What is Rock?

Introduction

Rocks are the naturally occurring aggregate of minerals and matter forming the Earth's crust, the final products of various geologic processes that ultimately lead to igneous, sedimentary, and metamorphic rocks. Each of these kinds of rocks has its features and processes, all of which are critical for understanding the Earth's history and the role of active processes in shaping our planet. Knowing the rocks and details of their formation is very crucial in many applications related to construction, resource extraction, or environmental management.

Rock types and their formations 

There are the following three categories that rocks can be divided according to their process of formation: igneous, sedimentary, and metamorphic. Defining rock types, typical for them, with formation processes that are important factors in learning the geological history and the processes that shape our planet.

Igneous Rocks

Igneous rocks are formed by solidification from a semifluid or molten mantle material called magma. It may take place beneath or on the Earth's surface, hence being divided into two subtypes: intrusive or plutonic and extrusive or volcanic igneous rocks.

1. Intrusive Igneous Rocks: These rocks are formed when magma cools and solidifies beneath the Earth's surface. With this slow cooling, large crystals grow, characterized by a coarse-grained texture. Examples include granite, diorite, and gabbro.

2. Extrusive Igneous Rocks: They are formed when magma goes out of the Earth's surface and into the atmosphere as lava, then cools rapidly. The fast cooling process leads to fine-grained and glassy textures. Examples include basalt, andesite, and rhyolite.

Process of Formation:

-Melting: Rocks from the mantle or crust of the Earth melt under high pressures and temperatures to form magma.

-Crystallization: Minerals, upon magma cooling, crystallize in a sequence particular to melting points.

- Solidification: Magma is solidified into igneous rock, either subtly under the surface of the Earth (intrusive) or on its surface (extrusive).

Sedimentary rocks

These are formed when sediments accumulate and get lithified. The sediments are derived from preexisting rocks or biological matter through meteorite impacts, weathering, erosion, transport, deposition, and diagenesis.

1. Clastic Sedimentary Rocks: These are rocks that are held firmly together with fragments, called clasts, of other rocks. These clasts are moved by water, wind, or ice and deposited into layers. Over time, they compact and start to cement into rock. Examples include sandstone, shale, and conglomerate.

2. Chemical Sedimentary Rocks: Such rocks originate from the precipitation of minerals from solution. The process takes place in water, wherein minerals precipitate out from the water due to changes in temperature, pressure, or chemical composition. Examples include limestone, chert, and evaporites, including rock salt and gypsum.

3. Organic Sedimentary Rocks: These are the result of an accumulation of plant or animal debris. The organic material compacted and lithified over time. Coal and some varieties of limestone—in general, shell fragments—fit into this category.

Formation Process:

- Weathering: Physical and chemical breakdown of pre-existing rocks to small particles.

- Erosion and Transportation: Water, wind, or ice-carrying sediments.

- Deposition: Sediment settles, generally with layered structures in a new location.

- Lithification: Compaction and cementation of sediments into solid rock.

Metamorphic Rocks

Metamorphic rocks come from the alteration of pre-existing rocks (igneous, sedimentary, or other metamorphic rocks) due to changes in temperature, pressure, and chemically active fluids. This process, called metamorphism, forms new mineral assemblages and textures.

1. Foliated Metamorphic Rocks: Those that have a layered or banded appearance due to the orientation of their minerals under directed pressure. Slate, Schist, Gneiss

2. Non-foliated Metamorphic Rocks: Those not occurring as foliated and exhibiting a homogeneous texture: marble from limestone; quartzite from sandstone.

Process of Formation:

- Heat and Pressure: Higher temperatures and pressures generally due to tectonism or burial are best owed on rocks.

- Recrystallization: Minerals of the rock recrystallize to form newer minerals and textures

- Chemically Active Fluids: Fluids act as a medium for chemical processes, enhancing the growth of new mineral assemblages.

Types of Rocks

Igneous Rocks

1. Granite: This is an intrusive coarse-grained igneous rock, essentially consisting of quartz, feldspar, and mica. It finds wide applications in building and monumental construction.

2. Basalt: Fine-grained extrusive igneous rock, basically consisting of plagioclase and pyroxene. It's the most common volcanic rock on Earth, forming the oceanic crust and many volcanic islands.

3. Diorite: An intrusive igneous rock whose composition falls between granite and gabbro in its composition, with plagioclase, hornblende, and biotite. It is a dimension stone.

4. Rhyolite: Fine-grained extrusive igneous rock rich in silica with about the same general composition as granite. It comes from very viscous lava and often forms explosive eruptions.

5. Gabbro: Coarse-grained intrusive igneous rock, mostly made up of plagioclase & pyroxene & including minor content of olivine; it includes crushed stone and is used as dimension stone.

Sedimentary Rocks

1. Sandstone: A clastic sedimentary rock composed mainly of quartz & feldspar sand-sized particles. It finds huge applications in structure building and as a petroleum reservoir rock.

2. Shale: Finely-grained clastic sedimentary rock of clay minerals and silt-sized particles; the most common sedimentary rock; used in making cement and bricks.

3. Limestone: A chemical or organic sedimentary rock that is mainly composed of calcium carbonate, mostly calcite. The sedimentary rock may result from the accumulation of shell, coral, algal, and fecal debris, or through precipitation of calcium carbonate out of water. This applies to building and agriculture. It is also a raw material used for cement.

4. Conglomerate: A clastic sedimentary rock consisting of rounded gravel-sized clasts that are bonded or cemented together. Conglomerates form through higher-energy processes, mostly in rivers and beaches.

5. Chert: A very fine-grained chemical sedimentary rock that essentially consists of microcrystalline quartz. It is formed by the accumulation of silica-rich organisms, like radiolarian protozoa or diatoms, or through chalcedony precipitation from water. It is used in tools and as a source of silica.

Metamorphic Rocks

1. Slate: Fine-grained foliated metamorphic rock; an advanced degree of metamorphism results in foliation in rocks derived from shale. It possesses perfect cleavage properties; this property is utilized for roofing, flooring, and in-writing slates.

2. Schist: A medium to coarse-grained, foliated metamorphic rock, primarily consisting of aligned or oriented platy minerals such as mica; it requires moderate to high-grade metamorphic conditions for its formation.

3. Gneiss: High-grade foliated metamorphic rock that has typical banding or layering from the segregation of minerals into distinct bands; it forms from the metamorphism of granite or sedimentary rock.

4. Marble: The non-foliated metamorphic rock predominantly consists of recrystallized carbonate minerals: calcite or dolomite. It forms from the metamorphism of limestone or dolostone and is extensively used in sculpture, architecture, and building.

5. Quartzite: A nonfoliated metamorphic rock; it's primarily a quartz-rich metamorphic rock derived from the metamorphism of sandstone. It is very, very hard and resistant to weathering, making it applicable for use as a building material and for ornamental stone purposes.