Life on Earth

About 1 to 1.5 billion years after the earth formed, geologists believe, simple living organisms appeared in the oceans. However, more complex forms did not appear in abundance until about 2.5 billion years later – at the beginning of the Cambrian period, only 550 million years ago. Not until the Devonian period, about 350 million years ago, did vegetation become widespread on the land. Land animals were scarce until even later.

Because life has evolved continuously from Precambrian times, the fossil remains of animals and plants succeed one another in a definite, known order. Geologists have classified rocks in groups based upon this succes­sion, as shown in table 1.1. Geologists have estimated the durations of the eras, periods, and epochs from studies of radioactive minerals. The pres­ence of life may be essential to the petroleum story because, according to the prevailing theory, organic matter is necessary for the formation of oil.

Table 1.1

Geologic time scale

Era	Period	Epoch	Duration (millions of years)	Dates (millions of years)
		Recent	0.01	0.00
	Quaternary	Pleistocene	1	0.01 1
		Pliocene	10
Cenozoic		Miocene	14	11 25
	Tertiary Cretaceous Jurassic	Oligocene Eocene Paleocene	15 20 10 65 30	40 60 70 ±2 135 ±5
Mesozoic	Triassic Permian		35 35	165 ± 10 200 ± 20 235 ± 30
	Pennsylvanian		30	265 ± 35
Paleozoic	Mississippian Devonian Silurian		35 50 40	300 ± 40 350 ± 40
	Ordovician Cambrian		70 90	380 ± 40 460 ± 40 550 ± 50
Precambrian			4,500 ±

(After R.M. Sneider)

Categorizing Rocks

Up to now, we have grouped all rock together as the material that the crustal plates are made of. But, of course, it is not all the same. Different kinds of rock contain different minerals, have different physical properties, and were formed in different ways. Geologists group the rocks of the earth's crust into three types according to how they were formed: igneous, sedimentary, and metamorphic.