Lessons 12–13 · 2 lessons · P. Gulomov, H. Vahobov, P. Baratov, M. Mamatqulov. Geography (Physical Geography of Central Asia; Physical Geography of Uzbekistan), Grade 7. Revised and expanded 5th edition. “O‘qituvchi” Publishing House, Tashkent, 2017
12
Geological structure of Central Asia and the geological time scale
Textbook: pp. 31–32
GoalUnderstand the geological time scale and the relative and absolute age of rocks.
New words
geological time scale · geologik sanaera · erarelative age · nisbiy yoshabsolute age · mutlaq yosh
Explanation
The time from the formation of the Earth’s crust to the present is called geological time, and its counting is the geological time scale. It is divided into the largest units, eons, then eras (Archean, Proterozoic, Paleozoic, Mesozoic, Cenozoic) and their periods. Sedimentary rocks lie in layers, so if the layers are undisturbed the lower one is older and the upper one younger; fossils of plants and animals in the layers also help to find their relative age. The time since a rock formed is its absolute age, found mainly by the radiometric method based on the decay rate of radioactive elements. With these methods scientists have built the geochronological table. The Earth is about 4.5 billion years old, and the Cenozoic Era began about 66 million years ago.
Worked examples
If in a gully wall the lower layer is limestone and the upper is sandstone, the limestone formed first and the sandstone later (if the layers are undisturbed).
If a rock contains the remains of a creature that lived 250 million years ago, the rock dates from the end of the Paleozoic–start of the Mesozoic.
Class activity
“Layer tower”: stack 5 colours of plasticine or paper strips in layers, then groups point out the “oldest” and “youngest” layer.
Practice
1
What is the geological time scale?
The system for counting the time of the Earth’s geological history.