Magma : Definition, Forming Process, Movement, Composition, Type and Classification of Magma

"flowing rock" by paul bica is licensed under CC BY 2.0

Definition of Magma

The definition of magma is a mixture of molten or semi-liquid rock located in magma chambers under the earth's crust. This mixture is usually composed of four materials, namely:

• The body of magma in the form of a super hot liquid, also called melt (melt / melt).
• Minerals crystallized by molten magma
• Solid rock that enters the magma liquid coming from the walls of the magma chamber, as well as
• Gaseous substances dissolved into magma

Although magma is basically confined in a magma chamber, magma can move so that it reaches the earth's surface. Liquid magma that comes out to the earth's surface is called lava. While magma that has cooled and crystallized is called igneous rock .

Magma is a very dynamic ultra hot liquid. With a temperature of about 700 to 1,300 degrees Celsius, magma is a very fluid and dynamic substance. Magma can undergo changes (evolution) into new, different landforms. Changes that occur physically and chemically are caused by changes in the environment they experience during movement.

Magma Formation Process

Magma comes from the magma chamber which is located between the layers of the Earth's mantle and the Earth's crust. Earth's mantle and crust are generally solid. So the presence of molten magma between the two is very important for geologists to study the structure and morphology that occurs in the Earth's mantle. Usually magma activity is also influenced by geological activity that occurs in the earth's mantle layer below it.

The origin of magma is formed in several different ways, this is due to differences in structure, temperature and pressure between the layers of the earth's mantle and the earth's crust. There are three processes of magma formation, namely decompression melting, heat transfer  and flux melting .

Decompression melting

Decompression melting is the upward movement of the hot components of the Earth's mantle. This hot material rises to the top of the lower pressure through heat convection. According to the laws of physics, pressure is directly proportional to the melting point. Regions with lower pressure also have lower melting points. Decompression (a decrease in pressure) above it makes the solid mantle layer melt into magma

Decompression melting generally occurs in divergent areas, where tectonic plates are separated from each other. The movement of the plate causes the magma beneath it to move to fill the empty space above it. The magma then solidifies to form igneous rock that composes new layers of the Earth's crust.

Decompression melting also occurs in the mantle plume, which are passageways consisting of hot material from the Earth's core to the lower-pressure crust. When below sea level, this mantle plume, called magma, pushes the magma to the ocean floor. This process that took place over millions of years formed a mid-ocean volcanic island.

Heat Transfer

Magma can also form when heat energy from molten rock intrudes into the cold crust of the Earth. As this molten rock solidifies, it also spreads heat around it. As a result, the surrounding rock melts and forms magma.

Heat transfer occurs in convergent areas, where tectonic plates move and collide. The molten rock beneath a tectonic plate affects the cold layer above it. This process generates heat and creates magma. Over millions of years, the magma beneath this subduction zone changes to form volcanic arcs, which are a series of active volcanoes in a convergent area.

Flux Melting

Flux melting occurs when water and carbon dioxide are added to rock. These two compounds cause rock to melt at a lower temperature than normal. Flux melting creates magma in these areas, which when viewed from the temperature and pressure should still be in the form of rock (not melted).

Like Heat Transfer, Flux melting also occurs in convergent areas (subdiction zones). In this case, the water covering the seabed in the subduction zone will lower the melting point of the Earth's mantle. This causes magma to rise to the surface.

Magma Movement

Magma can move out of the magma chamber that is between the Earth's mantle and the Earth's crust. The movement of magma occurs in two main ways, namely magma intrusion and magma extrusion .

Magma Intrusion

Magma intrusion is the process of penetrating magma through cracks or gaps in the surface layer of the lithosphere, but not reaching the earth's surface. Magma intrusion usually occurs due to increased pressure from the gases that make up the magma itself.

Here are the types of Magma Intrusion:

Flat intrusion (sill or plate intrusion) : Magma infiltrates between two rock layers, horizontally, and parallel to the rock layers.

lakolith : Magma that breaks through between the Earth's uppermost layers. The shape is similar to a convex lens or macaroon.

Folds : Rocks resulting from magma intrusion that infiltrate and freeze between the folds 

Diathermic : The hole (pipe) between the magma chamber and the volcanic crater. The shape is like an elongated cylinder.

Magma Extrusion

Magma extrusion is the release of magma from the bowels of the earth towards the earth's surface. Extrusion of magma is a process commonly known as volcanic eruption. This process can occur because of a gap, crack or hole that leads to the earth's surface plus an increase by the gases that make up the magma. Magma extrusion can occur on land or at sea.

