Time and change: dating methods Absolute time scales and absolute age
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It is possible to work out the absolute ages of rocks using . Some minerals in rocks contain radioactive elements. The of these elements break down at a steady rate to form new elements. Isotopes are substances with the same atomic number but different mass number. e.g. The isotopes of potassium are: K, K, K . The top number gives the (no. of protons + no. of neutrons). The bottom number gives the atomic number (no. of protons = no. of electrons) Radioactive elements are those which have unstable which break down and give off alpha, beta or gamma radiation. When an alpha or beta particle is lost from a radioactive atom it into a product which is different from its parent (the original material). The rate of decay is steady. The time for the original number of atoms to be reduced by one half is the .
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Number of 1 2 3 4 5 half lives
The age of a rock can be calculated if (1) the proportion of parent to daughter product can be measured (2) the is known.
The age calculated is a true measure of the age of formation if: (a) there have been no or gains of daughter products by other processes (b) there have been no losses or of parent products by other processes (c) the mineral or rock was formed in a period of time compared to its age. Events which can be dated are: (1) crystallisation of rocks from magma (2) recrystallisation of rocks to form rocks (3) uplift & cooling of mountain chains (4) time of deposition of rocks (provided the rock contains one mineral formed at the time of deposition).
Potassium - Argon Dating Method ( K changes to Ar)
This has a half life of million years. It is the most important dating method used today because many rocks contain - rich minerals. It can be used to date and metamorphic rocks which contain the minerals biotite, hornblende, plagioclase feldspar, and K- rich feldspar. (All these minerals contain potassium). The method can be used on rocks which contain the mineral glauconite. (This is a mineral which contains & which forms at the time of deposition). It can be used on whole samples e.g. fine grained lavas & slates. It can be used to date rocks from 1 million years to 3900 years old.
Problems in absolute dating
1. Isotopes with half lives within one or two orders of magnitude of the age of the Earth are ideal. This is so that the amounts of product can be measured accurately. 2. The calculated age is only correct if there is no loss or gain of material other than by radioactive decay. (n.b. Argon is a and it may be lost from the rock, or added, from other sources during metamorphism when the rock is heated up). 3. The calculated age is correct only if the is accurately known. Completed exercise: Time and change: dating methods Absolute time scales and absolute age
It is possible to work out the absolute ages of rocks using radioactive dating. Some minerals in rocks contain radioactive elements. The isotopes of these elements break down at a steady rate to form new elements. Isotopes are substances with the same atomic number but different mass number. e.g. The 39 40 41 isotopes of potassium are: 19 K , 19 K , 19 K . The top number gives the mass number (no. of protons + no. of neutrons). The bottom number gives the atomic number (no. of protons = no. of electrons) Radioactive elements are those which have unstable nuclei which break down and give off alpha, beta or gamma radiation. When an alpha or beta particle is lost from a radioactive atom it decays into a daughter product which is different from its parent (the original material). The rate of decay is steady. The time for the original number of atoms to be reduced by one half is the half life.
Original 1 1 1 1 1 content left 2 4 8 16 32
Number of 1 2 3 4 5 half lives
The age of a rock can be calculated if (1) the proportion of parent to daughter product can be measured (2) the half life is known.
The age calculated is a true measure of the age of formation if: (a) there have been no losses or gains of daughter products by other processes (b) there have been no losses or gains of parent products by other processes (c) the mineral or rock was formed in a short period of time compared to its age. Events which can be dated are: (1) crystallisation of igneous rocks from magma (2) recrystallisation of rocks to form metamorphic rocks (3) uplift & cooling of fold mountain chains (4) time of deposition of sedimentary rocks (provided the rock contains one mineral formed at the time of deposition).
Potassium - Argon Dating Method ( 40 K changes to 40 Ar )
This has a half life of 11900 million years. It is the most important dating method used today because many rocks contain K- rich minerals. It can be used to date igneous and metamorphic rocks which contain the minerals biotite, hornblende, plagioclase feldspar, and K- rich feldspar. (All these minerals contain potassium). The method can be used on sedimentary rocks which contain the mineral glauconite. (This is a mineral which contains potassium & which forms at the time of deposition). It can be used on whole rock samples e.g. fine grained lavas & slates. It can be used to date rocks from 1 million years to 3900 million years old.
Problems in absolute dating
1. Isotopes with half lives within one or two orders of magnitude of the age of the Earth are ideal. This is so that the amounts of daughter product can be measured accurately. 2. The calculated age is only correct if there is no loss or gain of material other than by radioactive decay. (n.b. Argon is a gas and it may be lost from the rock, or added, from other sources during metamorphism when the rock is heated up). 3. The calculated age is correct only if the half life is accurately known.