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Igneous rocks are formed from the cooling and solidification of magma or lava. Igneous rocks, sedimentary rocks, and metamorphic rocks are the three primary kinds of rocks. Rocks are formed when the small grains of various minerals are crushed by the pressure they are subjected to. Furthermore, they are subjected to heat, which produces compression.
What are Igneous Rocks?
Igneous rocks are formed from the magma and begin the rock cycle. They are thus also known as primary rocks. Different processes are used to create various types of rocks. When heated, molten rock crystallizes and hardens, it forms igneous rocks.
The melted material comes from deep beneath the Earth, near active plate borders or hotspots. The melt starts deep below the Earth, at active plate borders or hot areas, and then rises to the surface. Depending on where the molten rock hardens, igneous rocks are classified as intrusive or extrusive.
Rock Cycle
Igneous Rock Texture
The rock textures found in igneous rocks are known as igneous textures. Geologists utilize igneous textures to determine the method of formation of igneous rocks, as well as to classify them.
Phenetic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic are the six major textures. For a crystal to develop in magma, enough of the chemical components that will make up the crystal must be present at the same moment in the same area.
Once a nucleus is formed, chemical components must diffuse through the liquid to reach the developing crystal's surface. The crystal can then continue to develop until it collides with other crystals or runs out of chemical components.
All of these rates are substantially influenced by the system's temperature. To begin with, nucleation and growth are not possible until temperatures are lower than the temperature at which equilibrium crystallization occurs.
Characteristics of Igneous Rocks
- All igneous rocks, as we know, are formed when lava solidifies. It can happen either under or above the earth's surface. As a result, all igneous rocks have some properties.
- There are no fossils in them. Acid seldom reacts with these rocks, therefore there is generally no layering. They can also be made up of two or more minerals and come in a variety of colors, including light and dark.
- Mineral crystals of various sizes are generally used to make them. These rocks may have holes or glass fibers. Furthermore, some are fine-grained or glassy.
Types of Igneous Rocks
Extrusive igneous rocks are classified as pumice and obsidian. Pumice is used to make lightweight concrete, while Obsidian is used to make decorations. Intrusive igneous rocks categories include granite and diorite.
Since there is a wide range of igneous rocks available, it's difficult to tell them apart. Let's look at the different sorts of igneous rocks:
Intrusive Igneous Rock
These are the types of rocks that form under the earth's surface. As a result of the lengthy cooling process, they produce massive crystals. Examples of these rocks are diorite, granite, and pegmatite.
Extrusive Igneous Rock
These are the kind of rocks that erupt onto the surface and cool fast, resulting in tiny crystals. Some of the rocks cool at such a rapid rate that they become amorphous glass. Basalt, tuff, and pumice are all examples of these rocks.
Bowen’s Reaction Series
Bowen's Reaction Series represents the temperatures at which various common silicate minerals change from the liquid to the solid phase. Bowen's Reaction Series is an idealized order of cooling magma crystallization.
Not all of these minerals, however, will crystallize in the same rock. A mafic magma will first crystallize olivine, then pyroxenes, then calcium-rich plagioclase feldspar. Before the melt is depleted, certain amphiboles may crystallize.
There isn't enough silica in mafic melts to form potassium feldspar, quartz, and other minerals. There isn't enough iron, magnesium, or calcium in felsic melts to create olivine, pyroxene, or calcium plagioclase.
Amphiboles (hornblende) or biotite mica, as well as some intermediate or sodium plagioclase, are the first minerals to develop in a felsic melt. Potassium feldspar and quartz crystallize when the melt cools and becomes richer in silica (as the metal cations are used up preferentially in the double chain and sheet silicates).
Things to Remember
- The form of igneous rocks depends on the magma they cool from
- Two rocks from identical magma may become either rock depending on their cooling speed
- Extrusive rocks are formed from lava while intrusive rocks are formed from magma
- Intrusive rocks, also called plutonic rocks, cool slowly without ever reaching the surface
- We may learn about the igneous portion of geologic history by studying igneous rocks.
