By: Faisal Andrew Travis Adrian

 Geological Significance  Texture  Characteristics  Composition  Structure  Setting  Classification  Composition

Igneous rocks make up about 90% of the upper part of the earth’s crust. Their minerals and chemical make-up give information about the mantle. Their age can be obtained using radioactive dating which can then be compared to other aspects of the earth. They can give information about tectonic plates. Sometimes they contain important mineral deposits such as tin and uranium.

Texture When magma cools and hardens slowly, large, well- developed crystalline grains form out of the minerals in the magma. The result is an intrusive igneous rock that has a coarse-grained texture. e.g. Granite When lava cools quickly on the earth’s surface, large crystalline grains cannot form due to the rapid loss of heat. This produces a fine-grained rock. e.g. Basalt (The core of the earth’s continental crust is made up of granite) (The earth’s oceanic crust is made up of basalt)

Texture (cont’d) Igneous rock that have a porphyritic texture have a mixture of large crystals embedded in a mass of smaller crystals. This happens when magma cools slowly at first, then faster as it reaches the earth’s surface. When magma has a high viscosity, or is rich in minerals, there may not be any crystals. e.g. Obsidian (volcanic glass) If the magma contains large amounts of dissolved gases, the gases may get trapped in the rock, which results in a rock full of holes. e.g. Pumice

Some characteristics of igneous rocks are: Can look glassy or frothy Can be porous or full of bubbles (pumice) Can have large, small, or a combination of large and small crystal structures Can be light enough to float

Igneous rocks are formed after the cooling of molten magma. This process is called “crystallization”. The three types of igneous rock that will be discussed; basalt, granite and andesite all have different compositions.

Granite is formed from magma high in silica. Andesite contains feldspar, hornblende, quartz and micas which all look darker and formed from magma with moderate amounts of silica. Basalts share the same minerals as Andesites as they have feldspar, mica, and hornblendes but they rarely have any quartz.

The crystal structures of most Igneous rocks are very well-developed. Low viscosity and slow cooling helps increase the grain size of any igneous rock.

A microscope may be needed to see the crystal structures of Igneous rocks. Furthermore, various elements of Igneous rocks are formed depending on the magma. All sorts of minerals fall into this magma from rocks that have been pushed deep and melted under its fiery pressure Silica is added into the magma as it gets melted from underneath the continent and mixes itself with water and carbon dioxide.

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Setting: Igneous rocks begin as a hot fluid material. The word "igneous" comes from the Latin word for fire. This material may have been lava erupted at the Earth's surface, or magma at shallow depths, or magma in deep bodies (plutons). Rock formed of lava is called extrusive, rock from shallow magma is called intrusive and rock from deep magma is called plutonic.

People commonly think of lava and magma as a liquid, like molten metal, but geologists find that magma is usually a mush; a liquid carrying a load of mineral crystals. Magma crystallizes into a collection of minerals, and some crystallize sooner than others. Not just that, but when they crystallize, they leave the remaining liquid with a changed chemical composition. Magma cools and evolves as it moves through the crust, interacting with other rocks evolving further.

Igneous rocks form in three main places: where lithospheric plates pull apart at mid-ocean ridges, where plates come together at subduction zones, and where continental crust is pushed together, making it thicker and allowing it to heat and melt.

Igneous rocks are called fire rocks and are formed either underground or above ground. Underground, they are formed when the melted rock, called magma, deep within the earth becomes trapped in small pockets. As these pockets of magma slowly cool underground, the magma becomes igneous rocks.

Igneous rocks are also formed when volcanoes erupt, causing the magma to rise above the earth's surface. When magma appears above the earth, it is called lava. Igneous rocks are formed as the lava cools above ground.

Igneous rock formation can also happen when sedimentary rocks are remelted. This takes place at subduction zones. Subduction zones occur when one of the earth's plates moves under another. Any sedimentary rocks sitting atop the subducting plate is pulled down. Tremendous heat is generated as the two plates grind against each other. This heat and the heat of the underlying magma melts the rock.

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Intrusive: Coarse-grained rocks that cool beneath the earths surface, most common type Granite Extrusive: Fine-grained rocks that cool above the earths surface, most common type is Basalt BasaltGranite

Felsic : Contains high silica levels composed primarily of the minerals Potassium feldspar, Quartz, and Plagioclase Intermediat e : Moderate silica levels, composed of Plagioclase, Biotite, and Amphibole Mafic : Low silica levels, but rich in iron and magnesium. Composed mainly of Pyroxene, Olivine, and Plagioclase Ultramafic : Extremely low silica level, composed of the minerals Pyroxene, and Olivine

Obsidian (Extrusive Felsic ) Diorite (Intrusive Intermediate) Gabbro (Intrusive Mafic) Peridotite (Intrusive Ultramafic)

Cooling Rates: “When lava flows on the earths surface, it is exposed to air and moisture. Under these conditions, the lava cools quickly, and there is not enough time for large crystals to form. Thus, extrusive igneous rocks such as obsidian have no visible mineral grains” Texture: Geologist use texture from different igneous rocks to classify them. By slicing the rock into thin samples geologist can categorizes the type of crystals that have formed. During fractional crystallization specific crystals form under certain conditions

Works Cited Alden, Andrew. About.com. 10 March The New York Times Company.. Busbey III, Arthur B. The Nature Company Guides: Rocks & Fossils. San Francisco:Time-Life Books, Millage, Clayton. Earth Science: Geology, the Environment, and the Universe. New York: Mcgraw-Hill, “All About Gemstones.” March 2010.