Igneous RocksUnder certain conditions, rocks of the upper mantle and lower crust melt, forming a hot liquid called magma. In other words Magma is a hot, molten material found under the earth surface. An igneous rock is formed when magma solidifies. About 95 percent of the Earth’s crust consists of igneous rock. Granite and basalt are two common and familiar igneous rocks. IGNEOUS ROCKS: THE ORIGINS OF MAGMAIf you drilled a well deep into the crust, you would find that Earth temperature rises about 30ºC for every kilometer of depth. Below the crust, temperature continues to rise, but not as rapidly. In the asthenosphere (between depths of about 100 to 350 kilometers), the temperature is so high that rocks melt in certain environments to form magma.

CLASSIFICATION OF IGNEOUS ROCKSOn the Basis of routes of OccurrenceMagma can either rise all the way through the crust to erupt onto the Earth’s surface, or it can solidify within the crust. Intrusive Igneous RocksAn intrusive igneous rock forms when magma solidifies within the crust. The intrusive igneous rocks can be further subdivided into 1. Plutonic igneous rock Plutonic Rocks are those which a formed by solidification of magma under earth surface in great depth. The word plutonic is derived from Pluto, the Greek god of the underworld (under earth surface).2. Hypabasal Igneous rock.Hypabasal rocks are those which are formed by the solidification of magma near to earth surface.

Formation of Intrusive Rocks When magma solidifies within the crust, the overlying rock insulates the magma like a thick blanket. The magma then crystallizes slowly, and the crystals may have hundreds of thousands or even millions of years in which to grow. As a result, most plutonic rocks are medium to coarse grained. Granite, the most abundant rock in continental crust, is a medium- or coarse-grained plutonic rock.

Extrusive Igneous RocksGranite

An extrusive igneous rock forms when magma erupts and solidifies on the Earth’s surface. Because extrusive rocks are so commonly associated with volcanoes, they are also called Volcanic Igneous Rocks word volcano is derived from Vulcan, the Greek god of fire.

Formation of Volcanic Igneous Rocks After magma erupts onto the relatively cool Earth surface, it solidifies rapidly—perhaps over a few days or years. Crystals form but do not have much time to grow.The result is a very fine-grained rock with crystals too small to be seen with the naked eye. Basalt is a common very fine-grained volcanic rock If magma rises slowly through the crust before erupting, some crystals may grow while most of the magma remains molten. If this mixture of magma and crystals then erupts onto the surface, it solidifies

quickly, forming porphyry, a rock with the large crystals, called phenocrysts, embedded in a fine-grained matrix. In unusual circumstances, volcanic magma may solidify within a few hours of erupting. Because the magma hardens so quickly, the atoms have no time to align them selves to form crystals. The result is the volcanic glass called obsidian.

On the Basis of mineral CompositionOn the basis of mineral composition the Igneous Rocks can be subdivided into:Acidic Igneous Rocks and Basic Igneous RocksAcidic Igneous Rocks are those rocks which are rich in Silica, Sodium and Potassium, they are poor in Calcium, Magnesium and Ferrum. Acidic Igneous Rocks posses low specific gravity, they are light in weight as compared to basic igneous Rocks, and they are light in colour.Basic Igneous Rocks are those rocks which are rich in Calcium, Magnesium and Ferrum, they are poor in Silica, Sodium and Potassium. Basic Igneous Rocks posses high specific gravity, they are heavy in weight as compared to acidic igneous Rocks, they are dark in colour.On the Basis of Silica PercentageThere are four types of Igneous Rocks on the basis of Silica percentage:Acidic Rocks: when the percentage of silica in ay rock is more than 65% then it is said to be Acidic. These are also called Felsic rocks.Intermediate Rocks: when the percentage of silica in ay rock ranges from 65% to 55% then it is said to be Intermediate Rocks.Basic Rocks: when the percentage of silica in ay rock ranges from 55% to 45% then it is said to be Basic. These are also called Mafic rock.Ultramafic: when the percentage of silica in ay rock less than 45% then it is said to be Ultramafic.

