Interesting Facts about Rocks and 3 Different Types

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Updated on: Educator Review By: Michelle Connolly

Rocks are known as solid natural formations consisting of one or more minerals, and they are an essential part of the earth’s crust. Rocks are classified according to the way they are formed into three main types; igneous rocks, metamorphic rocks, and sedimentary rocks, each type of rock has unique characteristics that can distinguish it from the other two types.

rocks are non-living substances that occur naturally and consist of mineral grains that may be so small that they can only be seen through a microscope, or as large as a fingernail, or even the finger itself. It is worth noting here that changes in conditions around it may lead to its transformation into another type of rock or a different rock of the same type.

Rocks can be classified into various classifications and types according to their broad specifications and characteristics. Hereunder are the most famous classifications and types:

Igneous Rocks 

All igneous rocks were formed from the solidification of lava, molten fragments, and ash either inside the ground or on the surface, where the lava, molten fragments, and ash from volcanoes are called extrusive igneous rocks. They were formed from the freezing of magma on the surface of the earth. 

Some other igneous rocks such as granite were formed in the depths of the earth, and these rocks are called intrusive igneous rocks because they were formed from freezing magma in the ground, and the following is an explanation of the two types of igneous rocks: 

  1. Intrusive igneous rocks: They are formed by the slow crystallization of magma, either several kilometers below the Earth’s surface in the form of large, irregular plutons or near the surface in fissures and fractures such as simple shallow subductions in Neo-Cross or sills. 
  2. Extrusive igneous rocks: They are sometimes called igneous rocks, are formed when magma flows on the surface of the Earth or volcanic fragments are pushed either on land, as in Hawaii, or under the sea, such as mid-ocean recesses, where differences in the composition of the  magma, its mineral pattern and the temperature of its crystallization affect the way it flowed from the volcano.

Igneous rocks can also be classified as follows:

The type of rocks that are formed as a result of the solidification of molten materials (magma), and solidification may occur in the ground, so the resulting rocks are called igneous rocks. Some of them solidify at the surface of the earth, so they are called igneous rocks, and there are igneous rocks under the surface, and depending on the proportion of silica to the following types

  1. Acidic Rocks

They are rocks with a silica content of more than 66%, examples include: 

Granite: It is an igneous rock that forms at a great depth in the ground, It is often coarse-grained rock. That is, its mineral content can be distinguished with the naked eye and it may sometimes be medium or fine-grained. 

Granite consists mainly of quartz, feldspar, and mica, and there are rocks equivalent to granite, but different from it in the nature of the texture, namely rhyolite, obsidian, pitchstone, and pumice, which is known as a pumice stone. 

Granodiorite: A coarse-grained igneous rock, the igneous counterpart of granodiorite is called Andesite. 

  1. Intermediate Igneous Rock

It is an igneous rock with a silica content of 52-66%, examples of which are:

Syenite: an underground, often the coarse-grained rock, and the volcanic counterpart of syenite is trachyte. 

Diorite: An igneous rock that has a volcanic counterpart called andesite. 

  1. Basic Igneous Rock

It is igneous rocks with a percentage of silica range from 40-52%, including Gabbro, which is underground and its volcanic equivalent is basalt. 

  1. Ultrabasic Igneous Rock

It is igneous rocks with a silica content of less than 40%, examples include dunite and peridotite.

Characteristics of Igneous Rocks 

Igneous rocks are characterized as:

  1. Relatively strong rocks as a result of the cohesion of the grains that make up them during cooling. 
  2. Its main colors are black, white, and gray and there may be igneous rocks of pale colors.
  3.  It has a crystalline texture as a result of its exposure to melting and cooling. 
  4. Does not contain fossils because it was exposed to a high temperature that does not allow the existence of any kind of life in it. 
  5. They are not arranged in the form of layers, with the exception of volcanic ash rock, which may form in the form of layers when deposited within bodies of water. 

Sedimentary Rocks 

They are rocks that are formed from grains separated from pre-formed rocks. They are called source rocks as a result of multiple factors. Sedimentary rocks are made of materials that accumulate on the surface of the earth, such as the outcome of weathering, erosion, and other materials such as organic materials. The process of deposition of these unconsolidated materials in the form of different rocks is called ossification or fusion of rocks. 

