Igneous Rocks: Formation and Characteristics

• Igneous is derived from a Greek word meaning fire-formed rock.
• Petrology is the science of rocks, where petro means rock in Greek.
• Igneous rocks are also referred to as magmatic rocks because the term magmatic comes from magma.
• Magma is molten rock material that is hot, mobile, and usually contains dissolved gases, solids (crystals), and water vapor.
• Magma is generated in the lower crust and upper mantle due to partial melting of rocks under favorable pressure and temperature conditions.
• Droplets of magma combine to form large magma bodies or chambers, which move upward due to buoyancy and pressure reduction.
• Extrusion is the process of magma reaching the Earth's surface, which is called a volcano where eruption takes place violently or non-violently.
• When magma reaches the Earth's surface, it is called lava.
• The cooling of magma or lava results in the formation of crystals, a process called crystallization, which leads to the creation of solid igneous or magmatic rocks.

Textures in Igneous Rocks

• Texture refers to the overall appearance of a rock, related to the size, shape, and arrangement of minerals.
• The texture of an igneous rock is associated with its cooling history, not its chemistry.
• Phaneritic texture (coarse-grained) is plutonic, where individual grains are visible to the naked eye and can be identified using mineral properties without a lens.
• Aphanitic texture (fine-grained) is volcanic, where individual grains cannot be seen with the naked eye.
• Glassy texture is not crystalline.

Structures in Igneous Rocks

• Volcanic or extrusive rocks form when lava cools and solidifies on the Earth's surface.
• Plutonic or intrusive rocks form when magma cools and solidifies below the Earth's surface.

Extrusive Structures

• Lava flows are molten rock moving at varying speeds downhill, solidifying to form a volcanic rock.
• Pyroclastic structures are accumulations of explosively erupted volcanic materials solidified in the atmosphere.
• Ash deposits are pyroclastic deposits composed of very fine materials.
• Tuff deposits are explosively erupted, fine-grained materials that weld together when deposited hot.

Intrusive Structures

• Concordant intrusives are parallel to structures in the country rock, such as laccoliths, lopoliths, phacoliths, and sills.
• Discordant intrusives cross-cut structures in the country rock, such as batholiths, stocks, dikes, and veins.
• Intrusives are classified by shape, size, and emplacement mechanism.
• Sheet intrusives have two dimensions greater than the third, like dikes, which are sheet-like bodies vertical or nearly vertical.
• Sills are sheet-like bodies parallel to country rock, either horizontal or inclined if the rocks are disturbed.
• Lens-shaped intrusives include laccoliths (plano-convex, causing upward warping), lopoliths (convexo-concave, in synclines or basins), and phacoliths (concave-convexo, in anticlines).
• Subjacent intrusives are large with no visible floor, including batholiths (more than 40 sq. miles) and stocks (less than 40 sq. miles).

Classification of Igneous Rocks

• Four main classification systems exist: by formation and mode of occurrence, chemical composition, chemical and mineralogical criteria, and the Streckeisen classification.
• Igneous rocks are classified by formation and mode of occurrence as pyroclastic, volcanic, hypabyssal, and plutonic.
• Pyroclastic rocks form when volcanoes erupt violently, and lava cools rapidly in the atmosphere, consisting of ash (<2mm), lapilli (2-64mm), blocks (angular >64mm), and bombs (rounded >64mm).
• Volcanic rocks form when lava comes to the Earth's surface and cools rapidly, resulting in fine-grained rocks or sometimes volcanic glass.
• Hypabyssal rocks form from small igneous bodies like sills or dikes close to the Earth's surface, cooling at a rate resulting in medium-sized grains.
• Plutonic rocks form when magma cools in the upper crust at a very slow rate, leading to coarse to very coarse-grained rocks.
• Igneous rocks are classified by chemical composition based on SiO2 percentage.
• Ultra-basic rocks have less than 45% SiO2.
• Basic rocks have 45-52% SiO2.
• Intermediate rocks have 52-66% SiO2.
• Acid rocks have more than 66% SiO2.
• Ultra-basic rocks have less than 45% SiO2, with dominant mafic minerals, no quartz or feldspar, relatively heavy density (3.0-3.4gcm³), and colors ranging from dark green to black.
• Basic rocks have 45-52% SiO2, less than 60% feldspar, a larger amount of mafic minerals (45-50%), dark green to black color, and a density of 2.6-3.27gcm-³.
• Intermediate rocks have 52-66% SiO2, with feldspar being approximately 90% dominant, and quartz being very minimal or absent.
• Acid rocks have more than 66% SiO2, with quartz being the dominant mineral, a density of about 2.74gcm-3, and other major and minor minerals.
• The Streckeisen classification is also used (refer to the textbook).

Igneous Rocks in Sri Lanka

• Ambagaspitiya granite
• Tonigala granite
• Dolerite (dikes)
• Veins and Pegmatite