COMMON IGNEOUS ROCKSType on silica percentage

Acidic(Felsic rocks)

Intermediate Basic(Mafic rock)

Ultramafic

Intrusive Granite Diorite Gabbro Peridotite

Extrusive Rhyolite Adensite BasaltComposition Silica 72% Silica 59% Silica 50% Silica 45%

Aluminum Oxide 14 %

Aluminum Oxide 17 %

Aluminum Oxide 16 %

Aluminum Oxide 4 %

Rhyolite

Iron Oxide 3% Iron Oxide 8% Iron Oxide 11% Iron Oxide 12%

Magnesium oxide 1%

Magnesium oxide 3%

Magnesium oxide 7%

Magnesium oxide 31%

Others 10% Others 13% Others 16% Others 8%

Major Minerals QuartzPotassium feldsparSodium feldspar (plagioclase)

AmphiboleIntermediateplagioclase feldspar

Calcium feldspar (plagioclase)Pyroxene

OlivinePyroxene

Minor Minerals MuscoviteBiotiteAmphibole

Pyroxene OlivineAmphibole

Calcium feldspar (plagioclase)

Most Common Colour

Light colored Medium gray ormedium green

Dark gray toblack

Very dark greento black

TEXTURES OF IGNEOUS ROCKSThe texture of a rock refers to the size, shape, and arrangement of its mineral grains, or crystals. Some igneous rocks consist of mineral grains that are too small to be seen with the naked eye; others are made up of thumb-size or even larger crystals. Volcanic rocks are usually fine grained, whereas plutonic rocks are medium or coarse grained.

Grain size TextureNo mineral grains (obsidian) GlassyToo fine to see with naked eye Very fine grainedUp to 1 millimeter Fine grained1–5 millimeters Medium grainedMore than 5 millimeters Coarse grainedRelatively large grains in a finer-grained matrix

Porphyritic

ROCKS AND THE ROCK CYCLEGeologists group rocks into three categories on the basis of how they form: igneous rocks, sedimentary rocks, and metamorphic rocks.

Under certain conditions, rocks of the upper mantle and lower crust melt, forming a hot liquid called magma. An igneous rock forms when magma solidifies. About 95 percent of the Earth’s crust consists of igneous rock and metamorphosed igneous rock. Although much of this igneous foundation is buried by a relatively thin layer of sedimentary rock, igneous rocks are conspicuous because they make up some of the world’s most spectacular mountains. Granite and basalt are two common and familiar igneous rocks.Rocks of all kinds decompose, or weather, at the Earth’s surface. Weathering breaks rocks

into smaller fragments such as gravel, sand, and clay. At the same time, rainwater may dissolve some of the rock. Streams, wind, glaciers, and gravity then erode the weathered particles, carry them downhill, and

The Rock Cycle

deposit them at lower elevations. All such particles, formed by weathering and then eroded, transported, and deposited in layers, are called sediment. The sand on a beach and mud on a mud flat are examples of sediment that accumulated by these processes. A sedimentary rock forms when sediment becomes cemented or compacted into solid rock. When the beach sand is cemented, it becomes sandstone; the mud becomes shale. Sedimentary rocks make up less than 5 percent of the Earth’s crust. However, because sediment accumulates on the Earth’s surface, sedimentary rocks form a thin layer over about 80 percent of all land. For this reason, sedimentary rocks seem more abundant than they really are. A metamorphic rock forms when any preexisting rock is altered by heating, increased pressure, or tectonic deformation. Tectonic processes can depress the Earth’s surface to form a basin that may be hundreds of kilometers in diameter and thousands of meters deep. Sediment accumulates in the depression, burying the lowermost layers to great depths. When a rock is buried, its tem perature and pressure increase, causing changes in both the minerals and the texture of the rock. These changes are called metamorphism, and the rock formed by these processes is a metamorphic rock. Metamorphism also occurs when magma heats nearby rock, or when tectonic forces deform rocks. Schist, gneiss, and marble are common metamorphic rocks.No rock is permanent over geologic time; instead, all rocks change slowly from one of the three rock types to another. This continuous process is called the rock cycle. The transformations from one rock type to another can follow many different paths. For example, weathering may reduce a metamorphic rock to sediment, which then becomes cemented to form a sedimentary rock. An igneous rock may be metamorphosed. The rock cycle simply expresses the idea that rock is not permanent but changes over geologic time.

SEDIMENTARY ROCKSWeathering decomposes bedrock. Flowing water, wind, gravity, and glaciers then erode the decomposed rock, transport it down slope, and finally deposit it on the sea coast or in lakes and river valleys. Finally, the loose sediment is cemented to form hard sedimentary rock.