Following are the types of Magma Extrusion:

1. Linear Extrusion

Linear extrusion occurs when magma comes out through cracks or elongated faults to form a series of volcanoes. For example, the Laki Volcano in Iceland.

2. Extrusion Area

Area extrusion occurs when the magma is close to the earth's surface, so that the magma melts out in several places in a certain area. For example Yellow Stone National Park in the United States which covers an area of ​​up to 10,000 km2.

3. Central Extrusion

Central Extrusion occurs when magma escapes through a hole (magma channel) and forms separate mountains. For example Mount Krakatoa, Mount Vesucius, etc.

Magma can move infiltrate to a place with a lower density (low density), for example in sedimentary rock structures. When cooling occurs, magma remains frozen. This magma intrusion process is called pluton .

There are several forms of pluton, such as dikes and xenoliths. Dikes are slabs of magma (not yet melted) that infiltrate another rock body. A xenolith is a type of rock trapped in another rock. Most xenoliths are crystalline fragments (rocks) from deep within the earth that are trapped in magma during cooling.

The most common way of movement of magma to the earth's surface is through lava. Lava from this volcanic eruption can be in the form of a "fountain" consisting of thick molten rock and other material that melts and moves like a river. Lava will freeze to form volcanic rock and volcanic glass.

When an explosive volcanic explosion occurs, the movement of magma can reach the atmosphere. Magma in the air then solidifies to form tephra . Tephra is often called volcanic ash. As it falls back to earth, tephra can collect or combine with other rocks. For example, pumice stone.

Properties of Magma

The properties of magma are divided into two, the first is the physical properties of magma and the chemical properties of magma. The physical properties of magma include viscosity, specific gravity and temperature of magma, while the chemical properties of magma include volatile compounds, non-volatile compounds and other elements called trace elements.

Viscosity is the viscosity or tendency not to flow. A liquid with a high viscosity will have a lower tendency to flow than a liquid with a low viscosity. The same is true of magma.

Magma viscosity is determined by SiO2 content and magma temperature. The higher the SiO2 content, the lower the viscosity or the thicker it is. On the other hand, the higher the temperature, the lower the viscosity. Thus, basaltic magma flows more easily than andesitic or rhyolitic magmas. Likewise, andesitic magma flows more easily than rhyolitic magmas.

Magma Content and Classification Type

Magma is liquid rock that is inside the Earth. This hot magma has a lot of substances in it. However, research on the content in magma can only be studied through the igneous rocks that make up the magma. This is because of the impossibility of taking samples of magma that is in the bowels of the Earth and its very hot nature.

Some of the content in magma is known that it is a chemical substance. The composition that composes this magma is very complex. Approximately 99% of this magma is composed of 10 chemical elements, including silicon (Si), titanium (Ti), aluminum (AI), iron (Fe), magnesium (Mg), calcium 

(Ca), sodium (Na), potassium (K), hydrogen (H) and oxygen (O). Those are some of the main ingredients possessed by magma. These contents can still be varied so as to produce a content that combines with one another.

Although there are many elements that make up magma, in general we will find that SiO2 dominates in the composition of magma. If we percentage the amount, then SiO2 makes up the most magma, which is more than 50% of the magma's weight. Then 44% of the weight of the magma is occupied by AI2O3, FeO, MgO, CaO. The rest is Na2O, K2O, TiO2, and H2O which make up about 6% of the bert magma.

As one of the contents of the bowels of the Earth, this magma turns out to have several different types from one another. This type of magma is seen based on the content of SiO2. Based on this category, magma is divided into three types, namely:

• Basaltic Magma or Basaltic Magma, namely magma that has a SiO2 content of 45 to 55% by weight, high Fe and Mg content, and low K and Na content.
• Andesitic magma or Andesitic Magma, which is magma that has a SiO2 content of 55 to 65% by weight, contains moderate or moderate amounts of Fe, Mg, Ca, Na, and K.
• Rhyolitic magma or rhyolitic magma, which is magma that has a SiO2 content of 65 to 75% percent by weight, then also contains low amounts of Fe, Mg, and Ca, and has a high content of K and Na.

There is also another classification of magma, namely:

• Alkaline magma is magma containing 50% SiO2, high temperature between 9000-12000 C and low viscosity, flowing.
• Intermediate magma is magma that has a SiO2 composition of between 50 and 60% (in between alkaline and acidic magmas).
• Acid magma is magma that has a SiO2 composition between 60 to 70%, a temperature below 8000 C and a high viscosity so that the mobility is low.

