 to decay to nitrogen-14
It will take another 5370 years for half the remaining carbon-14 to decay into nitrogen-14. This is known as the second half-life.
This means it has taken 10740 years for ¾ of the original carbon-14 to decay. (two half-lives)
Using the half-lives from radioactive elements to calculate age is known as radiometric dating.
Dating rocks using radioactive elements can indicate a particular time of formation.
When minerals in igneous rocks are first formed after cooling, the amount of each radioactive mineral present at this time is 100%
Immediately after formation these unstable radioactive minerals begin to break down
Think about this:
If you were given two samples of rock, one sample containing 95% of the original radioactive parent element and the other sample had 30% of the same original radioactive parent element, which of these rocks do you think would be oldest? Why?
Radiometric dating methods have enabled scientists to determine the time of mineral formation of some of the oldest rocks on Earth.
The oldest minerals ever dated in Australia are zircon crystals found in quartzite rock at Mt. Narryer in the Murchison region of Western Australia.
The correct types of radiometric isotopes have to be used to date Precambrian rocks. Precambrian time starts about 4.7 billion years ago when the Earths crust began to form to the start of the Cambrian about 542 million years ago.
Because the Precambrian dates back to far, uranium/lead isotopes are used to date rocks within this period of time. Naturally occurring uranium contains two radioactive isotopes both with very long half-lives.
Uranium usually occurs as
trace elements in minerals
such as zircon.