Some sedimentary rocks are formed on the surface of the earth due to direct precipitation resulting from rainfall, while other types of sedimentary rocks form under the surface of the earth when sediments are buried and appear on the surface through tectonic uplift or erosion of the materials that cover them. The formation of sedimentary rocks goes through several stages, namely:

  1. Weathering: These are the processes that break the source rocks into small pieces. Weathering processes are divided into two parts:
  • Mechanical weathering: The disintegration of the source rocks without changing their chemical composition or mineral content. 
  • Chemical weathering: The fragmentation of source rocks with a change in their chemical composition, which occurs in the presence of water, and at relatively warm temperatures. 
  1. Erosion and Transport: The erosion of the source rocks and the transfer of crumbs to other places. The factors of erosion and transport are gravity, ice, wind, and water. 
  2. Deposition: The materials that were transported settle in low places, coalesce with each other and are arranged in the form of layers, some types may contain fossils. 
  3. Lithification: It means the solidification of sediments and turning them into rocks. 

Characteristics of Sedimentary Rocks 

Sedimentary rocks are characterized by the following: 

  1. They are arranged in the form of layers of clay or sandy materials, so they are also called stratified rocks. 
  2. Its colors range from light brown to light grey. 
  3. It contains fossils, footprints, and neem marks; marks that appear on rocks as a result of waves crashing into them. 

Types of Sedimentary Rocks 

Sedimentary rocks are classified into the following main types: 

Clastic Sedimentary Rocks: rocks consisting of grains of minerals, pebbles, and other remnants of rocks and shells, then separated in the formation of shells. It was carried away, deposited, buried, and then petrified as a result of exposure to pressure and somewhat low temperature, not much more than 100 degrees Celsius, it is the most common type of sedimentary rock, examples of which are: 

  1. Sandstone 
  2. Shale 
  3. Conglomerate 
  4. Breccia 

Organic sedimentary rocks: They are sedimentary rocks that were formed as a result of the deposition of animal remains, such as the shells of microscopic animals containing calcium carbonate or silica and plant remains on the sea floor in the form of thick layers, as time passes it gets hardened. The types of organic sedimentary rocks include:

  1. Limestone 
  2. Chert
  3. Peat 
  4. Coal 

Chemical Sedimentary Rocks: Sedimentary rocks formed when water in some shallow sea areas begins to evaporate, thus increasing the concentration of dissolved minerals in it and it begins to precipitate in the form of rocks. Examples include: 

  1. Gypsum. 
  2. Halite or rock salt. 
  3. Limestone. 

Metamorphic Rocks 

Metamorphic rocks are sedimentary or igneous rocks that are buried deep under the surface of the earth and have been exposed to harsh conditions, such as pressure, heat, and chemical processes, which affected their chemical composition, texture, and the type of minerals that make them up. 

Metamorphic rocks are formed as a result of exposure of the original rock to heat and pressure, which leads to the formation of a completely new rock, where the original rock can be sedimentary, igneous, or metamorphic. For example, when the igneous granite rock is subjected to sufficient pressure, the metal plates move in the same direction in which the metal plates move in the same direction and it turns into gneiss rock. 

Metamorphic rocks are classified as follows: 

Foliated metamorphic rocks: They are formed under direct pressure or shear stress and usually consist of large amounts of the minerals mica and chlorite. They come in several types, including the following:

  1. Slate: The rocks that have the slate texture have fine grains that are difficult to see with the naked eye and they have dull surfaces that do not show any luster, thus they can split easily.
  2. Phyllite: The rocks that have the limestone texture are similar to those that have the slate texture in that they consist of small grains, but they appear in the form of wavy or wrinkled lines with shiny surfaces, and they can split easily, and the limestone rocks are different in appearance from the original rocks Much more than slate rocks.
  3. Schist: The rocks are distinguished by their visible grains that are clear to the naked eye, and are mostly formed of lamellar minerals such as mica. The schistosogenic rocks differ in their appearance from the original rocks to a greater extent than the flite rocks, and these rocks may consist of a variety of parent rocks.
  4. Gneiss: The minerals in the gneiss rocks are arranged in light and dark bands.

Non-foliated metamorphic rocks: They are formed without direct pressure or relatively close to the Earth’s surface, where the pressure is very low, and they do not necessarily appear as metamorphic rocks despite direct pressure. This is due to its internal minerals, which quartz and calcite are not arranged in lines that show the petiole of the rock.

