Geologic Principles and Laws
Geologic Principles are fundamental rules or laws which guides a geologist on how to properly read rocks which are the pages of the book of Earth’s history.
These principles aid geologist place events in Earth’s history in their proper chronological order.
Listed below are some of the major geologic principles, they have been simplified for better understanding by scholars.
The Principle of Superposition
This principle states that all young rocks are laid on top of older rocks. This means that in a succession of strata or an outcrop, the lower beds were first deposited before the beds above them.
The Principle of Uniformitarianism
The Principle states that the present is the key to the past.
This means that all geologic activities currently taking place now (i.e. in the present), are still the same activities that took place in the past to form any geologic structure we see today.
For instance, in the past (perhaps 100,000 years ago) during volcanic eruption, magma was released to the surface of the earth as lava, this lava cooled and solidified forming large bodies of igneous rocks which we see today. The same process and result is what we see taking place in our present age. Therefore, when a geologist sees a body of igneous rock which looks very old, he or she can tell how it formed.
The Principle of Original Horizontality
This principle states that layers of sediments are initially laid or deposited in horizontal forms. This is because they are deposited in sedimentary basins.
The principle implies that the strata we see today either inclined or horizontal were initially deposited horizontally.
Note: Beds which are inclined have done so due to tectonism (folds, fractures, faults, etc.).
The Principle of cross-cutting Relationship
This principle states that an intrusion or fault is younger than the rock it cuts across. This means that before an intrusion or fault can occur in a strata, the strata has to have been deposited.
The Principle of Inclusion
This principle states that fragments of other rocks contained within the body of a rock must be older than the host rock.This means that as geologists in the field, if we come across an outcrop which contains fragments of another rock, we know immediately that those fragments had to have been formed long time ago, weathered and then transported to a new location where deposition or sedimentation was currently taking place. Therefore, we can conclude that the rock fragments is older than the bed or rock which hosts it.