Igneous Rocks
IGNEOUS ROCKS
Igneous (Latin, ignis or fire) rocks are formed by cooling and crystallization of molten rocks.
Magma
Hot, molten rock material, existing below the earth surface.
Acid Magma: It is rich in Si, Na and K and poor in Ca, Mg and Fe.
Basic Magma: It is rich in Ca, Mg and Fe and poor in Si, Na and K.
Lava
When magma reaches the surface of the earth, it is called lava.
Igneous Process
Most igneous process occur underground we can not see them, geologist often rely on geophysical studies of the earths interior, computer models, that simulate subterranean temperature and pressure conditions and laboratory experiment.
Melting
A crystalline solid melts when the bonds between its ions break, allowing the charged particles to move freely when under ground temperatures become hight enough bonds in minerals are broken, eventually the heated rock the heated rock no longer a crystalline solid becomes magma, since the various types of mineral melt at different temperatures, different minerals melt out of the rock as the heat gradually increases hence composition of the magma changes as each newly molten mineral enters and enriches it. Partial melting is described as melting is described as melting points of some but not all of its components minerals, factors control the melting of rocks and creating of magma.
1 – Heat = (a-heat produced due to radio active decay of elements, b-form the core of the earth,3-Frictional heat produced as each plate moves.
2 – Pressure = help to increase melting point of a substance.
3 – Water = Water lowers the melting point of substance.
Fluidity Of Magma
Its ability to flow, governed by its temperature and composition.
Viscosity
Fluids resistance to flow, viscosity increases with decreasing temperature, viscosity increases with silica content.
Crystallization
Where temperature decreases bonds start to form and tiny crystals begin to appear. As cooling progress, different minerals crystallize out of the magma at different temperatures.
Classification Of Igneous Rock
1 – Texture
2 – Mineral content
Texture
It means the size, shape and arrangement of mineral grains
Phaneritic
1 – State
Metamorphic rocks derived from shale or mudstone a strongly foliated rock known as state, state tends to break parallel to the basic planes into relatively thin, flat fragments, a pattern known as state cleavage.
2 – Phyllite
Low grade metamorphic rock with glossy and shinning lusture, as temperature increase to 300 C, the size of the microscopic mica, chlorite and graphite flakes in state increases, producing the foliated metamorphic rock phyllite.
3 – Schist
When the flakes become visible to the unaided eye, the metamorphic rock is classified as schist, a medium if coarse grain strongly foliated.
4 – Gnesis
It is composed of feldspars, quartz and some mafic minerals, Gneisses are distinguished by minerals that are present in comparatively large amount.
Nonfoliated Rocks
1 – Marble
When relatively pure limestone and dolostones are metamorphous, even by directed pressure, they tend to form a coarsely granular nonfoliated metamorphic rock marble.
2 – Quartz
A very durable Nonfoliated metamorphic rock come by the metamorphism of quartz sandstone.
Phaneritic
(Form the Greek Phaneros, or visible) When a magma’s minerals crystallize slowly over hundreds or thousands of years, there is ampte time for crystals to grow large enough to be seen by the unaided eye.
Aphanitic
(From the Greek a Phaneros, or “not visible”), when magma solidify so quickly that their crystals have little time to grow, and crystals are so small they can barely be seen by the naked eye.
Prophyritic
When large crystals are surrounded by small crystals.
Glassy
When magma cools so quickly that its ions don’t have enough time to form any crystals, then they are essentially frozen in place, randomly bonded to produce glassy texture.
Intrusive OR Plutonic Rocks (for Pluto, the Greek god of the underworld)
Plutonic Rocks are formed when magma cools slowly at great depth with the retention of the volatiles. The texture of such rocks are phaneric.
Extrusive OR Volcanic Rocks
Volcanic rocks are formed when the magma erupts at the earths surface and cools rapidly. The volatiles present in the magma escape into the atmosphere. The texture of such rocks is aphanitic or glassy.
Hypabyssal Rocks
Hypabyssal Rocks are formed when consolidation of magma takes place very close to the earth’s surface, fine grand with prophyritic texture.