These rocks are formed under conditions of low pressure or under the influence of equal confining pressure in all directions, which makes them not look leafy, and in most cases this is because they are not buried deep enough. 

It is worth noting that the heat of transformation in this type of rock comes from the magma that moved to the upper part of the crust, where the transformation that occurs due to the proximity of magma is called tangential transformation, there are many examples of non- foliated metamorphic rocks, including the following:

  1. Quartzite: A metamorphic rock of protolith sandstone, in which quartz grains swell from the original sandstone and intertwine by recrystallization.
  2. Marble: A metamorphic limestone or dolostone composed of calcite or dolomite. Recrystallization usually produces interlocking crystals larger than calcite or dolomite. Marble and quartzite often look the same, but are softer than quartz. Marble can also be distinguished from quartzite by using a drop. Of dilute hydrochloric acid, if marble is composed of calcite, the foam will appear on the marble.
  3. Horny: Can be known for their density, fine grains, hardness, agglomeration or splintered texture consisting of many silicate minerals. 
  4. Novaculite: A less common type of non-foliated metamorphic rock in nature.

Comparison between Foliated and Non-foliated Rocks

It was found that there are many differences between the two types of metamorphic rocks, the most prominent of which are the following:

Comparison between Foliated and Non-foliated Rocks
Foliated Metamorphic RocksNon-foliated Metamorphic Rocks
Complex composition that includes different types of minerals.Simple composition that includes only a few minerals, such as calcite or quartz.
Many new minerals were produced when temperature, pressure, or both changed.No new minerals are formed when temperature, pressure, or both change.
Texture appears in the form of leafy layers with bands, where the minerals are arranged in a specific direction.Texture appears as equal-dimensional grains, not arranged in a particular direction.

Characteristics of Metamorphic Rocks 

One of the characteristics that distinguish metamorphic rocks is that they:  

  1. Rigid and solid, due to the agglutination of the mineral grains that make up them. 
  2. Color diversity, due to the fact that it is made up of a mixture of different minerals. 
  3. Striped or arranged in the form of strips, as a result of being stretched and pressed again.

Rock Strength

Rocks vary greatly in strength, from quartz having durability in far more than 300 MPa to stone so softly that it may crumble with bare fingers. Relatively soft, easily worked stone was quarried for construction as early as 4000 BCE in Ancient Egypt, and stone was accustomed build fortifications in inner Mongolia as Early as 2800 BCE. The soft rock, tuff, is common in Italy, and therefore the Romans used it for several buildings and bridges. Limestone was widely employed in construction within the Middle Ages in Europe and remained into the 20th century. 

Most Well-known Rocks in Nature

  1. Reservoir Rocks

They are oil and natural gas container rocks located at great depths below the surface of the earth so that both oil and natural gas are stored in the pores between mineral grains, or in fractures in rocks with low porosity.

Important Physical Characteristics of the Reservoir 

One of the most important characteristics of reservoir rocks is that it contains porosity, permeability, sufficient hydrocarbon accumulation, and a sealing mechanism to form a reservoir from which commercially viable hydrocarbon fluxes can be produced.

Porosity and permeability are related to each other to the extent that there can be no permeability without porosity, although the opposite is not necessarily true, as all rocks have porosity, but in varying proportions. More storage, and the most effective rocks in retaining and transporting oil and natural gas.

Porosity: is a measure of the percentage of pore size, or the size of holes per unit volume of rock, for example, well-sorted sand placed in a 300 ml container will retain about 100 ml of water in its pores, meaning that its porosity is about 33 %. If petroleum is present in sandy rocks, it can also occupy this pore space, but as this sand is buried, the pressure reduces this porosity so much that only a small percentage of the porosity remains.

Permeability: is a measure of the interconnectedness of pores in the earth’s interior. The sand in the reservoir rocks contains narrow pore holes between the large pores that allow liquid to pass between them. Ranging from 100-500 mm.

Clastic sandstones rich in silica are likely to have porosity and vertical and lateral permeability over a relatively large distance. Frequently, carbonate rocks have the most complex porosity patterns, and often have little or no porosity in rock cavities, but bands containing rocks can be transformed into dolomite to have lateral porosity and permeability, and metamorphic sedimentary rocks, in addition to other sedimentary rocks can acquire secondary porosity, by weathering or groundwater movement. 

