​​The word geology means ‘Study of the Earth’. Also known as geoscience or earth science, Geology is the primary Earth science and looks at how the earth formed, its structure and composition, and the types of processes acting on it

The Principles of Geology

  • Uniformitarianism.
  • Original horizontality.
  • Superposition.
  • Cross-cutting relationships.
  • Walther’s Law.

What are the 4 laws of geology?

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The four laws are the law of superposition, law of original horizontality, law of cross-cutting relationships, and law of lateral continuity. Nicolaus Steno was a 17th-century Danish geologist.


Minerals are natural occurring elements and compounds with a definite homogeneous chemical composition and ordered atomic composition.

Each mineral has distinct physical properties, and there are many tests to determine each of them. The specimens can be tested for:

  • Luster: Quality of light reflected from the surface of a mineral. Examples are metallic, pearly, waxy, dull.
  • Color: Minerals are grouped by their color. Mostly diagnostic but impurities can change a mineral’s color.
  • streak: Performed by scratching the sample on a porcelian plate. The color of the streak can help name the mineral.
  • Hardness: The resistance of a mineral to scratching.
  • Breakage pattern: A mineral can either show fracture or cleavage, the former being breakage of uneven surfaces, and the latter a breakage along closely spaced parallel planes.
  • specific gravity the weight of a specific volume of a mineral.
  • Effervescence: Involves dripping hydrochloric acid on the mineral to test for fizzing.
  • Magnetism: Involves using a magnet to test for magnetism
  • Taste: Minerals can have a distinctive taste, such as halite (which tastes like table salt ).


The rock cycle shows the relationship between igneous sedimentary and metamorphic rocks A rock is any naturally occurring solid mass or aggregate of minerals or mineraloids Most research in geology is associated with the study of rocks, as they provide the primary record of the majority of the geological history of the Earth. There are three major types of rock: igneous sedimentary, and metamorphic. The  rock cycle illustrates the relationships among them .

innerscale of the Earth

The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon. The second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline.

Millions of Years (1st, 2nd, 3rd, and 4th)
Thousands of years (5th)

Relative dating;

Cross cutting relations can be used to determine the relative ages of  rock strata and other geological structures. Explanations: A – folded  rock strata cut by a thrust fault B – large intrusion  (cutting through A); C – erosion angular unconformity  (cutting off A & B) on which rock strata were deposited; D –  volcanic dyke (cutting through A, B & C); E – even younger rock strata (overlying C & D); F – normal fault  (cutting through A, B, C & E).

Methods for relative dating  were developed when geology first emerged as a natural science . Geologists still use the following principles today as a means to provide information about geological history and the timing of geological events.

The principle of uniformataism states that the geological processes observed in operation that modify the Earth’s crust at present have worked in much the same way over geological time. A fundamental principle of geology advanced by the 18th-century Scottish physician and geologist James Hutton  is that “the present is the key to the past.” In Hutton’s words: “the past history of our globe must be explained by what can be seen to be happening now.”

Absolute dating;

Geologists also use methods to determine the absolute age of rock samples and geological events. These dates are useful on the own and may also be used in conjunction with relative dating methods or to calibrate relative methods.

At the beginning of the 20th century, advancement in geological science was facilitated by the ability to obtain accurate absolute dates to geological events using radioactive isotopes  and other methods. This changed the understanding of geological time. Previously, geologists could only use fossils and stratigraphic correlation to date sections of rock relative to one another. With isotopic dates, it became possible to assign absolute ages  to rock units, and these absolute dates could be applied to fossil sequences in which there was datable material, converting the old relative ages into new absolute ages.

Methods of geology;

A standard  Brunton pocket transit commonly used by geologists for mapping and surveying Geologists use a number of fields, laboratory, and numerical modeling methods to decipher Earth history and to understand the processes that occur on and inside the Earth. In typical geological investigations, geologists use primary information related to petrology  (the study of rocks), stratigraphy (the study of sedimentary layers), and structural geology (the study of positions of rock units and their deformation).

Field methods

A typical USGS field mapping camp in the 1950

Today, handheld computers with  GPS and geographic information systems software are often used in geological field work digital geological mapping

A pertified log in pertified forest national park Arizona U.S.A.

Geological  field works varies depending on the task at hand. Typical fieldwork could consist of:

  • Geological mapping
    • Structural mapping: identifying the locations of major rock units and the faults and folds that led to their placement there.
    • Stratigraphic mapping: pinpointing the locations of sedimentary facies (lithofacies and biofacies) or the mapping of  isopachs of equal thickness of sedimentary rock
    • Surficial mapping: recording the locations of soils and surficial deposits
  • Petrology
  • In addition to identifying rocks in the field (lithology), petrologists identify rock samples in the laboratory. Two of the primary methods for identifying rocks in the laboratory are through optical microscopy and by using an electronic microprobe. In an optical minerology analysis, petrologists analyze thin sections of rock samples using a  petrograhic microscope .where the minerals can be identified through their different properties in plane-polarized and cross-polarized light, including their birefringence, pleochroism, twinning, and interference properties with a conoscopic lens In the electron microprobe, individual locations are analyzed for their exact chemical compositions and variation in composition within individual crystals. stable and radioactive isotope studies provide insight into the geochemical  evolution of rock units.

Economic geology;

Economic geology is a branch of geology that deals with aspects of economic minerals that humankind uses to fulfill various needs. Economic minerals are those extracted profitably for various practical uses. Economic geologists help locate and manage the Earth’s natural resources, such as petroleum and coal, as well as mineral resources, which include metals such as iron, copper, and uranium.

Mining geology;

Mining geology consists of the extractions of mineral resources from the Earth. Some resources of economic interests include gemstones metals such as gold and copper, and many minerals such as perlite, mica, phosphates, zeolites, clay, ,pumice,, quartz and silica, as well as elements such as sulfur, chlorine , and helium

Petroleum geology

Mud log in process, a common way to study the lithology when drilling oil well petroleum geologists study the locations of the subsurface of the Earth that can contain extractable hydrocarbons, especially petroleum and natural gas. Because many of these reservoirs are found in sedimentary basins they study the formation of these basins, as well as their sedimentary and tectonic evolution and the present-day positions of the rock units.

Engineering geology

Engineering geology is the application of geological principles to engineering practice for the purpose of assuring that the geological factors affecting the location, design, construction, operation, and maintenance of engineering works are properly addressed. Engineering geology is distinct from geological engineering, particularly in North America


Geology and geological principles can be applied to various environmental problems such as stream restoration, the restoration of  brown fields, and the understanding of the interaction between natural habitat and the geological environment. Groundwater hydrology, or hydrogeology, is used to locate groundwater, which can often provide a ready supply of uncontaminated water and is especially important in arid regions, and to monitor the spread of contaminants in groundwater wells.