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Do you know Trenches?
Trenches are long, narrow, depressions on the seafloor. They are typically 50 to 100 kilometers wide. Trenches are the result of convergent plate boundaries, in that where two plates collide and one plate will subduct. We know that most of you are familiar with Mariana Trench, the most known and deepest trench in the world situated in Pacific and Philippine Plate.
What is Mariana Trench?
What is Mariana Trench?
The Mariana Trench is the lowest point in the ocean, therefore you might want to visit it if you want to investigate its depths. This trench is over five times broader than it is deep, with approximate depth of 11 kilometers, or around 7 miles. All life in the Mariana Trench must withstand the frigid water and intense pressure, as it is twice as deep as Mount Everest. There are certain animals that lives there, such as the deep-sea crabs Hirondellea gigas, who recently had a human visitor. In 2012, James Cameron made a history by being the first person to complete a solo submarine dive to Challenger Deep, the trench's lowest point and, by extension, the globe.
The Mariana Trench is the deepest known sea trench on Earth, situated primarily to the east and south of the Mariana Islands in the western North Pacific Ocean. It is a component of the subduction zones, or places where two nearby tectonic plates clash and push one beneath the other, that make up the western Pacific system of oceanic trenches. The Mariana Trench is an arcing depression that is more than 1,580 miles (2,540 km) long and has a mean width of 43 miles (69 km). Challenger Deep, a smaller, steeply walled valley on the floor of the main trench southwest of Guam, is where the lowest depths are attained. The Mariana Trench is located inside the borders of Guam and the Northern Mariana Islands, two US dependent territories, and was designated a national monument in the United States in 2009.
Figure 1. A look at Mariana Trench
What is the Endogenic Process Involved in Mariana Trench?
What is the Endogenic Process Involved in Mariana Trench?
The Mariana Trench is formed primarily due to a Subduction Zone, an endogenic process where one tectonic plate is being forced beneath another. In this case, the Pacific Plate is subducting beneath the smaller Mariana Plate, creating the deepest part of the world’s oceans.
When two plates crash into each other, an oceanic plate plunges downward into the mantle, while the other plate rides up over the top. This movement creates a trench where the descending oceanic plate drags down the edge of the overriding plate. This movement also creates the largest known earthquakes, which often generate tsunamis.
Figure 2. A representation of how deep is the Mariana Trench
Due to the formation of the Mariana Trench, different rocks and minerals were formed.
1. GABBRO
Gabbro is a type of intrusive igneous rock with a coarse-grained texture. It is primarily composed of dark minerals, such as pyroxene and plagioclase feldspar. Gabbro forms beneath the Earth's surface through the slow cooling and solidification of magma, and it is often associated with the formation of oceanic crust. Its classification is basaltic, indicating an intermediate composition between mafic and felsic rocks.
Figure 3. A Gabbro rock
What minerals are in Gabbro?
In gabbro, the main minerals are plagioclase feldspar, pyroxene, olivine, amphibole, and opaque are present We can say that Gabbro is dominated by ionic and covalent bonds.
Opaque minerals such as magnetite (Fe3O4) are found in gabbro. Magnetite is an iron oxide mineral commonly found in basalt. It is often magnetic and involves the bonding of iron ions (Fe2+ and Fe3+) in magnetite to form ionic bonds with oxygen ions (O2-). This results in the creation of iron oxide.
Figure 4. Chemical bond of Magnetite
Figure 5. A Magnetite Mineral
Due to the chemical bonding of magnetite and some other minerals, the distinctive properties of gabbro and magnetite includes:
Composition - Gabbro is mainly composed of dark-colored minerals, including the magnetite. The specific mineral composition can vary, leading to variations in color and texture.
Color - Gabbro is typically dark green to black in color. The dark color is a result of the presence of minerals like pyroxene, magnetite, or in some cases, olivine.
Texture - Gabbro has a coarse-grained texture, meaning that its minerals are large enough to be easily visible to the naked eye. The coarse-grained nature is a result of the slow cooling of magma beneath the Earth's surface.
Density - Gabbro is a relatively dense rock due to its mineral composition. It has a density ranging from about 2.7 to 3.3 grams per cubic centimeter. It is because of magnetite that has a relatively high specific gravity, typically ranging from 4.9 to 5.2.
Association with Iron Ore Deposits: Magnetite is a significant source of iron ore and is commonly associated with large iron ore deposits. It is often found in igneous and metamorphic rocks.
Shape: Natural and synthetic magnetite occurs most commonly as octahedral crystals bounded by planes and as rhombic-dodecahedra. Twinning occurs on the plane.
