On average, a fossil takes thousands of years to form. Scientists have not yet formulated a time period. A hundred year old item would not be categorized as a fossil.
It takes ages to form a fossil under the compressive forces of layers of soil or rocks. That is why we discover fossils of pre-historic ages today.First there is decomposition, they the molecules of plant or body parts are replaced by surrounding minerals.And finally they are preserved in the layers of earth.
Contrary to popular belief, it doesn't always take millions or thousands of years for fossils to form. Depending on environmental influences, certain things can fossilized in as few as five to ten years.
This is method by which nature preserves the species under the rocks.So dead species are pressed between rocks and kept for millions of years.The formation may take atleast thousands of years minimum.It depends on the pressure exerted by the rocks and the depth in which they fell.How ever it may take million years for extinguished fossils.
