Geology Chapter 8

Key to the Past (Uniformitarianism)

- This concept means that the present is the key to the past. It means that the processes operation on the earth today (earthquakes, volcanoes, erosion, etc.) have been operating on the earth thorough its history. - Actualism is the reverse of uniformitarianism, which means that the processes operating in the past are operating today. - Can establish the ages of rocks using dating method based on radioactivity principles. - But to establish the ages of rocks in the field, not in terms of ages but based on comparison. This depends on the sequence in which events took place rather than the number of years. This concept is called relative time.

Intrusions

When a body of rock intrudes another one, the intruding body of rock is younger than the host rock.

Isotopes

Atoms of the same element having different number of neutrons but the same amount of protons.

Physical Continuity

Being able to trace physically the course of a rock unit.

Combining Relative and Numerical Ages

Combining relative and numerical ages gives a better perspective regarding the occurrence of geologic events and the ages of materials. This provides a good idea regarding the sequence of geologic events and the numerical dating can also confirm or deny the observations made by relative dating.

Similarity of Rock Types

Correlation between two rock types can be made on the assumption that the two rock types in two different regions were formed at the same time.

The different isotopic dating methods include U/Pb, Rb/Sr, K/Ar, Radiocarbon. Each of these radioactive isotopes have their own half lives. They can provide the ages within a given margin of error.

Curves Used to Establish the Age of the Rock by Comparing The Amount of Radioactive Isotope Remaining in Time to the Original Amount.

Isotopic Dating

Establishing the age of the rock using radioactive elements is known as isotopic dating.

Angular Unconformity

Is a contact in which the younger strata lie over the older ones that are tilted or folded.

Disconformity

Is the contact representing missing rock strata. The older rocks were eroded away which were parallel to the bedding surface.

Radioactive Decay

Is the spontaneous nuclear decay of certain isotopes.

Correlation

It is a method by which rocks are correlated against one another. This correlation of the rocks can be within a region, a continent or between continents.

Numerical Age

Only after the discovery of radioactivity have scientists been able to establish the age of rocks, which has provided them with the necessary tools to establish the ages of rocks, minerals, and formations.

Age of the Earth

Over time various people and various school of thought have established the age of the earth with varying figures. However, today the age of the earth is established at somewhere between 4.5 to 4.6 billion years, or 4,500 to 4,600 million years, much older than the oldest rock found. This observation was made on the basis of meteorites and lunar rocks, as most meteorites are regarded as fragments of materials that did not combine into a planet. It is likely that the planets and other bodies of the earth solar system, earth including formed at about the same time.

Nonconfomity

The erosion surface on a plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.

Correlation by Fossils

The fossils succeed one another in a predictable order. If the same fossils are found between two rock types then a correlation between them can be established. This is also called as the principle of faunal succession.

Cross-Cutting

The principle of cross-cutting mentions that the disrupted pattern is older than the disruption which caused it. When a fault cuts across layers of rocks the fault is younger than the layers that it cuts across.

Inclusions

The principle of inclusion states that the fragments included in the host rock are older than the host rock.

Lateral Continuity

The principle of lateral continuity mentions that the original sedimentary layer extends laterally until it tapers or thins out at edges.

Original Horizontality

The principle of original horizontality states that when material was initially deposited on the surface of the earth, it was deposited in a horizontal fashion. The folding-faulting observed in the rocks, was after its deposition.

Superposition

The principle of superposition states that the layers below are older than the layers deposited on top of them. The oldest is at the bottom and the youngest is at the top.

Half-life Period

The time taken by a given amount of radioactive isotope to be reduced by one half. The time taken for the unstable parent element (100 g. of Uranium, U-unstable) to decay to its parent (50 g. Uranium, U-unstable) and daughter element (50 g. Lead, Pb-stable). The half life for isotopes varies from a few seconds to billions of years.

Unconformities

Unconformity is a surface that represents the gaps in geologic record. There are three types of unconformities.