Question

key question: how do scientists model the age of the earth?
the age of the earth is currently accepted as around 4.5 billion years old. this was not always the way. until the renaissance period, starting around 500 years ago, the earth was accepted as being quite young (anywhere between 7500 years old to 6000 years old). however, discoveries over the last 500 years, and especially in the last 200 years, have produced evidence that the earth, all of the other planets, and the sun are very old.
charles lyell was a scottish geologist who helped demonstrate how natural events could be used to explain earths history. in the summer of 1828, lyell travelled through france and italy studying volcanoes. when lyell reached mount etna, in sicily, he observed numerous phenomena that could only be explained if the earth was much older than people thought. firstly, he noticed that mt etna was built up from many small cones formed by past eruptions, but only one of these cones, monti rossi (46 m high), had actually newly appeared in the last two hundred or so years. given that mt. etna was more than 3000 m high this meant mt etna must have been tens, if not hundreds, of thousands of years old.
lyell also discovered that shellfish sold in the fish markets of nearby towns were identical to fossil shellfish he found in limestone beds at the base of mt. etna. importantly, mt etna appeared to be on top of the limestone beds. so if mt etna was hundreds of thousands of years old, the fossil shellfish must have been much older than that.
this suggested to lyell that geological events happened very slowly. if the building of volcanoes and fossilization of shells happened so slowly, then the earth must have been at least tens of millions to hundreds of millions of years old. much older than was currently accepted.

1. lyell reasoned the age of the earth partially using a method called relative dating. describe how this method works:
2. explain why relative dating cannot produce a definitive age for the earth:
3. what would be needed in order to produce a definitive age for the earth?
4. which of the layers shown on the right is the oldest layer?

Explanation:

1. Relative dating works by comparing the positions of rock layers (strata). Younger layers are deposited on top of older ones. So, it determines the age of a layer relative to other layers.
2. Relative dating cannot give a definitive age because it only shows the order of events (which is older or younger than another) but not the actual numerical age (in years).
3. Absolute dating methods (like radiometric dating which measures the decay of radioactive isotopes in rocks) would be needed to produce a definitive age for the Earth.
4. According to the principle of superposition (in undisturbed sedimentary rock layers, the oldest layer is at the bottom), layer C is the oldest.

Answer:

1. Relative dating compares rock - layer positions; younger layers are on top of older ones.
2. Relative dating only shows age order (older/younger), not numerical age.
3. Absolute dating methods (e.g., radiometric dating).
4. Layer C.