What is relative geological age and absolute geological time?

Relative geological age – refers to the order in which the strata are formed and the relative relationship between the old and the new. It only indicates the relative order and stages of development of geological history, not the individual geological epochs.

The length of the unit.

Absolute geological age – refers to the radioactive isotopes in rocks.

The content is determined and the age of the rock is calculated according to its decay law. or in absolute astronomical units.

years", absolute geochronology can be used to determine the time of occurrence, continuation and end of geological events.

1. The method of determining the relative age.

1) Stratigraphic method (stratigraphic sequence: Nicolaus Steno (1638-1686), born in Copenhagen in 1669, concluded that between rock layers, there is the following law: after the formation of rock layers, if they are not affected by strong crustal movements and reverse their original positions, they should be deposited first below, then deposited on top, layer by layer, maintaining a state close to horizontal, extending to a distance before gradually extinguishing.

The formation of strata is horizontal or nearly horizontal, with the first formation located in the lower part and the later formation in the upper part. Note: The original output of the new and the old, not all the strata seen in the field now are new and old, and there are later crustal transportation transformations.

For the stratigraphic sequence of stratigraphic changes (tilting, inversion) caused by crustal movement in the later stage, the sedimentary structure can be used as the "bottom-showing structure" to determine the order after restoring the top and bottom.

2) Paleontological methods (fossil sequence law): biological evolution is from simple to complex, from low to high, and biological species from few to many, and this evolution and development is irreversible. Therefore, the species and types of organisms in different geological periods are different.

The older the age, the fewer creature types, the lower the creature, and the simpler the structure; The newer the era, the more creature types it has, the more advanced the creatures, and the more complex the structure. As a result, fossils of organisms preserved in older rocks are relatively low-grade and structurally simple; The fossils of organisms preserved in the newer rocks are relatively advanced and structurally complex.

3) Structural geology method (cutting law): The above two criteria are mainly applicable to determine the relative relationship between the old and the new of sedimentary rocks or layered rocks, but it is difficult to apply them to magmatic rocks or metamorphic rocks that are massively produced because they are not layered and do not contain fossils. However, these massive rocks are often interspersed and cut with layered rocks and between them, and at this time, the relationship between the old and the new between them is determined by the cutting law between geological bodies, that is, the newer geological bodies always cut or intersperse with the older geology, or the cutter is new and the cut is old.

2. Determination of isotopic age (absolute age).

1) Rubidium-strontium method, uranium (thorium)-lead method: mainly used to determine the age of older rocks;

2) Potassium-argon method: the effective range is large, almost applicable to most geological time, and potassium is a common element, many minerals are rich in potassium, so the determination of potassium-argon method is difficult to reduce and the accuracy is improved, so potassium-argon method is the most widely used;

3) 14C method: due to its short half-life of the same modified hormone, it is generally only suitable for age determination after 50,000 years;

4) Samarium-neodymium method, 40AR-39AR method: high precision and strong resolution.

There are two main ways to determine the relative age of strata:

Stratigraphic division method: According to the physical properties, geological genesis, geological age and other factors of the strata, the strata are divided into different levels. On the basis of the division of Kaizhihu strata, the relative age of the strata is determined according to the relative position relationship between the strata.

Biostratigraphic method: Determine the relative age of the stratum according to the biological remains in the strata, such as fossils and fossil groups. By systematically classifying and comparing the biological remains in the strata, the relative age of the strata was determined.

The main methods are fossil group chronology and stratigraphic chronology.

1. The geological age from ancient times to the present is: Cryptozoic Eon (now known as Precambrian Supereon) and Phanerozoic Eon.

2. The Cryptozoic has now been subdivided into the Phertozoic, Archean, and Proterozoic.

3. Phanerozoic is further divided into: Paleozoic, Mesozoic, and Cenozoic.

4. The Paleozoic Era is divided into: Cambrian, Ordovician, Silurian Period, Devonian, Carboniferous, and Permian.

5. The Mesozoic Era is divided into: Triassic, Jurassic, and Cretaceous.

6. The Cenozoic is divided into: Paleogene, Neogene and Quaternary.

We currently belong to the Quaternary Period, the most coarse and new era of the Cenozoic Era.

Geologic time is a term used in geology to divide the time of the Earth's history. Geological time is divided by geologists according to the age of the strata and geological events. The geological age is divided into three parts: the Paleozoic, Mesozoic, and Cenozoic.

The Paleozoic Era is the oldest geologic epoch, lasting about 4.6 billion years. The Mesozoic lasted about 2.8 billion years, and the Cenozoic lasted about 200 million years. The Paleozoic, Mesozoic, and Cenozoic are further subdivided into several epochs and periods.

The determination of the relative geological age of rocks (bodies) is based on three methods: stratigraphic sequence law, biological evolution law and contact relationship between geological bodies

a. The original occurrence of sedimentary rocks without the influence of tectonic deformation should be horizontal or approximately horizontal. And the rock layer formed first is below, and the rock layer formed later is above.

b. The law of biological evolution is due to the continuous development and evolution of organisms from low to high, from simple to complex. Therefore, the relationship between the old and the new of the rock layer can be judged according to the biological fossils preserved in the rock layer.

c. The contact relationship between geological bodies is judged according to the unconformity contact between sedimentary rock layers. The formation time of the rock strata with the same lithology as the bottom conglomerate on the undemolished Chunqi integration surface is earlier, and the formation time of the rock strata consistent with the occurrence of the angular unconformity is later.

If the rock layer is in sedimentary contact with the magmatic rock, the sedimentary forest hall rock is formed later, and if the rock layer is in intrusive contact with the magmatic rock, the early sedimentary rock formation time is earlier.

1. Relative geological age: Relative geological age refers to the sequence of rock formations. i.e., which rock formations are old and old; Which rock formations are post-generated and are new. Does not contain the concept of time expressed in chronology.

Regardless of the specific time, only the order of time is compared. For example, there must be a father and a mother first, and then a son, and this order is emphasized in the relative geological age.

2. Absolute geological time: Absolute geological age is generally determined by the metamorphosis law of radioactive elements contained in rocks.

It's all about absolute time. Let's take the example just now as an analogy, when my father and mother were 24 years old, their son was born, and the emphasis is on the concept of 24 years.

In popular understanding, relative geological age is only the order of comparative chronology, absolute geological age, we must know this specific time, how many years it is.

The relative geological age is mainly judged by the representative fossils in the strata, and the absolute geological age is determined by isotope dating.

Relative geological age is the sequence in which geological bodies are formed or geological events occur. Absolute geological age is the age at which a geological event occurred before the present day.

When you read some books or movies on the theme of dinosaurs or paleontology, you will always hear some unbelievable geological chronological names, such as Devonian, Silurian, Ordovician, Cretaceous and so on. So how exactly is the geological age of the earth divided? How did these names come about?

Determine the geological age in which tectonic movements occurred.