Primary rocks, also known as igneous rocks, are formed directly from the cooling and solidification of magma or lava. They can be classified into three main categories based on their texture and mineral composition: 1. Intrusive Igneous Rocks: Formed when magma cools beneath the Earth's surface, resRead more
Primary rocks, also known as igneous rocks, are formed directly from the cooling and solidification of magma or lava. They can be classified into three main categories based on their texture and mineral composition:
1. Intrusive Igneous Rocks:
- Formed when magma cools beneath the Earth’s surface, resulting in coarse-grained textures.
- Examples:
- Granite: a common type of intrusive igneous rock with a coarse-grained texture and a mixture of quartz, feldspar, and mica minerals.
- Gabbro: a dark-colored, coarse-grained rock with a high iron and magnesium content.
2. Extrusive Igneous Rocks:
- Formed when lava cools and solidifies above the Earth’s surface, resulting in fine-grained textures.
- Examples:
- Basalt: a dark-colored, fine-grained rock with a high iron and magnesium content.
- Andesite: a medium-colored, fine-grained rock with a mixture of quartz, feldspar, and pyroxene minerals.
3. Volcanic Glass (Obsidian):
- A type of extrusive igneous rock that forms when lava cools rapidly, resulting in a glassy texture.
- Characteristics:
- Vitreous luster
- Conchoidal fracture
- Often black or dark brown in color
Other Types of Primary Rocks:
- Kimberlite: an ultramafic intrusive rock that forms from magma rich in magnesium and chromium.
- Komatiite: an ultramafic extrusive rock that forms from highly viscous lava.
- Pumice: a light-colored, porous volcanic rock that forms when lava is ejected into the air and rapidly cools.
Folk’s classification of limestones is one of the most commonly used methods of limestone categorization that was introduced by Robert L. Folk and it categorizes limestones based on their composition and texture. This classification is useful when tracing the depositional environments and geologicalRead more
Folk’s classification of limestones is one of the most commonly used methods of limestone categorization that was introduced by Robert L. Folk and it categorizes limestones based on their composition and texture. This classification is useful when tracing the depositional environments and geological history of limestones.
Folk’s system divides limestones into three main components:
1. Allochems: These are the grains or particles mostly observed in the formation of limestone. They include:
Skeletal grains (bioclasts): Some of the smaller shells, corals, and other organisms recognized in the marine ecosystem.
Ooids: Shapes that are shell-like with concentric circles ranging from inside out.
Pellets: Small, spherical dust, likely to be generated by any of the above-mentioned organisms.
Intraclasts: Limestone chips of a small size or irregular shapes and some of the pieces are split.
2. Matrix: This is a very micro-grain size of the sedimentation material which is made up of infants of calcite in microcrystalline nature known as the micrite that encases the allochems.
3. Cement: The chemical part which is mainly sparite (sparry calcite) formed by the groundwater, cements both the allochems and matrix.
Folk subdivided limestones into classificational types with regard to the kind of the most ubiquitous allochems and matrix or cement. For example:
Oosparite: Limestone with mainly ooids and with sparry calcite cement.
Biomicrite: Limestone that is created mainly from the skeletal part of the micrite mud.
Pelmicrite: Mudstone with lime pellets in the sub-micritic fabric.
This system provides much information on the depositional environments of the limestones and their diagenetic evolution and as such assists the geologists.
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