What is Meant by Mafic Rocks?
The Earth, by weight, mostly consists of mafic or ultramafic rocks, but silicic rocks are considered to be more abundant on the surface of the Earth. The common type of mafic rocks is basalt, pyroxene, biotite, etc. In this article, we will talk about this type of rock. We will learn about what are mafic rocks, mafic minerals, their meaning, definitions, features, and examples, etc. This topic will be helpful to understand one of the types of igneous rocks which are formed from magma and which we usually study in Geography, Geography, Earth Sciences, Physical Geography, etc.
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Introduction
Rocks are said to be the hard matter which forms after the solidification of the magma and because of various other geographical phenomena. These are generally of three main types which include igneous rocks, sedimentary rocks, and metamorphic rocks. Among these, igneous rocks are said to be the primary rocks because they occur first and then sedimentary rocks form and then the formation of metamorphic rocks happens because of geological processes on the above two types of rocks. If we talk about Igneous rocks, these are formed after the solidification of magma and can be of two types i.e. Intrusive and Extrusive. These are being divided on the basis of where they actually occur. If rock formation is done below the surface of the Earth is known as intrusive igneous whereas if it occurs above the surface of the Earth, it is known as Extrusive rocks. Mafic rock is an example of Intrusive igneous rock.
Meaning of Mafic
It is a portmanteau term that is made up of magnesium and ferric. On the basis of silica content, igneous rocks can be classified into various types. Silica is found in abundance in igneous rocks because of which is considered the basis of classification. On this basis, igneous rocks can be divided into four major types i.e felsic, intermediate, mafic, and ultramafic ( written as per the silica content in decreasing order). Mafic is a kind of igneous rock which are rich in magnesium and iron. This type of rock contain 45 to 55 % silica content and 4 mafic igneous rocks minerals are pyroxene, amphibole, olivine, and mica. Mafic word is sometimes also used as a synonym for dark coloured minerals.
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Definitions
Some of the standard definitions are given below:
"In Geology, of or relating to rocks rich in dark, ferromagnesian minerals." - dictionary
"Mafic describes rocks containing more dark-coloured substances and iron than other rocks." - Cambridge dictionary.
"It is Relating to or denoting a group of dark-coloured, mainly ferromagnesian minerals such as pyroxene and olivine and often contrasted with felsic. ( Geology ). - Lexico definition.
"Mafic is a collective term for minerals present in igneous rocks Or "relating to igneous rocks or silicate minerals." Or "( Geology ) of or pertaining to igneous rocks that are rich in dark-coloured minerals and that contain magnesium and iron and a comparatively low level of silica." - Collins Dictionary.
Features
The basic features of mafic rocks are given below:
This is one of the important types of igneous rocks that are classified on the basis of silica content.
It is a type of rock that crystallizes from silicate minerals at high temperatures. Sometimes, these rocks are also called basaltic.
Olivine, pyroxene, amphibole, and biotite are common rock-forming minerals of mafic.
Common examples of mafic rocks are basalt, gabbro, diabase, etc.
They are dark in colour ( whereas felsic rocks are light in colour).
Chemically, these rocks are consist of magnesium, iron, and calcium.
Mafic magma has a low content of silica because of which before cooling, mafic lava has a low viscosity.
The volcanic eruptions consist of mafic lava are less explosively violent as compared to felsic volcanic eruptions.
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Additional Information
On the basis of the texture of rocks, different names of mafic rocks are given below in the table:
Different Names of Mafic Rocks
Conclusion
To sum up we can say that mafic is a term which is made up of magnesium and ferric thus used to describe the rocks which are rich in two whereas now it is also used to describe the igneous rocks which are rich in high content of magnesium as well as iron. It is a dark coloured rock that consists of silica content less than felsic and intermediate whereas more than ultramafic igneous rocks. On the basis of the texture of the rocks, different names of mafic rocks can be there. This topic is useful to study whenever we talk about igneous rocks and their types.
FAQs on Mafic Rocks
1. What are mafic rocks?
Mafic rocks are a classification of igneous rock that are rich in magnesium and iron. The name itself is a portmanteau of Magnesium and Ferric (iron). They are characterised by their dark colour, high density, and a relatively low silica content, typically between 45% and 55%.
2. What are the main characteristics of mafic rocks?
The defining characteristics of mafic rocks are:
- Composition: They are rich in silicate minerals like pyroxene, olivine, amphibole, and calcium-rich plagioclase feldspar.
- Colour: Typically dark, ranging from black to dark grey or greenish-black due to the high iron and magnesium content.
- Density: They are denser than light-coloured felsic rocks.
- Viscosity of Magma: Mafic magma has a low viscosity (it is more fluid) because of its low silica content, allowing it to flow easily.
- Formation Temperature: They crystallise from magma at high temperatures (around 1000°C to 1200°C).
3. What is the key difference between mafic and felsic rocks?
The key difference lies in their mineral composition, which affects their colour, density, and silica content. Mafic rocks are dark, dense, and rich in magnesium and iron with low silica content (45-55%). In contrast, felsic rocks are light-coloured, less dense, and rich in feldspar and silica with high silica content (over 65%). Common examples are basalt (mafic) and granite (felsic).
4. What are some common examples of mafic rocks?
The two most common examples of mafic rocks demonstrate how cooling location affects texture:
- Basalt: An extrusive (volcanic) mafic rock. It cools quickly on the Earth's surface from lava flows, resulting in a fine-grained texture with tiny crystals. It forms most of the world's oceanic crust.
- Gabbro: An intrusive (plutonic) mafic rock. It cools slowly deep beneath the Earth's surface, allowing large, visible crystals to form, giving it a coarse-grained texture.
5. How does the cooling rate of magma determine the type of mafic rock formed?
The cooling rate is the primary factor determining the rock's texture. When mafic magma erupts onto the surface, it cools very quickly, preventing large crystals from growing. This creates a fine-grained extrusive rock like basalt. If the same magma is trapped and cools slowly deep within the Earth's crust, it has ample time to grow large, interlocking crystals, forming a coarse-grained intrusive rock like gabbro. Both have the same chemical composition but different textures.
6. Why do mafic lavas have low viscosity and flow easily?
Mafic lavas are fluid and have low viscosity because of their low silica (SiO₂) content. In magma, silica molecules link together to form long chains (polymers). Fewer silica molecules in mafic magma mean fewer and shorter chains, making the liquid less 'sticky' and more fluid. This allows mafic lava, like that seen in Hawaii, to flow across vast distances before solidifying into rock.
7. Where on Earth are mafic rocks most commonly found and why?
Mafic rocks are predominantly found in specific tectonic environments where the Earth's mantle material rises towards the surface. These include:
- Mid-Ocean Ridges: Where tectonic plates pull apart, allowing mafic magma to well up and form new oceanic crust, which is primarily basalt.
- Oceanic Hotspots: Volcanic island chains like Hawaii are formed by massive outpourings of fluid mafic lava from a mantle plume.
- Continental Rift Zones: Areas where continents are splitting apart, such as the East African Rift, also experience mafic volcanism.
8. How do mafic rocks differ from ultramafic rocks?
While both rock types are rich in iron and magnesium, the primary difference is their silica content. Mafic rocks contain 45-55% silica. Ultramafic rocks have an even lower silica content, less than 45%. They are composed almost exclusively of dark mafic minerals like olivine and pyroxene. Peridotite, the main component of the Earth's upper mantle, is a prime example of an ultramafic rock.