Changes in Magma Composition

The process of freezing magma into rock begins with the formation of mineral crystals. According to their chemical composition, the formation of mineral crystals occurs at different temperatures. It should be understood that with the formation of crystals, it means that there are chemical elements from the magma solution that are taken and bound into the crystal, so that the content of these elements in the liquid or magma solution is reduced.

If the crystals that form in magma have a density greater than that of magma, the crystals will settle and the liquid will separate from the crystals. On the other hand, if the crystals formed are of lower density than magma, the crystals will float. When the liquid magma comes out due to pressure, the crystals will be left behind.

This situation will change the chemical composition of the remaining magma liquid. If a lot of the chemical composition is reduced from the initial magma due to the formation of mineral crystals, it will form a new magma with a different composition from the initial magma. Such a change in the chemical composition of magma is referred to as magma differentiation by crystal fractionation. This process can cause basaltic magma in a volcano to change from basaltic to andesitic and even rhyolitic. This change in magma composition can change the type of eruption of a volcano.

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Metamorphic Rocks: Definition, Types and Impacts

"Gneiss 1" by James St. John is licensed under CC BY 2.0

 Definition of Metamorphic Rock

There are many types of rock. One type of rock is metamorphic rock. This metamorphic name is a name for the type of rock that complements igneous and sedimentary rocks. This metamorphic rock is often referred to as metamorphic rock. Metamorphic rock or metamorphic rock is a group of rocks that are the result of the alteration or transformation of a pre-existing rock type (protolith) by a process called metamorphosis or changing shape.

This metamorphic rock has very important uses for humans. Through research conducted on these metamorphic rocks can be obtained very important information about the temperature and pressure that occurs deep in the earth's surface. However, nowadays many metamorphic rocks have been exposed on the earth's surface due to soil erosion and uplift.

The Process of Occurrence of Metamorphic Rocks

This metamorphic rock is not a type of rock that directly exists in this world. To turn into metamorphic rock, it takes several processes. The process of the occurrence of these metamorphic rocks comes from pre-existing rocks, namely protoliths. Protolith or original rock that is subjected to heat of more than 150 degrees Celsius and also extreme pressure will experience major physical or chemical changes. There are many types of protolith rocks. Included in this protolith rock are igneous rock, sedimentary rock, or it could be other metamorphic rocks of older age such as Gneiss, slate, marble, and schist stones.

Types of Metamorphic Rocks

 There are various types of Metamorphic Rocks and not just one. This metamorphic rock can be divided into three types, namely contact metamorphic rock, dynamo metamorphic odor, and pneumatolist contact metamorphic rock. To get to know each of these metamorphic rocks closer, we will discuss them one by one.

Contact metamorphic rock

The first type of metamorphic rock we will discuss is the contact metamorphic rock type. Contact metamorphic rock is a type of metamorphic rock that undergoes metamorphosis as a result of very high temperatures or as a result of magma activity. There are also those who state that these contact metamorphic rocks are rocks formed due to the influence of magma intrusion at very high temperatures. The existence of very high temperatures originating from this magma activity causes changes in the shape and color of the rock. This high temperature is also due to its proximity to magma. Examples of these contact metamorphic rocks are limestone or limestone into marble, then batholiths, lacolitic rocks, and sill rocks. One thing we need to know about this type of rock,that is, this type of rock is influenced by the location of the intrusion, where the farther it is from the intrusion, the degree of metamorphosis will decrease.

Dynamo metamorphic rock

The second type of metamorphic rock is dynamo metamorphic rock. Dynamo metamorphic rock is a type of rock that undergoes metamorphosis as a result of high pressure originating from endogenous forces for a long time, and is produced in the process of forming the earth's crust due to endogenous forces. This dynamo metamorphic rock usually occurs or is in the upper part of the earth's crust. The existence of pressure in the opposite direction causes changes in mineral grains, some are flat and some are back into crystal form. Some of these types of metamorphic rocks turn into crystals. Examples are pulverized rock and shale. Another example of dynamo metamorphic rock is mud stone or mud stone into slate or slate. This type of rock is often found in areas of faults or folds.

Pneumatolytic contact metamorphic rocks

The next type of metamorphic rock is pneumatolist contact metamorphic rock. This type of rock is a rock that undergoes a metamorphic process as a result of the influence of the gases present in the magma. The influence of this hot gas causes changes in the chemical composition of the minerals of this rock. An example of a pneumatolist contact metamorphic rock is quartz with borium gas turning into tourmaline or a kind of gemstone. Another example of this type of stone is quartz stone with fluorium gas and housed into topaz.

Those are the kinds or types of metamorphic rocks that are around us or that we often encounter. Metamorphic rock is basically a type of rock that has undergone a metamorphic process. Metamorphosis that occurs in rock itself is a process in which an object changes form from one form to another. In this stone metamorphosis, the metamorphosis process consists of various kinds and not just one.