Sedimentary rocks make up only about 5 percent of the Earth’s crust. However, because they form on the Earth’s surface, they are widely spread in a thin veneer over underlying igneous and metamorphic rocks. As a result, sedimentary rocks cover about 75 percent of continents.Many sedimentary rocks have high economic value. Oil and gas form in certain sedimentary rocks. Coal, a major energy resource, is a sedimentary rock. Limestone is an important building material, both as stone and as the primary ingredient in cement. Gypsum is the raw material for plaster. Ores of copper, lead, zinc, iron, gold, and silver concentrate in

certain types of sedimentary rocks.

Formation of Sedimentary Rock1. Weathering & ErosionAfter weathering creates clastic sediment, flowing water, wind, glaciers, and gravity erode it and carry it down slope. Streams carry the greatest proportion of clastic sediment. Because most streams empty into the

Horizontally layered sandstone in eastern Utah has been eroded to produce spectacular towers.

oceans, most sediment accumulates near continental coastlines. However, some streams deposit their sediment in lakes or in inland basins.Streams and wind modify sediment as they carry it down slope. The rounded cobbles shown in originally formed as angular rubble in the Bitterroot Range of western Montana. The rubble became rounded as the stream carried it only a few kilometers. Water and wind round clastic particles as fine as silt by tumbling those against each other during transport. Finer particles do not round as effectively because they are so small and light that water and even wind, to some extent, cushion them as they bounce along, minimizing abrasion. Glaciers do not round clastic particles because the ice prevents the particles from abrading each other.Weathering breaks bedrock into particles of all sizes, ranging from clay to boulders. Yet most clastic sediment and sedimentary rocks are well sorted—that is, the grains are of uniform size. Some sandstone formations extend for hundreds of square kilometers and are more than a kilometer thick, but they consist completely of uniformly sized sand grains. Sorting depends on three factors: the viscosity and velocity of the transporting medium and the durability of the particles. Viscosity is resistance to flow; ice has high viscosity, air has low viscosity, and water is intermediate. Ice does not sort effectively because it transports particles of all sizes, from house-sized boulders to clay.In contrast, wind transports only sand, silt, and clay and leaves the larger particles behind. Thus, wind sorts particles according to size. A stream transports only small particles when it flows slowly, but larger particles when it picks up speed. For example, a stream transports large and small particles when it is flooding, but only small particles during normal flows. As a flood recedes and the water gradually slows down, the stream deposits the largest particles first and the smallest ones last, creating layers of different sized particles. Finally, durability of the particles affects sorting. Sediment becomes abraded as it travels downstream. Thus a stream may transport cobbles from the mountains toward a delta, but the cobbles may never complete the journey because they wear down to smaller grains along the way. This is one reason why mountain streams are frequently boulder choked but deltas are composed of mud and sand.

2. SedimentationThe process by which sediments accumulate at the base of Deposition.

Sedimentation

3. LithificationLithification refers to processes that convert loose sediment to hard rock. Two of the most important processes are compaction and cementation.If you fill a container with sand, the sand grains do not fill the entire space. Small voids, called pores, exist between the grains. When sediment is deposited in water, the pores are usually filled with water. The

proportion of space occupied by pores depends on particle size, shape, and sorting. Commonly, freshly deposited clastic sediment has about 20 to 40 percent pore space, although well-sorted and well-rounded sand may have up to 50 percent pore space. Clay-rich mud may have as much as 90 percent pore space

occupied by water.As more sediment accumulates, its weight compact the buried sediment, decreasing pore space and forcing out some of the water. This process is called compaction. Compaction alone may lithify clay because the platy grains interlock like pieces of a puzzle.Water normally circulates through the pore space in buried and compacted sediment. This water commonly contains dissolved calcium carbonate, silica, and iron, which precipitate in the pore spaces and cement the clastic grains together to form a hard rock. The red sandstone in gets its color from red iron oxide cements. In some environments, sediment lithifies quickly, whereas the process is slow in others. In the Rocky Mountains, calcite has cemented glacial deposits less than 20,000 years old. In contrast, some sand and gravel deposited between 30 and 40 million years ago in south-western Montana can be dug with a hand shovel. The speed of lithification depends mainly on the availability of cementing material and water to carry the dissolved cement through the sediment.

