age of earth

indirect estimates

During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques. What methods can you think of for doing this? One example is that by measuring how much sediment a stream deposited in a year, a geologist might try to determine how long it took for a stream to deposit an ancient sediment layer. Not surprisingly, these methods resulted in wildly different estimates. A relatively good estimate was produced by the British geologist Charles Lyell, who thought that 240 million years had passed since the appearance of the first animals with shells. Today scientists know that this event occurred about 530 million years ago. In 1892, William Thomson (later known as Lord Kelvin) calculated that the Earth was 100 million years old, which he later lowered to 20 million years. He did this systematically assuming that the planet started off as a molten ball and calculating the time it would take for it to cool to its current temperature. This estimate was a blow to geologists and supporters of Charles Darwins theory of evolution, which required an older Earth to provide time for geological and evolutionary processes to take place. Kelvins calculations were soon shown to be flawed when radioactivity was discovered in 1896. What Kelvin didnt know is that radioactive decay of elements inside Earths interior provides a steady source of heat. He also didnt know that the mantle is able to flow and so convection moves heat from the interior to the surface of the planet. Thomson had grossly underestimated Earths age.

more quantitatively

Radioactivity turned out to be useful for dating Earth materials and for coming up with a quantitative age for Earth. Scientists not only date ancient rocks from Earths crust, they also date meteorites that formed at the same time Earth and the rest of the solar system were forming. Moon rocks also have been radiometrically dated. Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well- defined timeline of Earth history. With information gathered from all over the world, estimates of rock and fossil ages have become increasingly accurate. This is the modern geologic time scale with all of the ages. Click image to the left or use the URL below. URL:

instructional diagrams

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about how old is planet earth?

a) 6,000 years

b) 4.6 million years

-->  c) 4.6 billion years

d) 6.0 billion years

how have scientists tried to calculate the age of earth?

a) measure how much sediment a stream deposited in a year and calculate how long it would take to deposit a sediment layer.

b) calculate how long it would take for the earth to cool to its current temperature from a molten ball.

c) calculate the rate of evolution and determine how long it would take for modern animals to evolve.

-->  d) all of these.

scientists can determine an exact number for the age of earth materials using radioactivity.

-->  a) true

b) false

this material is thought to have formed when the solar system was forming ______.

a) earths oldest rocks

-->  b) meteorites

c) earths atmosphere

d) all of the above

earths age is calculated using ages from

a) meteorites

b) earths oldest rocks.

c) lunar rocks.

-->  d) all of the above

lord kelvin calculated earths age at 100 million years. what he didnt know was that

-->  a) radioactive decay adds heat.

b) the cooling rate of earth materials is slower than he thought.

c) earth started out much hotter than he thought.

d) all of these.

geologists have pieced together earth history using all of the scientific tools available to them.

-->  a) true

b) false

earth had to be older than 100 million years because of

a) the amount of time it takes one stream to deposit a sediment layer.

b) the amount of time it would take for earth to cool.

-->  c) the length of time it would take for organisms to evolve into modern animals.

d) the amount of time that passed since the dinosaurs died out.

many lines of evidence indicate that earth is 4.6 billion years old.

-->  a) true

b) false

without radiometric dating, scientists would still know the absolute age of earth.

a) true

-->  b) false

diagram questions

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