Types of Metamorphic Rock Metamorphic

Metamorphic rock is a type of rock that undergoes a metamorphic or metamorphic process. The process of rock metamorphism itself consists of three kinds, namely thermal metamorphosis, dynamo metamorphosis, and also regional metamorphosis. Then, what exactly is the meaning of each type of metamorphosis?

Thermal Metamorphosis

Thermal metamorphosis is also known as touch metamorphosis, where this type of metamorphosis is a metamorphosis that occurs when rocks are touched by hot magma around the magma chamber or intrusive rock bodies. An example of this thermal or touch metamorphosis is limestone turning into marble.

Dynamo's Metamorphosis

The next type of metamorphosis is dynamo metamorphosis or also known as pressure metamorphosis. This type of metamorphosis is a metamorphosis that occurs where there is rock that is exposed to pressure from tectonic events (on the earth's crust only occurs at the top) so that it will undergo metamorphosis. An example of this type of metamorphosis is in the fault plane a friction mirror or milonite flour is formed.

Regional Metamorphosis

The next type of metamorphosis is regional metamorphosis. Regional metamorphosis is also known as dynamic metamorphosis. Regional metamorphosis is a metamorphosis that affects a very wide area that occurs at the bottom of the earth's crust as a result of pressure throughout the formation of schist, mica, phyllite, and gneiss. Rocks can undergo metamorphosis only with or if they are at great depths below the earth's surface, experience high temperatures, and also experience great pressure caused by the enormous weight of the rock layers above and will disrupt the structure of the earth.. This regional metamorphosis tends to make the rock harder, and at the same time causes the formation of a foliated, schistos, or gneiss texture consisting of a mineral palanar arrangement. This causes plate or prismatic minerals such as mica and hornblende to have the longest axes that are parallel to each other. The main characteristic of metamorphic rocks that undergo this type of metamorphosis is the presence of a shiny color and also not fossilized.

Cataclastic Metamorphosis

Next there is a type of cataclastic metamorphosis. This cataclastic metamorphosis occurs as a result of mechanical deformation, such as when two bodies of rock slide past each other along a fault zone. Friction that occurs along the shear zone will generate heat, and the rock is mechanically deformed. The rock is then crushed and crushed as a result of the shift. This type of metamorphosis is not common confined to a narrow zone where a horizontal fault would occur.

Hydrothermal Metamorphosis

Next there is hydrothermal metamorphosis. Hydrothermal metamorphosis occurs when there are rocks that are changed at high temperatures and medium pressures due to hydrothermal fluids. This means that the rock is undergoing hydrithermal metamorphosis. This is common in basaltic rocks which generally lack hydrate minerals. This hydrothermal metamorphosis causes alteration to hydrate minerals rich in Mg – Fe such as talc, chlorite, serpentine, actinolite, tremolite, zeolite, and clay minerals rich in ore deposits are also often formed as a result of hydrothermal metamorphism.

Overlapping Metamorphosis

Then there is another type of metamorphosis, namely overlapping metamorphosis. Overlap metamorphosis will occur when sedimentary rocks are buried to a depth of several hundred meters, and temperatures greater than 300 degrees Celsius can develop in the absence of differential stress. New minerals grow, but the rock does not appear to be metamorphosed, the main mineral usually produced from this process is zeolite. This overlapping metamorphosis is an overlapping metamorphosis with diagnosis to some extent. and this metamorphosis can turn into regional metamorphosis with increasing temperature and pressure.

Those are some types of metamorphic processes that can occur in metamorphic rocks. The process of metamorphosis that occurs in this stone occurs because it is natural and occurs because of natural processes. The metamorphosis process that occurs in these rocks occurs due to natural processes involving certain elements such as water, wind, air temperature, sunlight, and so on. All that are involved in the metamorphosis process of this rock are natural materials or those that can be found in nature and cannot be made by humans.

The process of metamorphosis itself takes time. To be able to turn into a good stone and a perfect stone, the metamorphosis process itself requires supportive conditions, including the surrounding environment and time. The time required is also quite long and even quite long. Without human knowledge, these stones already exist and we ourselves are not aware of it. This is what is called a natural process.

Impact of Metamorphosis

The metamorphosis process experienced by these rocks not only produces certain stones, but also we will get certain impacts from this metamorphosis process.

When extraterrestrial material, such as types of solar system  types such as meteorites or comets, falls to earth, or in the event of a very large volcanic explosion, very high stresses can occur in the affected rocks.