Types of Sedimentary RocksThey are divided into two main types

1) Clastic Sedimentary Rocks 2) Non-Clastic Sedimentary RocksClastic Sedimentary RocksClastic sedimentary rocks are composed of fragments of weathered rocks, called clasts, which have been transported, deposited, and cemented together. Clastic rocks make up more than 85 percent of all sedimentary rocks. This category includes sandstone, siltstone, and shale.Clastic sediment consists of grains and particles that were eroded from weathered rocks and then were transported and deposited in loose, unconsolidated layers at the Earth’s surface. The sand on a beach, boulders in a river bed, and mud in a puddle are all clastic sediments. Clastic sediment is named according to particle size. Gravel includes all rounded particles larger than 2 millimeters in diameter. Angular particles in the same size range are called rubble. Sand ranges from 1/16 to 2 millimeters in diameter. Sand feels gritty when rubbed between your fingers, and you can see the grains with your naked eye. Silt varies from 1/256 to 1/16 millimeter. Individual silt grains feel smooth when rubbed between the fingers but gritty when rubbed between your teeth. Clay is less than 1/256 millimeter in diameter. It is so fine that it feels smooth even when rubbed between your teeth. Geologists often rub a small amount of sediment or rock between their front teeth to distinguish between silt and clay. Mud is wet silt and clay.

SIZES AND NAMES OF SEDIMENTARY PARTICLES ANDCLASTIC ROCKS

Diameter (mm) Sediments Clastic Sedimentary Rock

More than 256 Boulders Gravel(Rubble)

Conglomerate(rounded particles)64 to 256 Cobbles

or breccia2 to 64 Pebbles

1/16 to 2 Sand Sandstone1/256 to 1/16 Silt

MudSlit Stone }MudstoneLess than 1/256 Clay Shale

Types of Clastic Sedimentary RocksThey are classified on the basis of particle sizeRudaceous The particles size varies from 2mm to 10mm. e.g. Conglomerate & Bressica.Arenaceous The particles size varies from 0.1mm to 2mm. e.g. Sandstone.Argillaceous The particles size is 0.01mm. e.g. Shale.

Non-Clastic Sedimentary RocksThey are further subdivided into two types.

1. Chemically formed 2. Organically formed

Chemically Formed Chemically Formed sedimentary rocks form by direct precipitation of minerals from solution. Rock salt, for example, forms when salt precipitates from evaporating seawater or saline lake water. Some common elements in rocks and minerals, such as calcium, sodium, potassium, and magnesium, dissolve during chemical weathering and are carried by ground water and streams to the oceans or to lakes. Most lakes are drained by streams that carry the salts to the ocean. Some lakes, such as the Great Salt Lake in Utah, are landlocked. Streams flow into the lake, but no streams exit. As a result, water escapes only by evaporation. When water evaporates, salts remain behind and the lake water becomes steadily more

salty. The same process can occur if ocean water is trapped in coastal or inland basins, where it can no longer mix with the open sea.

Organically FormedOrganic sedimentary rocks consist of the remains of plants or animals. Coal is an organic sedimentary rock made up of decomposed and compacted plant remains.Organic sedimentary rocks, such as chert and coal, form by lithification of the remains of plants and animals. CHERT Chert is a rock composed of pure silica. It occurs as sedimentary beds interlayered with other sedimentary rocks and as irregularly shaped lumps called nodules in other sedimentary rocks.

Lime Stone with Shell Fragments

Red nodules of chert in light-colored limestone

COAL When plants die, their remains usually decompose by reaction with oxygen. However, in warm swamps and in other environments where plant growth is rapid, dead plants accumulate so rapidly that the oxygen is used up long before the decay process is complete. The undecayed or partially decayed plant remains form peat. As peat is buried and compacted by overlying sediments, it converts to coal, a hard, black, combustible rock.

Some Geological termsDike: Its is a discordant Igneous Rock Body which is formed by Injection of magma by cutting the bedding planesSill: It is a concordant igneous Rock body which is formed by following of magma along bedding plane without cutting the bedding plane or it is parallel to bedding plane.

Batholith: A very large Plutonic igneous rock mass that has been exposed by erosion and with an exposed surface area of over 100 square kilometers. A batholith has no known floor. It is Spread & Irregular its small

part is known as Boss. Lypolith: It is an igneous rock body which forms Saucer like shape when lava comes out of earth surface.Laccolith: It is an igneous rock body which forms concave like shape when lava comes out of earth surface. Or it is an igneous intrusion that has been forced between two layered rock units. The top of the intrusion is arched upwards and the bottom of the intrusion is nearly flat.Conolith: It is an igneous rock body which forms spoon like shape when lava comes out of earth surface.Pillow Lava: It is Extrusive igneous rock body which is formed by solidification of lava & forming a pillow like shape.