Sample Questions
Ques. How did the word "igneous rock" come to be? (2 Marks)
Ans. The term "igneous" comes from the Latin word ignis, which means "fire." Vulcan is the Roman name for the God of Fire, and volcanic rocks are named after him. Intrusive rocks are sometimes known as "plutonic" rocks, after Pluto, the Roman god of the underworld.
Ques. Extrusive igneous rocks are extrusive igneous rocks. (2 Marks)
Ans. These are the kind of rocks that erupt onto the surface, resulting in tiny crystals due to fast cooling. Some of the rocks cool at such a rapid rate that they form an amorphous glass. Basalt, tuff, and /pumice are examples of these rocks.
Ques. Explain the formation of igneous rocks based on Irish landscapes. (3 Marks)
Ans. When lava spills out onto the earth's surface and swiftly cools and solidifies due to contact with air or water, extrusive or volcanic rock is formed. An extrusive igneous rock that occurs at constructive plate boundaries is basalt. As a result, all of our seabeds are formed of Basalt, a dark brown or black volcanic rock.
Since the lava cools so fast, mica, feldspar, and quartz crystals do not have time to form and are consequently tiny. The lava generally enters the surface by a fissure or rupture in the crust, although it can also enter through a vent, as Mt. Helka in Iceland. The Antrim Derry plateau (The Giant's Causeway) contains basalt.
Ques. How are igneous rocks formed? (3 Marks)
Ans. Intrusive or plutonic igneous rocks develop deep inside the earth's crust, under the surface. Magma is generated when descending plates are subducted into the heated mantle at convergent plate boundaries.
Magma plumes rise to the surface and embed themselves in the rocks above. The magma cools slowly at the core of the fold mountains, surrounded by a mass of insulating rock. Large crystals, such as glassy quartz, develop as a result.
Granite is an intrusive igneous rock that forms along plate convergent boundaries. Granite is a grey-colored rock that is made up of quartz, feldspar, and other minerals. The Wicklow and Donegal mountains are rich in granite.
Ques. How would you classify the igneous rocks based on the size and nature of crystals? (4 Marks)
Ans. The igneous rocks are classified based on the size and nature of crystals viz.
- Rocks with phaneritic textured crystals
- Rocks with orphan textured crystals
- Equigranular textured rocks, and
- Porphyritic textured rocks.
The crystals large enough to be seen with naked eyes or with the help of a hand lens are called phaneritic textured crystals, whereas those too small to be distinguished without the aid of a microscope are – called orphan textured crystals of the igneous rocks.
Where crystals in the rocks are all within the same size range, the texture is described as equigranular, whereas few large crystals are embedded in a matrix of smaller crystals, the texture is porphyritic.
Ques. Describe the formation of igneous rocks, giving suitable examples of various types. (5 Marks)
Ans. The igneous rocks are divided into extrusive rocks and intrusive rocks. When the magma solidifies on the surface of the earth, we call it an extrusive rock. A typical example of this kind of rock is basalt. It is a very fine-grained rock, usually Sf black in color.
On the other hand, when the magma solidifies in the crust, below the surface, it is called an intrusive rock. The few most common examples of intrusive rocks are granite and dolerite. More frequent, however, is the solidification of magma below the earth’s surface leading to the formation of intrusive rocks.
Igneous rocks are classified based on chemical composition and texture. Chemical differentiation of magma gives rise to mafic and felsic types of igneous rocks.
Texture relates to the sizes and patterns of the mineral crystals present in the rock. The size of mineral crystals in an igneous rock depends largely upon the rate of cooling of magma. As a general rule, rapid cooling results in small crystals, and slow cooling results in large crystals.
Extremely sudden cooling results in the formation of natural glass which is non-crystalline. Large bodies of magma trapped beneath the surface cool very slowly because the surrounding rocks conduct the heat slowly. Rapid cooling occurs in lava that loses heat rapidly to the atmosphere or the overlying ocean water.
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