1. Earth’s History in Fossils
P.Lobosco

2. Fossils
A fossil if the remains or evidence of a living thing. A fossil can be the bone of an organism or the print of a shell. A fossil can be a tunnel lefts by a worm. The most common fossils are bones, shells, pollen grains and seeds.
Most fossils are buried in sediments. Plants and animals that live near water are preserved more often. Most fossils are found in sedimentary rock.

3. Fossil Identification
Fossils are identified according to the process by which it was formed.
Petrification
Molds and Casts
Imprints
Preservation of Entire Organism
Trace Fossils

4. Petrification
Petrification means turning into stone. When the dinosaurs died, the soft parts of their bodies decayed quickly. Only the hard parts, the bones, were left. Many of these bones were buried in sediments of mud and wet sand. The water dissolved minerals in the mud and as the water flowed through pore in the bones, minerals were deposited. The water evaporated and the bones were turned to stone.

5. Molds and Casts
Two types of fossils are formed when an animal or plant is buried in sediments that harden into rock. If the soft parts of the organism decay and the hard parts are dissolved by chemicals an empty space will be left in the rock. The empty space, called a mold, has the same shape as the organism. Sometime the mold is filled in by minerals and they harden to form a cast in the same shape as the organism.

6. Imprints
Sometimes a fossil is formed before the sediments harden into rock. Thin objects, such as leaves and feathers, leave imprints or impressions in soft sediments such as med. When the sediments harden into rock, the imprints are preserved as fossils.

7. Preservation of Entire Organism
Freezing prevents substances from decaying. Some animals have been preserved by freezing. Wooly mammoths have been found in large blocks of ice. A furry rhinoceros was found in the loose frozen soil in the arctic. When the resin form evergreen trees hardens it forms amber. Insects have been trapped and preserved in amber. Some animals have been found in tar pits.

8. Trace Fossils
Trace fossils are fossils that reveal much about an animal’s appearance without showing any part of the animal. Trace fossils area the marks or evidence of animal activities. Tracks, trails, footprints, and burrows are trace fossils.

9. Interpreting Fossils
Fossils indicate that many different life forms have existed at different times throughout the Earth’s history. Some scientists believe that for every organism that exists today, 100 have become extinct. When fossils are arranged according to age, they show that living things have evolved. Fossils also indicate for the Earth’s surface has evolved. Fossils give clues about past climate.

10. A History in Rocks and Fossils
The Law of Superposition states that in a series of sedimentary rock layers, younger rocks normally lie on top of older rocks. This idea was proposed by James Hutton in the 18th century. The processes of weathering, erosion and deposition have always acted on the Earth’s surface. Scientists use this law to determine whether a fossil or layer or rock is older or younger than another fossil or layer of rock.

11. Index Fossils
The law of superposition helps scientists put in order the record of Earth’s past for one particular location. Index fossils are fossils of organisms that lived during only one short period of time. Scientists assume that index fossils of the same type of organism are all nearly the same age. So a layer of rock with one type of index fossil in it is close in age to another layer of rock with the same type of index fossil in it.

12. Unconformities
Sedimentary rock layers and the fossils found within them may be disturbed by powerful forces within the Earth. The rock layers may be folded, bent or twisted. Sometimes older layer are uplifted to the Earth’s surface. At the surface, the exposed rocks are weathered and eroded. Sediments are then deposited on top of the eroded surface of the older rocks. The deposited sediments harden to from new horizontal sedimentary layers. The old eroded surface beneath the newer rock layer is called an unconformity. There is a wide gap in the ages of the rock layers above and below an unconformity.

13. Faults
A break or crack along which rocks move is called a fault. The rock layers on one side of a fault may shift up or down relative to the rock layers on the other side of the fault. Because faults can occur only after rock layers are formed, rock layers are always older than the faults they contain. The relative age of a fault may be determined form the age of the sedimentary rock. Scientists can determine the forces that have changed the Earth’s surface by examining the faults in rock layers.

14. Intrusions
The relative ages of igneous rock formations can also be determined. Magma often forces its way into layers of rocks. The magma hardens in the rock layers and forms an intrusion. An intrusion is younger than the sedimentary rock layers it passes through.

15. Extrusions
Sometimes magma reaches the surface of the Earth as lava and hardens. Igneous rock that forms on the Earth’s surface is called an extrusion. Extrusions are younger than the rock layers beneath them.

16. Radioactive Dating
The discovery of radioactive elements in 1896 led to the development of an accurate method of determining the absolute age of rocks and fossils. An atom of radioactive element has an unstable nucleus, or center, that breaks down or decays. During radioactive decay, particles and energy called radiation are released by the radioactive element. As some radioactive elements decay, they form decay elements. A decay element is stable. This breakdown occurs at a constant rate.

17. Half-Life
Scientists measure the decay rate by a unit called a half-life, the time it takes for half of the radioactive element to decay. If certain radioactive elements are present in a rock or fossil, scientists can find the absolute age of the rock or fossil. Many different radioactive elements are used to date rocks and fossils. One radioactive element used to date the remains of living things is carbon-14. Nitrogen is the decay element of Carbon –14.

18. Other Methods of Absolute Dating
There are other methods of absolute dating.
Varve
Splitting or fission of the nuclei of radioactive atoms

19. Varve
A varve is sediment that shows a yearly cycle. These sediments are often deposited in glacial lakes. Each year a light-colored sandy layer is deposited in the summer and a dark-colored layer is deposited in the winter. Be counting the pairs of layers it is possible to tell haw many years a a glacier was in a particular place before it disappeared.

20. Splitting of Radioactive Atoms
The splitting or fission of the nuclei of radioactive atoms is another method of dating. The high-energy particles that are split off leave microscopic tracks that can be counted. The older the rock, the greater the number of tracks. This method can be used to date moon rock samples and meteorite samples as well as samples of Earth rocks.

21. The Age of the Earth
Scientists use radioactive dating to determine the age of rocks. By finding the age of rocks, the can estimate the age of the Earth. Scientists have found some rocks in South Africa that are more than 4 billion years old. Radioactive dating of moon rocks brought back by the Apollo missions show them to be 4 to 4.6 billion years old.