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Igneous rocks are called fire rocks and are
formed either underground or above ground.Igneous rocks are also formed when volcanoes erupt, causing the
magma to rise above the earth's surface.Igneous rocks are formed as the lava cools above ground.Most PopularRock Picture GalleryRock Identification TablesMineral Picture GalleryEarthquake LightsAbout Igneous Rocks
About Igneous RocksFrom Andrew Alden,Your Guide to Geology.Igneous rocks are the first great class.Latin for fire, and all igneous rocks began as hot, fluid material.To learn more about how igneous rocks form, see About Volcanism.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.This makes igneous petrology a very complex field, and this article is only the barest outline.Igneous Rock Textures
Tell the three types of igneous rocks apart by their texture, starting with the size of the mineral grains.Extrusive rocks cool quickly (over periods of seconds to months) and have invisible or very small grains, or an aphanitic texture.Because they solidified from a fluid state, igneous rocks tend to have an equigranular texture, a uniform fabric without layers, and the mineral grains are packed together tightly.Think of the texture of a piece of bread as a similar example.Phenocrysts are minerals that solidified earlier than the rest of the rock, and they are important clues to the rock's history.Obsidian, formed when lava hardens quickly, has a glassy texture.Pumice and scoria are volcanic froth, puffed up by millions of gas bubbles giving them a vesicular texture.And pillow lava is a lumpy formation created by extruding lava underwater.It is rich in feldspar and quartz (silica) and hence is called a felsic rock.So granite is felsic and plutonic.These two categories cover the great majority of igneous rocks.Ordinary people, even ordinary geologists, use the names freely.But igneous petrologists use many more names.True granite and true basalt are narrow subsets of these categories.Basalts are also erupted above the Earth's great subduction zones, either in volcanic island arcs or along the edges of continents.However, continental magmas tend to be less basaltic and more granitic.The continents are the exclusive home of granitic rocks.In general, granitic rocks are less dense than basaltic rocks, and thus the continents actually float higher than the oceanic crust on top of the ultramafic rocks of the Earth's mantle.Get more help identifying igneous rocks in the Rock Identification Tables.ALERT site that teaches us about volcanoes.Make your own miniature Lava Lamp!Everything you ever wanted to know.Magma, Rocks and Planetary Development, A book that puts in all together.Not for the beginner, but rather someone who wants to catch up on what has been going on with igneous rocks.The eruption of a volcano is an awesome process.Volcanic rocks, the solidified products of volcanic eruptions are part of a larger group of rocks called igneous rocks.IGNEOUS ROCKS
Igneous rocks are crystalline or glassy rocks formed by the cooling and solidification of molten magma.Igneous rocks comprise one of the three principal classes of rocks, the others being metamorphic and sedimentary.Igneous rocks are formed from the solidification of magma, which is a hot (600 deg.F) molten or partially molten rock material.The Earth is composed predominantly of a large mass of igneous rock with a very thin covering of sedimentary rock.Because magma is less dense than the surrounding solid rocks, it rises toward the surface.Rocks formed from the cooling and solidification of magma deep within the crust are distinct from those erupted at the surface mainly owing to the differences in conditions in the two environments.The slow cooling promotes the growth of minerals large enough to be identified visually without the aid of a microscope (called phaneritic, from the Greek phaneros, meaning "visible").On the other hand, magma erupted at the surface is chilled so quickly that the individual minerals have little or no chance to grow.This results in two groups of igneous rocks: (1) plutonic or intrusive igneous rocks that solidified deep within the earth and (2) volcanic, or extrusive, igneous rocks formed at the Earth's surface.Extrusive rocks occur in two forms: (1) as lava flows that flood the land surface much like a river and (2) as fragmented pieces of magma of various sizes (pyroclastic materials), which often are blown through the atmosphere and blanket the Earth's surface upon settling.Both intrusive and extrusive magmas have played a vital role in the spreading of the ocean basin, in the formation of the oceanic crust, and in the formation of the continental margins.Igneous processes have been active since the formation of the Earth some 4.CLASSIFICATION
Igneous rocks are classified on the basis of mineralogy, and texture.In any classification scheme, boundaries between classes are set arbitrarily; however, if the boundaries can be placed closest to natural divisions or gaps between classes, they will seem less random and subjective, and the standards will facilitate universal understanding.The most commonly used scheme was devised by the International Union of Geological Sciences (IUGS)(See image).That classification takes advantage of simple associations that occur among the various silicate minerals.We do not need to know percentages of the various mineral phases, merely which minerals are present.We will be utilizing this classification as our basis for identifying igneous rocks.Volcanic rocks present a greater challenge.Since many of the mineral grains are not visible, using a mineralogical classification becomes problematic.Ideally we would like to have a chemical analysis.We shall attempt to rely on texture, color and phenocryts to identify our volcanic rock specimens.To learn more about textures and mineralogy of igneous rocks click on the links below.Now you are ready to identify some common igneous rocks.He specializes in mineralogy and ore deposit geochemistry.He has written many articles on ore deposits in the Mojave and lead numerous field trips to the Mojave and eastern Sierra Nevada.He specializes in stratigraphy and structural geology.Describe how the rate of cooling influences the size of crystals in igneous rocks.List the different igneous rock textures (aphanitic, phaneritic, porphyritic, pegmatitic, vesicular, glass, and pyroclastic) and explain their origins.Bowen to the understanding of igneous rocks.Be able to list the minerals of Bowen's Reaction series in order.Discuss the various types of magma and how they relate to igneous rocks.List the various kinds of intrusive igneous bodies (dike, sill, laccolith, stock, batholith) and describe each in terms of the criteria used to classify plutons.Igneous rocks are classified on their texture and their composition.Mixture of grain sizes caused by mixed cooling history; slow cooling first, followed by a period of somewhat faster cooling.The groundmass may be either aphanitic or phaneritic.Early Mesozoic dikes in eastern North America.The industrial name for the rocks is "blue pearl".You have to know the series AND understand the concepts of how Bowen's Reaction Series relates to melting and to crystallization, and to the origin of igneous rocks of various composition.Igneous rocks are formed from molten magma or lava.All igneous rock starts deep in the earth as hot,
molten magma.This material is copyrighted, but may be used by anyone for personal or education purposes as long as the source is acknowledged.
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