Chemical Composition
The earth’s magmas consist largely of the most common elements: oxygen, silicon, aluminum, iron calcium, magnesium, sodium, potassium and sulfur. The relative proportions of these components within a body of magma, added to the prevailing temperature, pressure and its water contents, distinctly characterize magma into following types.
Ultramafic Igneous Rock
Ultramafic Igneous Rocks contain less than 45% silica, here both feldspars and quartz are essentially absent and one or more of the ferromagnesian mineral is the dominant component. These rocks are darker in colors, have a relatively higher specific gravity Gabbro, Dolerite.
Intermediate Igneous Rocks
These rock contain silica in between about 55% 65%. They consist of iron and magnesium silicate, along with sodium plagioclase and mica and small amount of quartz. These rocks are lighter in color although a little darker than color of acidic rocks: Example, andesite and diorite.
Felsic Igneous Rock
These rocks contain more then 65% silica, these rocks are rich in potassium feldspar, aluminum rich micas and quartz but poor in iron magnesium and calcium silicates. These rocks are light in color and low in specific gravity granite, rhylite.
Intrusive Rock Formations
Magma moves forcefully into the cracks in preexisting rock, actually pushing the rocks aside to create its own space. I n a similar way, rising magma may force overlying rocks to bulge upward.
Mode Of Occurrence
Sill
A sill is a sheet like body, lying parallel to layers of preexisting rocks. Sills are produced when intruding magma enters a space between layers of rocks, melting and incorporating adjacent sedimentary material.
Phacolith
Magma under low pressure will not exert much pressure to make space for itself, in such cases the magma may be pushed into the existing crests and troughs of a fold in the introduced rocks, and get solidified there. Such structures are in form of crescentric.
Laccolith
When thick, viscous magma intrudes between two parallel layers of rock and lifts the overlying rock layers and the form of a dome of arch, it is called Laccolith.
Dike
Dike is a vertical wall like igneous body that cuts across preexisting rocks. Dike s are generally steeply inclined or nearly vertical, suggesting that they formed from rising magma, which tend to follow the most direct route upward.
Batholith
Batholiths are massive discordant igneous bodies with surface areas of 100 square kilometers or more. They typically form intermediate to high viscosity magmas that, because they flow so slowly, are more likely to remain underground and crystallize here.
Lava Flow
Tabular shaped volcanic rock bodies, formed by lava flows.
Description Of Common Rocks
Granite: It is coarse grained rock which is composed of quartz, alkali feldspar and small amount hornblende. The granites are mostly light in color with a white or pink tint according to the color of the feldspar.
Andesite: These are light colored volcanic rocks. In nature, they are intermediate between acidic and basic rocks. They are general fine grained aphanitic rock with typical prophyritic textures.
Rhyolite: Rhyolite is similar to granite in mineral composition but different texture. The texture is often prophyritic, which contains phenocryst of quartz and feldspar, set in a finely crystalline or glassy groundmass. The color of the rock is generally white, grey or pink.
Obsidian: Obsidian is glassy rock which is acid in composition. It is generally dark colored but show brilliant lusture.
Diorite: It is light colored coarse grained rock of plutonic origin, which is mainly composed of plagioclase feldspar and hornblende. Most diorites contain little or no quartz.
Syenite: It is coarse grained rock light colored rock of plutonic origin, which is mainly composed of orthoclase, soda-plagioclase and or more mafic minerals such as biotite and hornblende.
Gabbro: It is a coarse grained plutonic rock which is dark green or black in color, it is composed of calsic plagioclase, augite and magnetite.
Dolerite: Dolerite is usually dark being almost black when fresh. Its texture is medium to fine grained. It is mainly composed of plagioclase, augite and iron oxide, it is a hypabyssal rock.
Basalt: Basalt is dense looking black volcanic rock. Its texture is fine grained to glassy. It is composed of augit, plagioclase and iron oxide. Sometimes it contains vesicles which have been filled with secondary material.
Peridotite: It is granular, massive, dark colored of plutonic origin, which is composed almost entirely of ferromagnesian minerals like olivine and augit. Quartz and felspars are mostly absent.
Dunite: It is a coarse grained rock which is composed almost entirely of living.