Agriculture Reference
In-Depth Information
Box 1.2 The Age of Rocks
Compared with the 6,000 years or so of recorded human history, the earth's age
of 4.6 billion years is unimaginably long. Nevertheless, even events that occurred
millions of years ago have relevance today because of the unending cycles of rock
formation, weathering, erosion, and deposition that have laid the foundations
of, and helped to shape, the present earth's surface. As the science of geology
developed, the history of earth's rocks was divided into a time scale consisting of
eras, periods, and epochs. Most information is available for rocks dating from
the appearance of the first multicellular life forms about 650 million years ago,
through many subsequent epochs to the present. Periods within eras are usually
associated with sequences of sedimentary rocks that were deposited in the area
now known as Europe and recognized there for the first time. Although examples
of these rocks are found elsewhere, the European time divisions have generally
been accepted worldwide, except in cases such as the Ediacaran period of the
Pre-Cambrian era whose defining fossils were first identified in the Flinders Ranges
of South Australia. Table B1.2.1 gives a simplified geological time scale from
the Cambrian period, when life forms exploded in diversity and number, to the
present.
Table B1.2.1
The Geological Time Scale
Start time
(million
years ago)
Era
Period
Epoch
Cainozoic (“young life”)
Quaternary
Recent
0.011
Pleistocene (the ice ages)
2
Tertiary (the age
of mammals)
Pliocene
5
Miocene
23
Oligocene
36
Eocene
53
Palaeocene
65
Mesozoic (“middle life”
and the age of reptiles)
Cretaceous
Jurassic
Triassic
145
205
250
Palaeozoic (“old life”)
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
290
360
405
436
510
560
Pre-Cambrian
4,600