These very high pressures produce minerals that can only be stable at very high pressures, such as SiO2 polymorphs such as coesite and stishophyte.

In addition they can also produce textures known as shock lamellae in mineral grains and also textures like or resembling broken cones in impacted rock.

That is the impact of the metamorphosis process. That's a little information about metamorphic rocks. Although not very complete but hopefully can provide useful knowledge.

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Sedimentary Rocks: Definition, Process, and Types

"Sedimentary rocks" by thy is licensed under CC BY 2.0

Definition of Sedimentary Rock

Sedimentary rock is one type of rock which is formed as a result of compaction of sediment in the form of loose material. Sedimentary rocks or often also referred to as sediments are rocks that are formed from deposited materials carried by water or wind. There is another understanding of sedimentary rocks, namely rocks formed due to the process of petrification or lithification from the results of weathering processes and also soil erosion.which has been carried away by the current and then deposited. An expert, namely Hutton (1875) stated that this sedimentary rock is a rock formed by sediment consolidation, as loose material, which is transported to the depositional location by water, wind, ice and also gravitational avalanches, soil movements or landslides. Besides being formed from this, these sedimentary rocks are also formed by the evaporation of a solution of calcium carbonate, silica, salt, and other materials. This is what is called sedimentary rock.

Did you know that these sedimentary rocks are very numerous and widely spread on the earth's surface in this world? Even according to Tucker (1991), that 70% of the rocks found on the entire surface of the earth is a type of sedimentary rock. But the rock is only 2% of the volume of the entire earth's crust. This indicates that sedimentary rocks are spread very widely on the earth's surface, but their thickness is only relatively thin. The earth's crust is indeed composed of various kinds of materials, not only rocks but also layers of soil, sand, and others. And these rocks also include elements that make up the composition of the earth's crust . The rocks that make up the composition of the earth's crust are divided into various types and one of them is this sedimentary rock.

Sedimentary Rock Formation Process

 This sedimentary rock undergoes a process of compaction and also compaction of loose material (sediment) to become a solid sedimentary rock. This process is known as diagenesis. The diagenetic process itself can occur at atmospheric temperatures and pressures up to a temperature of 300 degrees Celsius and also a pressure of 1-2 kilobars which takes place from sediment experiencing burial to being lifted and also exposed again above the surface   of the  earth's atmosphere . Based on this, there are 3 types of diagnosis, namely:

1. Eogenic diagnesa, namely the initial diagnesa that occurs in sediments below the water surface.
2. Mesogenic diagnosis, namely the diagnesa that occurs when the sediment undergoes a deeper burial.
3. Telogenic diagnosis, which occurs when sedimentary rocks are exposed back to the earth's surface due to uplift and erosion.

Those are the various types of diagnesa that occur in sedimentary rocks. Because there are various kinds of this diagnosis, the degree of compactness of these sedimentary rocks also varies or varies. Various kinds of compactness of these sedimentary rocks include:

• Loose materials, namely those that are still in the form of deposits or sediments.
• Solid or indurated. At this stage the consolidation of the material occurs in dry conditions. However, it will decompose when put into water.

Those are some types of compactness that occur in sedimentary rocks. After we study the compactness of sedimentary rocks, then we will learn about the texture of sedimentary rocks.

Sedimentary Rock Texture

As previously stated, these sedimentary rocks have a variety of textures. Sedimentary rock can be textured clastic or non-clastic. However, if the rock is very compact and if recrystallization or re-crystallization has occurred, then this sedimentary rock has a crystalline texture. Sedimentary rocks that have a crystalline texture generally occur in limestone and also silica-rich sedimentary rocks which are very compact and hard.

That's some information about sedimentary rocks that are widely available around us. As rocks that are widely found around us, these sedimentary rocks have many uses, especially for building materials or to decorate homes and buildings today. Thus information about sedimentary rocks that we can learn so that we can distinguish this type of rock from other rocks.

Types of Sedimentary Rocks

Sedimentary rocks are formed from igneous rock or other solid substances which are then eroded in certain places and then settle and eventually become hard. Sedimentary rocks are usually in the form of horizontal layers. Did you know that this type of sedimentary rock is still further divided into several types? The types of sedimentary rocks are classified according to several categories. Many experts categorize or classify these rock types with different amounts. Then, what are the types of these sedimentary rocks?

1. According to Pettijohn (1975), O'Dunn and Sill (1986) –  divide these sedimentary rocks based on their texture which is divided into two major groups, namely clastic sedimentary rocks and non-clastic sedimentary rocks.

