Igneous rocks absolute dating

Magma that erupts from a volcano behaves according to its viscosity , determined by temperature, composition, crystal content and the amount of silica. High-temperature magma, most of which is basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma, such as andesite , tends to form cinder cones of intermingled ash , tuff and lava, and may have a viscosity similar to thick, cold molasses or even rubber when erupted.

Felsic magma, such as rhyolite , is usually erupted at low temperature and is up to 10, times as viscous as basalt. Volcanoes with rhyolitic magma commonly erupt explosively, and rhyolitic lava flows are typically of limited extent and have steep margins, because the magma is so viscous. Felsic and intermediate magmas that erupt often do so violently, with explosions driven by the release of dissolved gasesótypically water vapour, but also carbon dioxide.

Explosively erupted pyroclastic material is called tephra and includes tuff , agglomerate and ignimbrite. Fine volcanic ash is also erupted and forms ash tuff deposits, which can often cover vast areas. Because lava usually cools and crystallizes rapidly, it is usually fine-grained. If the cooling has been so rapid as to prevent the formation of even small crystals after extrusion, the resulting rock may be mostly glass such as the rock obsidian. If the cooling of the lava happened more slowly, the rock would be coarse-grained.

Because the minerals are mostly fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous rocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a microscope , so only an approximate classification can usually be made in the field. Classification Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body.

The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. Two important variables used for the classification of igneous rocks are particle size, which largely depends on the cooling history, and the mineral composition of the rock.

Feldspars , quartz or feldspathoids , olivines , pyroxenes , amphiboles , and micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks. All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessory minerals. Types of igneous rocks with other essential minerals are very rare, and these rare rocks include those with essential carbonates.

In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence of quartz , and in rocks with no feldspar or quartz, the type of iron or magnesium minerals present. Rocks containing quartz silica in composition are silica-oversaturated.

Rocks with feldspathoids are silica-undersaturated, because feldspathoids cannot coexist in a stable association with quartz. Igneous rocks that have crystals large enough to be seen by the naked eye are called phaneritic ; those with crystals too small to be seen are called aphanitic. Generally speaking, phaneritic implies an intrusive origin; aphanitic an extrusive one. An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is termed porphyry.

Porphyritic texture develops when some of the crystals grow to considerable size before the main mass of the magma crystallizes as finer-grained, uniform material. Igneous rocks are classified on the basis of texture and composition. Texture refers to the size, shape, and arrangement of the mineral grains or crystals of which the rock is composed. Rock microstructure Texture is an important criterion for the naming of volcanic rocks.

The texture of volcanic rocks, including the size, shape, orientation, and distribution of mineral grains and the intergrain relationships, will determine whether the rock is termed a tuff , a pyroclastic lava or a simple lava. However, the texture is only a subordinate part of classifying volcanic rocks, as most often there needs to be chemical information gleaned from rocks with extremely fine-grained groundmass or from airfall tuffs, which may be formed from volcanic ash.

Textural criteria are less critical in classifying intrusive rocks where the majority of minerals will be visible to the naked eye or at least using a hand lens, magnifying glass or microscope. Plutonic rocks also tend to be less texturally varied and less prone to gaining structural fabrics.

Textural terms can be used to differentiate different intrusive phases of large plutons, for instance porphyritic margins to large intrusive bodies, porphyry stocks and subvolcanic dikes apophyses. Mineralogical classification is most often used to classify plutonic rocks. Chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, e. If the approximate volume fractions of minerals in the rock are known, the rock name and silica content can be read off the diagram.

This is not an exact method, because the classification of igneous rocks also depends on other components than silica, yet in most cases it is a good first guess. Chemical classification also extends to differentiating rocks that are chemically similar according to the TAS diagram, for instance: An idealized mineralogy the normative mineralogy can be calculated from the chemical composition, and the calculation is useful for rocks too fine-grained or too altered for identification of minerals that crystallized from the melt.

For instance, normative quartz classifies a rock as silica-oversaturated; an example is rhyolite. A normative feldspathoid classifies a rock as silica-undersaturated; an example is nephelinite. History of classification In , a group of American petrographers proposed that all existing classifications of igneous rocks should be discarded and replaced by a "quantitative" classification based on chemical analysis.

They showed how vague, and often unscientific, much of the existing terminology was and argued that as the chemical composition of an igneous rock was its most fundamental characteristic, it should be elevated to prime position. Geological occurrence, structure, mineralogical constitutionóthe hitherto accepted criteria for the discrimination of rock speciesówere relegated to the background.

The completed rock analysis is first to be interpreted in terms of the rock-forming minerals which might be expected to be formed when the magma crystallizes, e. The most important criterion is the phenocryst species, followed by the groundmass mineralogy. Often, where the groundmass is aphanitic , chemical classification must be used to properly identify a volcanic rock. For intrusive, plutonic and usually phaneritic igneous rocks where all minerals are visible at least via microscope , the mineralogy is used to classify the rock.

This usually occurs on ternary diagrams , where the relative proportions of three minerals are used to classify the rock. The following table is a simple subdivision of igneous rocks according to both their composition and mode of occurrence.


Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet, surrounded by pre-existing rock (called country rock); the magma cools slowly and, as a result, these rocks are coarse-grained. How the absolute age of rock is measured ABSOLUTE DATING study guide by ChloeMohan includes 17 questions covering vocabulary, terms and more. Quizlet flashcards, activities and games help you improve your grades. Used for IGNEOUS VOLCANIC ROCKS. URANIUM LEAD DATING - good for dating rocks between MILLION YEARS and BILLIONS of YEARS.

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