The sedimentary rocks were laid horizontally to the so-called stability layers, with the oldest being at the bottom. This is the basis of a relative dating system called Geological Timescale. But until scientists figured out how to detect radioactive elements, it was impossible to determine the exact age of the rock. This has enabled scientists to determine not only the age of the rocks but also the age of the planet Earth.

Rock records

Where rocks are formed today, usually at the bottom of lakes or at the bottom of the ocean, they are formed from soft sediments settled in horizontal, parallel layers. A new level may be the same as the one on which it settles, or it may be different in appearance due to some changes in the environment. A huge volcanic eruption can emit a huge amount of ash that settles in a certain layer. The oldest such layers are at the bottom.

Bend and tilt

The moving plates of the earth’s crust bend and fold at the level of the rock, so they are not horizontal. Often, they appear to be bent or even vertical, because the folds in the layers are so large that we can only see part of them. But sometimes the surface of the rocks is crushed under extreme pressure so much that they form dramatic zigzag patterns.

Snap and slip

Movements that cause earthquakes cause rocks to be stretched or compressed so they can snap. The strata move along the fault line. Because they are relative to each other, the connected layers are offset. So the appearance and thickness of each layer is usually recognizable, even if they fall from a very long distance.

(Fossil markers:- The order of the layers of rocks is mixed by folds and defects, so it is not clear how old the rocks are. But in the 18th century, geologist William Smith observed that if rocks in different places had the same fossils, the rocks would be of the same age. This allowed geologists to create geological maps).

Radiometric dating

Many elements in rocks are unstable. Their atoms are destroyed over time, they emit radiation. Scientists know how long it takes for an element to decay. As uranium forms lead at a steady rate. This radioactive dating technique has enabled geologists to keep dates in all eras and eras.

(From the formation of the Earth 4.6 billion years ago to the current quarter of time beginning 2.6 million years after the Neogene period, geological chronology spans billions of years).

Grand Canyon

Seen from the arid landscape of Arizona, this vast area is 6,000 feet (1,800 m) deep. It was formed by the Colorado River cutting rocks that were pushed up by the moving plates of the Earth’s crust. The canyon reveals about 40 layers of silt rock. The redwall limestone in the foreground was deposited in the tropical seas 330 million years ago.

See more

Different types of igneous rocks and their functions
Description of the Dynamic Landscape
Case of sedimentary rocks and their characteristics
The effect of the weathering rocks
The process of fossil formation and their characteristics

Metamorphic rocks and the process of their formation