Clastic sedimentary rocks are also called detritus, mechanical, exogenous sedimentary rocks which are sedimentary rocks consisting of clastics or crushed rocks that settle naturally or mechanically by their own gravity. This type of rock is formed as a result of reworking or reworkin from pre-existing rocks. The reworking process that occurs as rock formation includes weathering, erosion, transportation, and also redeposition or re-deposition. To support this process, several media are needed, namely water, wind, ice, and also the effects of gravity or its own weight. Especially for the latter media or this gravitational medium as a result of rock slides that have existed before.What we need to know from this type of rock group is that this rock group is fragmental or consists of grains or rock fragments so that it has a clastic texture. Examples of clastic sedimentary rocks include breccia, conglomerate, sandstone, and slab rock. Breccia is a sharp-angled gravel deposit that is still close to its place of origin. Conglomerate stones are gravel deposits whose corners are rounded (the corners that are far away are carried by the river). While sandstone is sedimentary rock derived from rock fragments measuring 1/16 to 2 millimeters.and also slate. Breccia is a sharp-angled gravel deposit that is still close to its place of origin. Conglomerate stones are gravel deposits whose corners are rounded (the corners that are far away are carried by the river). While sandstone is sedimentary rock derived from rock fragments measuring 1/16 to 2 millimeters.and also slate. Breccia is a sharp-angled gravel deposit that is still close to its place of origin. Conglomerate stones are gravel deposits whose corners are rounded (the corners that are far away are carried by the river). While sandstone is sedimentary rock derived from rock fragments measuring 1/16 to 2 millimeters.

Non-clastic sedimentary rock. Next we will discuss the types of non-clastic rock groups. This non-clastic rock is a type of sedimentary rock that is formed as a result of the evaporation of a solution or the deposition of material that is in the same place. The process of formation of this type of rock can occur by chemical, biological or organic processes, or a combination of the two, namely a combination of chemical and organic or biological. The process which is a combination of the two is called biochemistry. The process of rock formation that occurs biologically or organically is the formation process carried out by certain natural activities, namely by plants and animals. An example of this organic rock formation process is the formation of the home of marine animals or corals,accumulation of animal shells (fossils), and the burial of wood as a result of the land subsidence into the sea.

2. According to Sanders in 1981 and Tucker 1991 -  classify or divide these sedimentary rocks into four kinds, namely:

Detritus or clastic sedimentary rock

Chemical sedimentary rocks – Chemical sedimentary rocks are sedimentary rocks that are formed through chemical reactions, such as evaporation, precipitation, and concentration. Examples of these chemical sedimentary rocks are rock salt, gypsum rock, stalactites, and also stalagmites.

Organic sedimentary rocks – These organic sedimentary rocks are also known as sedimentary rocks of bodily origin. Organic sedimentary rock is sedimentary rock derived from the remains of living bodies or made by living organisms. This type of rock group can be broken down into two types, namely biomechanical sediments and biochemical sediments. Biomechanical sediments are deposits of the remains of living body parts that settle naturally under their own weight, for example, limestone, shells, numilites, and layered limestone. While biochemical sedimentary rocks are rocks that occur due to the deposition of limestone and silicium elements with living rocks.Examples of these rocks are limestone reefs or shellfish animal houses and also diatomaceous earth or the deposition of silicon elements due to the presence of carbon dioxide in the water which is absorbed by diatomaceous algae.

Volcanic clastic sedimentary rock. – Especially for this type of rock, it is a sedimentary rock that has a clastic texture with the main constituent materials derived from volcanic activity.

3. According to Graha (1987) –   This type of sedimentary rock is a sedimentary rock which is generally non-classical in texture. Graha divides these sedimentary rocks into four groups as well, namely:

• Detritus sedimentary rock (clastic/mechanical)
• Coal sedimentary rock (organic/vegetable)
• Silica sedimentary rock, and
• Carbonate sedimentary rock

Especially for this type of rock and also this silica sedimentary rock can be a clastic sedimentary rock or a non-clastic sedimentary rock. Then based on the composition of the main constituent, clastic sedimentary rocks or clastic textured, can be further divided into three kinds. Namely:

• Siliciclastic sedimentary rock. This type of rock is a type of clastic sedimentary rock whose main constituent minerals are quartz and feldspar.
• Then there are volcanic clastic sedimentary rocks. Volcanic sedimentary rock is one type of sedimentary rock with the main constituent materials derived from volcanic activity, such as glass, crystal, and/or lithic.
• The last one is clastic carbonate sedimentary rock, also known as clastic limestone. This type of rock is a clastic sedimentary rock with the main constituent minerals being carbonate material (calcite).