Factors Affecting Reservoir Rocks

Among the factors that can affect the characteristics of reservoir rocks are the following:

  1. Grains shape: the grains that have high sphericity tend to be somewhat compact, so the porosity of the rock decreases and the rocks with angles increase the porosity of the rock.
  2. Sorting or uniform granule size: granule size affects the characteristics of the reservoir. The more closely the size of the grains, the greater the porosity of the rock and therefore, the mixture of grains of different sizes tends to reduce the total volume of the empty space.
  3. Compaction: The more compacted and compacted the particles are, the smaller the voids are. However, sand compaction is less effective than the clay method.

Types of Reservoir Rocks

In order for a rock to be described as a reservoir rock, it must have a porous rock structure and it must have high permeability, and sedimentary rocks are the best types of rocks as reservoirs, the most prominent of which are:

  1. Sandstone reservoir rocks: which can be composed of quartz sand, or arkosic sandstone, with sand grain sizes ranging between 62 μm-2 mm. Sandstone is chosen in combination to combine porosity and permeability, that is the volume dominated by the sand on the rock, the size of the large and small grains should be, unlike in sandstone reservoirs they are those that consist sandstone usually about 25 meters.
  2. Carbonate reservoir rocks: One of the best features of carbonate rocks is their mineral content or composition. They usually consist of fossils ranging in size from a very small single cell to animals with larger shells. Most of the carbonate rocks are deposited at or near the site of their formation.
  3. Dolomite rock: Dolomite rock is often considered one of the good reservoirs as reservoir rock; Because dolomite rock contains magnesium, which is 13% smaller in size than calcium, which is formed during the dolomitization process, and this difference between calcium and magnesium provides volume between an increase in the porosity of the rock by about 13%.
  1. Source Rocks

Source rocks are rocks rich in organic matter that produce oil and gas when exposed and heated enough, and ideal source rocks contain 1% of organic matter, and at least 0.5% of total organic carbon (TOC).

The ideal source rock is usually limestone or shale, and it is worth noting that ensuring the formation of good source rock requires good retention of organic matter, which is essential and fundamental to building a complete petroleum system.

Types of Source Rocks

The source rocks are mainly divided into four groups, namely:

  1. Potential Source Rocks: are immature rocks that have the ability to produce and expel hydrocarbons if their maturity level is high.
  2. Effective Source Rocks: are rocks that contain sufficient quantities of hydrocarbons, but expelled them.
  3. Relic effective source rocks: are the effective source rocks that have stopped producing or expelling hydrocarbons, but have not exhausted their organic matter content. Due to the cooling and hypothermia occurring to the point where the rock cannot produce carbohydrates.
  4. Spent source rocks: are active rocks that were producing hydrocarbons for a specific period of time, but lost their ability to produce, either because their content of organic matter ran out, or because they reached a degree of excessive thermal maturity.

Types of Organic Matter in Source Rocks

The type of organic matter kerogen is one of the most important variables that determine the source rock, and the type of source rock is determined using a light microscope or through other physical and chemical methods, as the difference in the type of organic matter is due to the original source of the organic matter that the rock was formed, and organic materials are divided into several types, including: 

Equivalent sapropelic type: it consists of two types of organic matter, namely:

  1. First type: it is organic materials and light gases that tend to produce oil, and it consists of the remains of algae deposited in deep lakes under conditions of severe oxygen deficiency.
  2. Second type: It is a mixture of oil with organic materials that tend to produce gas, and it consists of marine plankton, and the remains of bacteria preserved in the depths of the sea in an environment dominated by a lack of oxygen.

Equivalent humic type: it consists of the third type of organic matter, which is:

  1. Third type: organic materials that mainly produce gases, and this type consists of the remains of terrestrial plant materials decomposed by bacteria and fungi under severe or medium oxidation conditions, and often the source rocks that produce coal contain organic materials of this type.
  2. Mixed type: represents a mixture of the second and third types.