Figure 6. A Diabase rock
2. DIABASE
Diabase is an intrusive igneous rock with the same mineral composition as basalt. It cools under basaltic volcanoes, like those at mid-ocean ridges. Diabase cools moderately quickly when magma moves up into fractures and weak zones below a volcano.
What minerals are in Diabase?
In diabase, the main minerals are plagioclase feldspar, pyroxene, biotite, amphibole, and opaques are present. We can say that Diabase is dominated with the ionic and covalent bonds.
Focusing on Plagioclase Feldspar which is the most content of diabase, it is an ionic and covalent bond. Plagioclase feldspar also contains metal ions such as calcium (Ca) and sodium (Na). These metal ions interact with oxygen ions through ionic bonding. Also, silicon (Si) and aluminum (Al) form covalent bonds with oxygen (O).
Figure 7. A Plagioclase Feldspar
Diabase looks like that because of the distinctive properties of the minerals, especially the plagioclase. Here is the distinctive properties of plagioclase:
Cleavage - Plagioclase feldspar exhibits two directions of cleavage at nearly right angles, resulting in a characteristic blocky appearance.
Hardness - It has a hardness of around 6 on the Mohs scale, making it moderately hard. Luster - The mineral has a vitreous to pearly luster on its cleavage surfaces.
Color - Plagioclase feldspar varies in color, ranging from white and gray to shades of green and blue. The color is often influenced by impurities.
Streak - Its streak is usually white.
Transparency - Plagioclase feldspar is typically translucent to opaque.
Twinning - It commonly exhibits twinning, particularly the albite pericline twins, which contribute to its unique appearance.
Striations - Striations on certain crystal faces are sometimes observed, providing additional identification features.
Density -The density of plagioclase feldspar varies depending on its composition but is generally in the range of 2.6 to 2.8 g/cm³.
Associations - Plagioclase feldspar is commonly found in igneous rocks such as basalt, gabbro, and granite.
Dolerite or also called diabases can also contain quartz. Quartz are covalent bonds as they are only composed of covalent bonded silicon-oxygen tetrahedra, making covalent bonding the predominant type of bonding in this mineral.
Figure 8. Chemical bond of Quartz
Figure 9. A Quartz Mineral
Due to covalent bonds, quartz has the following characteristics that helped in the formation of dolerite:
Hardness: Quartz is a hard mineral and ranks 7 on the Mohs scale of hardness, making it resistant to scratching.
Crystal Structure: It has a hexagonal crystal structure and often forms six-sided prismatic crystals.
Transparency: Quartz can be transparent, translucent, or opaque depending on impurities. Pure quartz is transparent.
Color: While pure quartz is colorless, it can come in a variety of colors due to impurities, including purple (amethyst), pink (rose quartz), yellow (citrine), and more.
Luster: Quartz has a glassy or vitreous luster on its crystal surfaces.
Cleavage: It lacks cleavage and instead breaks along fractures.
Conchoidal Fracture: When broken, quartz often exhibits a conchoidal fracture, meaning it breaks with smooth, curved surfaces.
Density: Quartz has a relatively low density for a hard mineral.
Chemical Composition: It is composed of silicon and oxygen (silicon dioxide, SiO2), with a repeating tetrahedral structure.
Piezoelectric Properties: Quartz exhibits piezoelectricity, meaning it can generate an electric charge under mechanical stress and vice versa.
Common Occurrence: Quartz is one of the most abundant minerals in the Earth's crust and is found in various geological environments.
3. BONINITE
Boninite is an extrusive rock high in both magnesium and silica, thought to be usually formed in fore-arc environments, typically during the early stages of subduction. This rock contain certain minerals such as the orthopyroxene, clinopyroxene, olivine, plagioclase feldspar, and spinel.
Figure 10. A Boninite rock
Olivine minerals can be ionic and covalent. Olivine's crystal structure involves ionic bonding between metal cations, primarily magnesium (Mg^2+) and iron (Fe^2+), and oxygen anions (O^2-). This ionic bonding results from the transfer of electrons between the metal and oxygen ions, creating a stable crystal lattice.
Figure 11. Chemical bond of Olivine
Figure 12. An Olivine Mineral
Distinctive Properties of Olivine that contribute to the structure of Boninite:
Crystal Structure - Olivine has a crystal structure based on a three-dimensional framework of linked silicate tetrahedra. In its common form, olivine consists of isolated tetrahedra arranged in a simple cubic close-packed structure. The isolated tetrahedral arrangement, composed of a central silicon atom surrounded by four oxygen atoms, forms the basis of olivine's crystal lattice, contributing to its characteristic properties and behavior in geological processes.