Those are the types of clastic sedimentary rocks according to their main constituent composition. Next we will discuss the color of sedimentary rocks.

Sedimentary Rock Color

If we generally find that the rock is dark, i.e. black or slightly grayish, then this sedimentary rock has a slightly subdued or light color. This sedimentary rock has a distinctive color. The color of these sedimentary rocks is generally light or bright, such as white, yellow, or light gray. However, this sedimentary rock is not always bright or light in color. These sedimentary rocks have dark colors, namely dark gray to dark black, as well as red and brown. Thus this sedimentary rock has a variety of colors. The color variation of sedimentary rocks is caused by the composition of the constituent materials.

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Igneous Rocks: Definition, Texture, Structure, Composition and Classification of Igneous Rocks

"Obsidian 1" by James St. John is licensed under CC BY 2.0

Definition of Igneous Rock

Igneous rock or igneous rock (from Latin : ignis , "fire") is a type of rock formed from cooled and hardened magma , with or without a crystallization process , either below the surface as intrusive (plutonic) rock or above the surface as rock. extrusive (volcanic). This magma can come from semi-molten rock or rock that is already present, either in the mantle or crust. Generally, the melting process occurs by one of the following processes: temperature rise , pressure drop, or a change in composition. More than 700 types of igneous rock have been described, most of which form beneath the surface of the Earth's crust .

According to scholars such as Turner and Verhoogen (1960), F. F Groun (1947), Takeda (1970), magma is defined as a naturally occurring viscous, incandescent, viscous silicate liquid, with a high temperature between 1,500–2.5000C and mobile (can move). and is found in the lower crust of the earth. In the magma there are several soluble, volatile materials (water, CO2, chlorine, fluorine, iron, sulfur, etc.) found in igneous rocks.

When magma experiences a decrease in temperature due to travel to the earth's surface, minerals will form. This event is known as the scattering event. Based on the crystallization of silicate minerals (magma), by NL. Bowen compiled a series known as Bowen's Reaction Series.

Texture of Igneous Rock

Texture is defined as a condition or a tight relationship between minerals as part of a rock and between minerals with a glass mass that forms the foundation mass of the rock.

The texture of igneous rocks is generally determined by three important things, namely:

Crystallinity

Crystallinity is the degree of crystallization of an igneous rock at the time the rock was formed. Crystallinity in its function is used to indicate how much is in the form of crystals and which is not in the form of crystals, but it can also reflect the speed of magma freezing. If magma continues to freeze slowly then the crystals are coarse. Meanwhile, if the freezing continues quickly then the crystals will be smooth, but if the cooling continues very quickly then the crystals are amorphous.

In its formation, there are three classes of degrees of crystallization, namely:

• Holocrystalline, i.e. igneous rock where everything is composed of crystals. The holocrystalline texture is characteristic of plutonic rocks, which are microcrystalline rocks that have frozen near the surface.
• Hypocrystalline, ie when some rocks consist of glass mass and some consist of crystal mass.
• Holohyaline, i.e. igneous rock which is all composed of glass mass. Most holohyaline textures are formed as lava (obsidian), dike and sill, or as facies smaller than rock bodies.

Granularity

Granularity is defined as the grain size (size) in igneous rocks. There are two types of grain size textures known, namely:

• Phaneric/phanerocrystalline

Most of the crystals of this group can be distinguished from each other megascopically with the naked eye. The crystals of this faneric type can be divided into:

• Fine, if the grain diameter is less than 1 mm.
• Medium (medium), if the grain diameter is between 1-5 mm.
• Coarse, if the grain diameter is between 5-30 mm.
• Very coarse, if the grain diameter is more than 30 mm.

Afanitik

Most of the crystals of this group are indistinguishable with the naked eye, so a microscope is needed. Rocks with an aphanitic texture can be composed of crystal, glass or both. In microscopic analysis can be distinguished:

• Microcrystalline, if the minerals in igneous rocks can be secured with the help of a microscope with a grain size of about 0.1 – 0.01 mm.
• Cryptocrystalline, when the minerals in the igneous rock are too small to be secured even with the aid of a microscope. The grain size ranges from 0.01 to 0.002 mm.
• Amorphous / glassy / hyaline, if the igneous rock is composed of glass.

Crystal Form

Crystal form is the nature of a crystal in rock, so it is not a rock property as a whole. From a two-dimensional view, there are three forms of crystals, namely:

• Euhedral, if the boundary of the mineral is the original form of the crystal member.
• Subhedral, when some of the crystal boundaries are no longer visible.
• Anhedral, if the mineral no longer consists of original crystals.

