BY ADITI ATMASIDHA

FINDING THE AGE OF EARTH

How Rock Layers Formed ?

Rock layers are formed by sedimentation.

Existing rocks weather and erode, are

deposited, covered with additional sediments,

compacted and cemented together by the

precipitation of minerals from the fluids

between the particles. There are other

instances where layers can be created, such

as repeated volcanic eruptions, but the bulk of

rock layers are sedimentary in origin.

Naming the rock layers

When scientist realized that the rock layers

could help them understand changes in the

Earth’s crust, they gave them names. Most of

the deeper layers were named after an area

where they form a major part of the landscape.

The names of the two most recent layers of

rocks and time periods come from another way

of dividing rock layers. This way was to divide

geological time into four periods- primary ,

secondary , tertiary and quaternary.

Name of rock layer and

time period

Meaning Time when the layer

began to form millions of

years ago

Quaternary The 4th period About 2.5

Tertiary The 3rd period About 65

Cretaceous Named after chalk rocks

found in France

About 145

Jurassic Named after the Jura

mountains, found in

France , Germany and

Switzerland

About 200

Triassic Named after 3 distinctive

layers of rocks found in

Germany

About 251

Permian Named after region in

Russia

About 300

Carboniferous Named after the large

amounts of coal found in

layer

About 360

Devonian Named after region of About 416

Cambrian An ancient name for

Wales

About 542

Ordovician Named after an ancient

tribe in Wales

About 488

Silurian Named after an ancient

tribe in Wales

About 444

Fossils

There are thousands upon thousands of layers

in the earth's crust. However, scientists have

grouped the layers into major groups. The

most recent three layers are the Paleozoic,

Mesozoic, and Cenozoic. These layers

represent the last 500 million years of life on

earth.

Fossils

What are Fossils ?

The word fossil comes from the Latin word fossilis, which means, "dug up". Most fossils are excavated from sedimentary rock layers (Sedimentary rock is rock that has formed from sediment, like sand, mud, and small pieces of rock).

Over long periods of time, these small pieces of debris are compressed (squeezed) and are buried under more and more layers of sediment that piles up on top of it. Eventually, they are compressed into sedimentary rock.

The fossil of a bone doesn't have any bone in it! A fossilized object has the same shape as the original object, but is chemically more like a rock.

How are Fossils formed ?

Some animals were quickly buried after their death (by sinking in mud, being buried in a sandstorm, etc). Over time more and more sediment covered the remains. The parts of the animals that didn't rot (usually the harder parts like bones and teeth) were encased in the newly formed sediment. In the right circumstances (when there is no scavengers, quick burial, not much weathering) parts of the animal turned into fossils over time.

After a long time the chemicals in the buried animals bodies underwent a series of changes. As the bone slowly decayed, water infused with minerals seeped into the bone and replaced the chemicals in the bone with rock-like minerals. The process of fossilization involves the dissolving and replacement of the original minerals in the object with other minerals (and or permineralization - the filling up of spaces in fossils with minerals, and /or recrystallization in which a mineral changes its form).

How are Fossils formed ?

In the end we get a heavy, rock-like copy of

the original object - a fossil. The fossil has the

same shape as the original object, but is

chemically more like a rock!

What are Index Fossils ?

Index fossils (also known as guide fossils, indicator fossils or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil. If the species concerned were short-lived (in geological terms, lasting a few hundred thousand years), then it is certain that the sediments in question were deposited within that narrow time period. The shorter the lifespan of a species, the more precisely different sediments can be correlated, and so rapidly evolving types of fossils are particularly valuable. The best index fossils are common, easy-to-identify at species level, and have a broad distribution—otherwise the likelihood of finding and recognizing one in the two sediments is minor.

Finding the age of rocks

How can you tell the age of a rock or to which geologic time period it belongs? One way is to look at any fossils the rock may contain. If any of the fossils are unique to one of the geologic time periods, then the rock was formed during that particular time period. Another way is to use the "What's on top?" rule. When you find layers of rocks in a cliff or hillside, younger rocks are on top of older rocks.

But these two methods only give the relative age of rocks--which are younger and which are older. How do we find out how old a rock is in years? Or how do we know how long ago a particular group of fossilized creatures lived?

Finding the age of rocks

The age of a rock in years is called its absolute age. Geologists find absolute ages by measuring the amount of certain radioactive elements in the rock. When rocks are formed, small amounts of radioactive elements usually get included. As time passes, the "parent" radioactive elements change at a regular rate into non-radioactive "daughter" elements. Thus, the older a rock is, the larger the number of daughter elements and the smaller the number of parent elements are found in the rock.

Finding the age of rocks

A common "parent-daughter" combination that geologists use is radioactive uranium and non-radioactive lead. As shown in the diagram above, uranium is trapped in a newly formed rock. As the rock ages, more and more of the uranium changes into lead.

The age of the rock in years can be found by measuring the rate at which a parent element decays and then measuring the ratio of parent element to daughter element in the rock. The ages in years of the different geological time periods are found by measuring the absolute ages of many rocks from all of the different periods. The absolute ages of some of the different geologic time periods are shown along the right side of the Staircase of Time.

Finding the age of rocks

The steps of the Staircase of Time are drawn

to be almost the same size, so you might think

that the time periods are the same length, but

they are not. The absolute ages of rocks taken

from the different time periods have shown

that the time periods were of greatly differing

lengths. Some were very short, like the

Quaternary period (only 2 million years), while

others were very long, like the Proterozoic Era

(almost 2 billion years)

Just pictures

Important sites

http://wiki.answers.com/Q/How_are_rock_layers_formed#

http://www.prehistoricplanet.com/news/index.php?id=48

http://en.wikipedia.org/wiki/Index_fossil

http://www.sciencekids.co.nz/sciencefacts/earth/fossils.html

http://www.cotf.edu/ete/modules/msese/earthsysflr/ages.html

Thank you