Color - Olivine has a crystal structure based on a three-dimensional framework of linked silicate tetrahedra. In its common form, olivine consists of isolated tetrahedra arranged in a simple cubic close-packed structure. The isolated tetrahedral arrangement, composed of a central silicon atom surrounded by four oxygen atoms, forms the basis of olivine's crystal lattice, contributing to its characteristic properties and behavior in geological processes.
Cleavage - Olivine typically exhibits no distinct cleavage due to its orthorhombic crystal structure, which lacks planes of weakness along which the mineral can easily split. Instead, olivine tends to fracture irregularly, displaying conchoidal or uneven patterns, making it brittle but not characterized by well-defined cleavage planes.
Two Distinct End-Members - Forsterite represents the magnesium-rich end member (Mg2SiO4), while fayalite represents the iron-rich end member (Fe2SiO4), reflecting the range of compositional variations within the olivine solid solution series.
Variability in Composition - Due to the presence of various metallic elements like iron (Fe) and magnesium (Mg), olivine's composition can vary inside lherzolite. The exact Mg to Fe ratio in olivine crystals can affect the color; a higher magnesium content produces a brighter color, while a higher iron content gives the crystals a deeper, more greenish tone.
Influence on Rock Classification - As a magnesium-rich silicate mineral, olivine contributes to the ultramafic nature of these rocks, influencing their overall color, texture, and physical properties. Additionally, olivine's resistance to weathering can impact the long-term stability of the rock, affecting its durability and resistance to chemical alteration.
In exploration of the Mariana Trench in 2016, tall hydrothermal vents spill out black chemical bacteria in the depths of Mariana Trench. Hydrothermal vents have minerals that have metallic bonding. Those are:
Sphalerite (Zn,Fe)S - A zinc sulfide mineral
Chalcopyrite (CuFeS2) - A copper iron sulfide mineral.
Pyrite (FeS2) - An iron sulfide mineral.
Bornite (Cu5FeS4) - A copper iron sulfide mineral.
Galena (PbS) - A lead sulfide mineral.
Figure 13. Hydrothermal vents in Mariana Trench
Bornite is one of the mineral that shows metallic bonding. It is also known as peacock ore, has a complex chemical composition with the formula Cu5FeS4. Bornite contains copper (Cu) and iron (Fe) as its major elements. The metallic bonding in bornite involves the delocalization of electrons, allowing them to move freely throughout the structure. This imparts electrical conductivity and the characteristic metallic luster to the mineral. Bornite also shows covalent bonding as sulfur bonds with copper and iron.
The combination of metallic and covalent bonding in bornite contributes to its unique properties, including its metallic luster, electrical conductivity, and distinctive iridescent colors.
Figure 14. A Bornite
Distinctive Properties of Bornite:
Color: Bornite exhibits a striking iridescence with colors ranging from blue and purple to red and gold. This iridescence is caused by surface tarnishing.
Luster: It has a metallic luster, contributing to its shiny appearance.
Crystal System: Bornite crystallizes in the orthorhombic crystal system, forming a variety of crystal habits.
Hardness: Bornite has a hardness ranging from 3 to 3.25 on the Mohs scale, making it relatively soft.
Streak: The streak of bornite is brownish-black.
Cleavage: Bornite has poor to indistinct cleavage and generally breaks with uneven to subconchoidal fracture surfaces.
Density: It has a density typically ranging from 4.9 to 5.3 g/cm³.
Association: Bornite is commonly associated with other copper minerals and is often found in copper ore deposits.
Tarnish: Over time, bornite may tarnish, developing a brownish tarnish on its surface.
Conductivity: Bornite is a good conductor of electricity due to its metallic bonding.
Nicknames: Its colorful appearance has led to nicknames such as "peacock ore" or "peacock copper."
At the end of the journey into the depths of the ocean, the trench reveals a geological ballet of immense importance. Here, the Earth's crust sinks to melting depths, triggering an alchemical dance that creates unique rocks and minerals. Subduction zones such as the Mariana Trench are crucibles of metamorphosis where deep-lying peridotites and mineral-rich hydrothermal vent layers are formed.
These grooves are silent architects, contributing to the living fabric of our planet's rocky and mineralogical wonders. As we bid farewell to the hidden spectacle of subduction, may we retain a deep appreciation for the geological wonders revealed in the deep sea.
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