Finding the age of earthChapter 12Chemistry

Layers of rock

How fossils form in rocks- Fossils are the remains of once living things.- Most fossils are formed from plant and animal

bodies.

1) Animals or plants covered quickly after death (stops scavengers ripping up the body)

2) Covering contains little air and oxygen (decomposers cannot thrive and rot down the body)

3) Sediments harden to form rock

4) Tissues of the body in the rock is replaced by chemicals in

water passing through rocks (replacement)

5) Water containing dissolved minerals seeps into the tissues.

The minerals strengthen the tissues into rock (petrification)

OR

Trace fossils- Things left behind EX. Eggs, droppings and tracks.

How ideas changed about fossils

Ammonite

- This ammonite fossil was first thought to be curled up bodies of stone snakes.

- Later they were found to be the shells of creatures related to the octopus, which swam in ancient seas.

- The word fossil was first used by Georgius Agricola (1494 – 1555), a German doctor.

- He used the word to describe anything that was dug out of the ground.

- Some people thought that it was a coincidence that the stones looked like the bodies of animals.

- Some triangular shaped stones had been widely known as tongue stones.

- Nicolas Steno (1638 – 1686), a Danish geologist, remembered tongue stones when he was examining the teeth of a shark.

- This led him to believe that tongue stones where teeth of ancient sharks.

- Once scientists had a clear idea that fossils were the remains of plants and animals that lived in the past, they started to examine them thoroughly.

- They found out: a) Some fossils where found in many layers.b) Some where found in one or few layers.c) Some where widespreadd) Others found in only few places.e) Some where easy to recognise.f) While others needed close and thorough examination to

identify them.

- Geologists discovered that some fossils were just found in just one layer (organism lived in one time period)

- Some fossils were found in many parts of the world and they were easy to recognise (useful in indicating the age of the rock). They are known as indicator or index fossils.

- Index fossils are useful where sedimentary rocks have formed in the same time period but in different ways. Ex. rock formed from mud an rock formed from limestone.

The fossil record- Each group of living things has a fossil record

(shows when the members of the group existed).

- Some living things like algae and bacteria have a long fossil record (found in rocks from Precambrian onwards).

- The mammal group has a much shorter fossil record (found in Jurassic period onwards)

The fossil record and the age of the earth- Fossils found in a particular layer

can be used to find out about the conditions on Earth at that time.

- Example 1:Fossils of plants and animals in Carboniferous times are of living things that lived in damp conditions (shows that may areas where swamps)

- Example 2:Fossils of living things in Permian times show that there where deserts.

- Fossils show that the features of some living things has changed over time.

- Scientists think that a change in feature happens in one individual in a species and then passed on to the following generations.

- In some circumstances the individual with the new feature forms a new species . Over a long period of time the new species might replace the old species (becomes extinct). This process of species change is called evolution.

- The fossil record does not give a time when scientists think the Earth formed

but it does suggest that the Earth is very old.

Finding the age of rocksBy using:

Varves Radioactive materials

Finding the age of rocks- 1st method used to tell the actual age of

rocks by (Gerard De Geer 1858 – 1943):

- He studied the building up sediments in lakes that received water from melting glaciers.

- The glaciers have a period of melting every year during the summer months, and the water from them carries sand and silt down the streams into rivers and lakes.

- In winter water slows down and carries only clay particles which settle over the sand and silt layer. Together the two bands count as a year.

Radioactive materials