Approximately 90% of the earth’s crust is composed of igneous rocks, but their abundance is hidden on the earth’s surface by a relatively thin layer of sedimentary and metamorphic rocks. The cooling and solidification of magma forms igneous rocks.
‘Magma’ is a hot, viscous, siliceous melt containing water vapor and gases. It comes from great depth below the earth’s surface. It is composed mainly of O, Si, Al, Fe, Ca, Mg, Na, and K. When magma comes out upon the earth’s surface, it loses its gases. Such magma is called “lava”.
Types of Igneous Rocks:
It is lighter than the surrounding rocks and works towards the surface. On consolidation, it produces Two major types of igneous rocks:
- Extrusive Rocks
- Intrusive Rocks
Extrusive Rocks:
- When magma reaches the earth’s surface, it causes a volcanic eruption, which generates extensive Lava Flows. The rocks formed due to the solidification of lava are called “extrusive rocks.”
- The extrusive rocks are also called “volcanic rocks.” As are generally fine-grained or glassy. During the cooling of lava, the volatiles present in it escape into the atmosphere.
- Volcanic rocks often contain gas cavities called “vesicles.” These rocks sometimes show a “flow structure” resulting from movement in viscous lava. This structure is seen as lines or streaks of different colors in a rock.
Intrusive Rocks
Intrusive rocks are formed when magma crystallizes beneath the earth’s surface. Depending on the depth of formation, intrusive rocks are divided into two groups:
- Plutonic rocks, and
- Hypabyssal rocks.
Plutonic Rocks:
Rocks crystallized at great depths are called “plutonic rocks.” A magma which is deeply buried in the earth’s crust. Cools slowly with the retention of the volatiles. As a result, the mineral constituents crystallizing from it take time to grow to a considerable size, giving the rock a coarse-grained texture.
Hypabyssal Rocks:
Hypabyssal rocks are formed when magma solidifies close to the earth’s surface. They occur as injections within the country rocks. Their textures are usually finer-grained than those of plutonic rocks but coarser than those of volcanic rocks. Hypabyssal rocks commonly show a porphyritic texture.
Structure of Igneous Rock:
The structures of igneous rocks are large scale features, which are dependent on several factors like
- Vesicular and Amygdaloidal Structure
- Block Lava and Ropy Lava
- Pillow Lava
- Jointing, Sheet Structure
- Columnar Structure
- Pegmatites
- Spherulitic
- Orbicular
Vesicular and Amygdaloidal Structure:
When lavas heavily charged with gases and other volatiles erupted on the surface, the gaseous constituent escapes from the magma as the pressure decreases.
- Near the top of the material’s flows, empty cavities, bubbles, and vesicles are formed, which may be elliptical, spherical, cylindrical, or irregular in shape.
- The individual openings are known as vesicles, and the structure is known as vesicular structure.
- If, however, the vesicles thus formed are subsequently filled in with some low-temperature secondary minerals, such as calcite, zeolite, chalcedony, etc., these infillings are called “amygdales.”
Block Lava and Ropy Lava:
- Since lavas of acidic composition, due to their high viscosity, do not flow to greater distances, they offer a rough surface after solidification. Such lava flows are known as block lava. It is also known as ‘aa’ structure.
- Lavas of elemental composition are quite mobile because of their low viscosity, and they can flow to greater distances and, after solidification, offer a very smooth surface. Such lava flows are known as ropy lava and are also known as ‘pahoehoe’ structures.
Pillow Lava:
- A distinctive form of lava flow in which the solidified rock forms rounded masses with a glassy exterior.
- Pillow lava is found where lava flows into the ocean or a lake and quickly cools.
- Due to surface tension, liquid lava forms a sphere when it enters cold water. These hot balls of quickly solidifying lava settle to the ground soon.
- As they settle, pillow lava flattens and becomes pillow-like in form. As more pillows settle to the seafloor, their weight flattens them even more. Pillow lavas, therefore, form piles around the vent or entry point of a lava flow. They make up much of the oceanic crust.
Jointing, Sheet Structure:
- Joints are divisional planes found in all kinds of igneous rock impressed upon them mainly by force acting from outside. Joint plains may be vertical or horizontal.
- The horizontal joint planes are sometimes closely spaced to produce a sheet structure. The sheets are commonly thinner as the surface of the ground is approached, and they usually show some degree of parallelism.
Columnar Structure:
- With uniform cooling and contraction in a homogenous magma, the parting planes tend to take on a regular or prismatic form, characterized by the development of four, five, or six-sided prisms, which cross-joints may intersect.
Pegmatites:
- Pegmatites are extreme igneous rocks that form during the final stage of magma’s crystallization. They are extreme because they contain exceptionally large crystals, and they sometimes contain minerals that are rarely found in other types of rocks.
- To be called a “pegmatite,” a rock should be composed almost entirely of crystals of at least one centimeter in diameter. Granite and Syenite more often show this type of structure.
Spherulitic:
- Spherulitic aggregate (Spherulites) are radiating arrays of fibrous (needle-like, acicular) crystals that are common in glassy felsic volcanic rocks.
- Spherulites are typically two-mineral aggregates (mainly quartz and feldspar) formed by initial. Spherulitic growth of one mineral and later crystallization of a second mineral from the liquid
Orbicular:
- Orbicular granite (also known as orbicular rock or orbiculate) is an uncommon plutonic rock type that is usually granitic in composition.
- These rocks have a unique appearance due to orbicular – concentrically layered, spherical structures, probably formed through nucleation around a grain in a cooling magma chamber
Characteristics of Igneous Rocks
- The igneous rocks don’t contain any fossil deposits because any fossils deep inside the crust would be destroyed when the rocks erupt due to their intense heat.
- The majority of igneous formations contain multiple mineral deposits.
- They can appear smooth and glassy or rough and grainy.
- These typically don’t interact with acids.
- The mineral deposits come in various sizes and are distributed in patch formations.
Conclusion:
Igneous rocks are essential in shaping the Earth’s surface and offer valuable insights into the planet’s internal processes. Their various forms, from volcanic to plutonic rocks, provide significant information about the history of tectonic activity and magma movement.
These rocks are also the source of many mineral deposits, making them practical for various industries. Studying igneous rocks adds to our knowledge of geology and deepens our understanding of the dynamic processes that continuously reshape our planet.
