M1 Earth’s resources
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11EES
Date: / /2021
analyse evidence of the Earth’s age, including: formation and age of zircon crystals, radiometric
techniques, meteorite evidence
1. Radiometric techniques
Youtube: How Does Radiometric Dating Work? | Ars Technica. https://www.youtube.com/watch?v=oe45GegJUvM
 The geological time scale and age of the Earth rely on radiometric dating techniques. Rocks
naturally contain radioactive elements that breakdown or decay. This is the basis for all
radiometric dating techniques.
 Radioisotopes are different forms of atoms because they contain a different number of neutrons.
This can make these isotopes unstable and therefore radioactive. To become stable, the original or
parent radioisotope decays into a stable form, called the daughter isotope. As this radioactive
decay occurs, the parent atoms reduce in number as they decay into daughter atoms.
 The time required for one-half of any original number of parent atoms to decay is the half-life. All
rocks and minerals contain radioisotopes that can be radioactively dated. This is because these
radioisotopes were incorporated into Earth when the Solar System was still being formed. These
radioisotopes act as ‘clocks’ that allow geologists to determine the age of the rocks in which they
occur based on how many half-lives have passed.
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 There are many different radiometric dating techniques as
there are different radioisotopes.
E.g. Uranium-lead dating, potassium-argon dating or
thorium-lead dating
 The age of the Earth has been radiometrically dated based
on:
-
Some of the oldest known rocks on Earth found in
western Greenland, have been dated by four
independent radiometric dating techniques. These
methods revealed that the rocks were 3.7-3.8 billion years old. Rocks in Western Australia and
other regions of the world have also been dated to a similar age at 3.4-3.6 billion years. The
debris from which these rocks would have originally formed are hypothesised to have come
from even older crustal rocks, however they have deformed over time making it difficult to
date.
2. Formation and age of zircon crystals
YouTube: How scientists found the oldest rock on Earth. https://www.youtube.com/watch?v=Lnqb_5JOpP8
 Zircons are minerals found in mostly in igneous rocks and some metamorphic rocks as tiny
crystals. Zircons contain trace amounts of radioactive elements uranium and thorium.
 During the Hadean eon (hypothesised to have intense volcanic activity), zircon crystals would have
originally formed by crystallisation from magma in cooling rocks.
 Zircons are highly resistant to erosion and chemical break down, therefore they can be found
surviving in the oldest rocks today of continental crust –allowing geologists to date them.
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 E.g. In the Jack Hills, Western Australia, zircon crystals
have been found and dated with an age of 4.4 billion
years. Because of the radioisotopes they contain, zircon
can be dated by radiometric dating techniques. To
determine the age of the zircon fragment, uranium-lead
dating is used. The zircon samples dated to 4.4 billion
years old.
 To put that age in perspective, the Earth itself formed 4.5
billion years ago as a ball of molten rock, meaning that its
crust formed relatively soon thereafter, 100 million years
later. The age of the crystal also means that the crust
appeared just ~160 million years after the very formation of the solar system.
3. Meteorite evidence
 Meteorites can be made of stone or iron. They are rocky debris in space that are pulled in by
earth’s gravity and strike the earth’s surface. The ages of meteorites are measured by radiometric
dating techniques
 There are approximately 70 well-dated meteorites that have ages of 4.4-4.6 billion years. These
meteorites represent some of the most primitive material in the solar system and have been dated
by 5 independent radiometric dating methods.
 E.g. Canyon Diablo iron meteorite was radiometrically dated using uranium-lead. This "model
lead age" is 4.54 billion years, suggesting this is the age of the Earth. Because meteorites and the
Earth contain a similar chemical composition, meteorites can be used to provide and estimate for
the age of the Earth.
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