From a three-dimensional perspective, four crystal forms are known, namely:

• Equidimensional, if the three-dimensional crystal form is the same length.
• Tabular, if the two-dimensional crystal form is longer than one of the other dimensions.
• Prismatic, if the crystal form in one dimension is longer than the other two dimensions.
• Irregular, if the crystal form is chaotic.

Relationship Between Crystals

The relationship between crystals or also called the relationship is defined as the relationship between crystals / minerals with one another in a rock. Broadly speaking, relations can be divided into two,

Equigranular

Equigranular is, if the relative size of the crystals that make up the rock are of the same size. Based on the ideality of the crystals, therefore equigranular is divided into three, namely:

• Panidiomorphic granular, ie when most of the minerals consist of euhedral minerals.
• Hipidiomorphic granular, ie when most of the minerals consist of subhedral minerals.
• Allotriomorphic granular, ie when most of the minerals consist of anhedral minerals.

Inequigranular

Inequigranular is if the size of the crystal grains as rock-forming is very different. The large mineral is called the phenocryst and the other is called the ground mass or matrix which can be mineral or glass.

Structure of Igneous Rock

Structure is the macro appearance of rocks which includes the location of a clear / general layer of rock layers. Most igneous rock structures can only be seen and observed in the field, for example:

• Pillow lava or pillow lava, which is a very typical structure of underwater volcanic rock, forms a pillow-like structure.
• Joint structure, is a structure characterized by the presence of joints that are arranged regularly perpendicular to the direction of the current. While the structure that can be seen and observed in rock samples (hand speciment samples), namely:
• Massive, that is, if it does not show the presence of currents, traces of gas (does not indicate the presence of holes) and does not indicate the presence of other fragments embedded in the igneous rock body.
• Vesicular, which is a structure with holes caused by the release of gas at the time of magma freezing. The holes show a regular direction.
• Scoria, which is the same structure as the vesicular structure but the holes are large and show a chaotic direction.
• Amygdaloidal, namely the structure where the gas holes have been filled by secondary minerals, mostly silicate or carbonate minerals.
• Xenoliths, namely structures that show the presence of other rock fragments / fragments that enter the intrusive rock.
• Most igneous rocks are without structure (massive), while the structures that are present in igneous rocks are made by joints or fractures and magma freezing, for example: columnar joints and sheeting joints.

Mineral Composition of Igneous Rock

To determine the mineral composition of igneous rock, it is enough to use the color index of the crystalline rock. On the basis of the color of minerals as a constituent of igneous rocks can be grouped into two, namely:

• Felsic minerals, which are light colored minerals, mainly consist of quartz, feldspar, feldspathoid and muscovite minerals.
• Mafic minerals are dark colored minerals, especially biotite, pyroxene, amphibole and olivine.

Igneous Rock Classification

Igneous rocks can be classified according to the manner in which they occur, the content of SiO2, and the color index. Thus, different rock names can be determined, even though they are in the same rock type, according to their classification basis.

Classification by way of occurrence

According to Rosenbusch (1877-1976) igneous rocks are divided into:

Effusive rock, for igneous rock that forms on the surface.

Dike rock, for igneous rock that forms near the surface.

Deep seated rock, for igneous rock deep in the earth. By WT Huang (1962), this rock type is called plutonic, while effusive rock is called volcanic rock.

Classification based on SiO2 

According to (CL Hugnes, 1962), namely:

Acid igneous rock, if the SiO2 content is more than 66%. For example, rhyolite.

Intermediate igneous rock, if the SiO2 content is between 52% - 66%. For example, dasit.

Alkaline igneous rock, if the SiO2 content is between 45% - 52%. For example, andesite.

Ultra-alkaline igneous rock, when the SiO2 content is less than 45%. For example, basalt.

Classification by color index

According to (SJ Shand, 1943), namely:

Leucoctaris rock, if it contains less than 30% mafic minerals.

Mesococtic rock, if it contains 30% - 60% mafic minerals.

Melanocratic rock, if it contains more than 60% mafic minerals.

Meanwhile, according to SJ Ellis (1948) also divides igneous rocks based on their color index as follows:

Holofelsic, for igneous rocks with a color index of less than 10%.

Felsic, for igneous rocks with a color index of 10% to 40%.

Mafelsic, for igneous rocks with a color index of 40% to 70%.

Mafic, for igneous rocks with a color index of more than 70%.

Types of igneous rock

Igneous rocks can be divided into 3 categories:

• Deep igneous rocks, for example: granite, diorite, and gabbro
• Igneous rock alley / middle, for example: porphyry granite
• External igneous rocks, for example: andesite, obsidian, and basalt

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