The Nature of Source Rocks

The source rocks are characterized by special characteristics that determine their nature, as follows: 

  1. Included in its composition are sediments rich in organic matter that were deposited in different environments, and have passed through all stages of rock formation, forming the source rocks.
  2. The presence of source rocks at the site is a preliminary indicator that calls for exploration and is the starting point for the petroleum system.
  3. The characteristics of the source rocks determine the economic viability of the rock.
  4. The source rocks must contain organic materials of good quality and sufficient to produce hydrocarbons, and keep them or expel them to settle in a suitable reservoir.
  5. Ideal source rocks contain at least 1% organic matter and not less than 0.5% organic carbon content.
  6. Rich source rocks contain 10% of the organic matter.
  7. Source rocks of marine origin, that is, formed from marine sediments in origin, tend to produce oil.
  8. The source rocks of terrestrial origin, that is, formed from sediments of land origin such as coal – tend to produce gas.
  9. The source rocks are characterized by high porosity and low permeability, which enable them to expel hydrocarbons.

The Importance of Examining Source Rocks

There are several ways through which it is possible to explore the kerogen present in the source rock and determine the feasibility that can be obtained from it and the most important of these methods is the organic rock examination, as this method does the following: 

  1. Describes the kerogen present in the source rock accurately by providing information on the mineral composition, including information on heat-sensitive kerogen and heat-resistant kerogen.
  2. Identifies the mixtures of organic materials found in the rock.
  3. Determines the differences in the concentration of organic matter in the vertical and lateral surfaces within the source rock.
  4. Provides evidence of the ability of the source rock to generate petroleum by determining the amount of oil, oil films, and the amount of pyrolytic carbon.

The Outcrop

The Outcrop is the visible part of the rocks, which appears as a result of erosion and weathering factors. These rock detectors are found in mountainous areas, on steep slopes and also at areas or heights where trees do not grow, or at areas that are not covered by soil such as the effects of glacier runoff where they look like roads and sediment accumulation and plant growth may obscure these rock discoveries, especially the basic rocks from these rock discoveries, and the rock detectors facilitate mapping and sampling of rocks.

How to Describe Rock Detectors?

Some rock detectors are noticeable to the eye and a worker in the field may wish to provide a more detailed or accurate description than is possible using a diagram. One way to do this is to map out and describe the outcrops or rock detectors and describe them using photographs. There is plenty of information from maps made from pictures, but here are some things to keep in mind:

  1. Boundaries between Rock Layers

In some cases, the layers may be metamorphic sedimentary layers, but they can be lava flows or layers formed by segregation in igneous intrusions and it is often difficult to know the origin of the layers, so it is important to describe them independently of trying to explain their origin, in addition to defining layer boundaries and measuring its direction.

  1. Layers of Rock that Exist Singly or in Groups

The shape, lateral extent, and contact with adjacent layers must be shown, as some layers may show the lateral extent and may show contact with adjacent layers. In addition, the rock texture is described, the size of the grains is determined, and any characteristic description of the layer that distinguishes it from the adjacent layers is described.

  1. Geological Structures

It mainly includes faults and folds, as it is necessary to determine the direction of movement of the faults and the direction of the axis of the folds.

  1. Veins and Dikes

They are the thin structures that cut through layers of rock, some of which may be parallel to the layer, sometimes found either in groups or singly.

  1. Fractures 

Fractures are voids within rock layers that may be filled with secondary minerals such as oxides, whether iron or manganese oxides, in this case, their direction is described and the spacing within the boundaries of these fractures or voids is described, the filled material and thickness.

Types of Boundaries between Rock Layers

The boundaries between the layers differ according to the conditions to which the rock discoveries are exposed, and the following are the types of those boundaries that may have escaped when describing the rock discoveries: 

  1. Angular Unconformity: It arises when the newer layers are immersed within the older layers that are at the bottom and appear in a non-parallel way, meaning that the two layers are angular.
  2. Nonconformities: They arise when sedimentary layers are deposited over igneous layers or exposed to igneous penetration, where the sedimentary layers are often the most recent.
  3. Disconformity: It is when the sedimentary layer surfaces are subjected to erosion or erosion so that they are parallel to the layers.

Determination of Stratigraphic Boundaries in Rock Discoveries

The process of setting boundaries between rock layers is important to distinguish the types of rock layers from each other, for example, a change may appear between clay layers to lime layers, and here it is necessary to set boundaries between these two layers, sometimes the change between layers is gradual and here is placed. 

An appropriate boundary can separate these layers quite well, for example, if the bottom layer is mainly composed of clay rocks with some sandy layers, and above the sandy layers a layer of secondary claystone, here the boundary is placed at the sandstone that makes up more than 50% of the layers. 

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