Iron is low in the Komatiite suite because the ultramafic components Mg, Ni, and Cr are so high. Suite Fractionation: Igneous rock evolution can occur both within and among the suites. Within suite evolution occurs when, for example, a calcalkaline suite evolves from a diorite to a granite, or a komatiite suite evolves from a peridotite to a basalt to an andesite. That is, tholeiitic, followed by calcalkaline, and finally alkaline suites. Cross section.
Another evolutionary process occurs when one fractionated igneous rock is re-fractionated at a later time. This would occur, for example, if a fractionated diorite magma emplaced and solidified into a batholith. If this batholith is later heated, a second, more felsic, fractional product granite could be sweated out of it, leaving behind a more mafic residue. Also, a rock of one suite may re-fractionate to a melt with the characteristics of another suite.
Suite Tectonic Association: One important feature of the suites is their association with particular tectonic regimes go to table. This knowledge is valuable in understanding and reconstructing ancient tectonic events when most of the evidence is destroyed or otherwise unavailable. By analyzing the chemistry of the rocks we can reconstruct the processes by which they formed.
The cross section shows typical tectonic conditions under which each suite forms. A divergent plate boundary rift is on the right and a subduction on the left. The large arrows rising on the right, pointing horizontally across the middle, and descending into the subduction zone on the left marks the path the igneous rocks take. From step to step the evolutionary processes described above occur in sequence.
Fractionation takes place in two primary tectonic regimes. First is at rifting centers. Silica over saturated parent rocks komatiites in the Archean, other ultramafics since then rise to the surface and fractionally melt. The melt is tholeiitic and rises to the surface to form the pillow basalts and sheeted dikes of ocean crust.
The unmelted residue is usually silica under saturated ultramafics which stay in the mantle as layer 4 in the ophiolite suite. Note that the ophiolite "suite" is NOT a suite in the same sense as calcalkaline, etc. The second fractionation takes place at convergent boundaries. The tholeiitic oceanic crust moves away from the rifting center until it is subducted.
It heats up during subduction and fractionally melts. Typically the first melts erupting closest to the trench are still tholeiitic, but in time the melts evolve to the calcalkaline suite, which build most of the volcanic arc.
Figure 7. To see how Figure 7. An igneous rock can be represented as a vertical line drawn through the diagram, and the vertical scale used to break down the proportion of each mineral it contains. For example, a felsic intrusive rock is called granite , whereas a felsic extrusive rock is called rhyolite. Granite and rhyolite have the same mineral composition, but their grain size gives each a distinct appearance.
The key difference between intrusive and extrusive igneous rocks—the size of crystals making them up—is related to how rapidly melted rock cools. The longer melted rock has to cool, the larger the crystals within it can become. Notice that in Figure 7. A rock with individual crystals that are visible to the unaided eye has a phaneritic or coarse-grained texture.
The extrusive rocks in the second row have much smaller crystals. The crystals are so small that individual crystals cannot be distinguished, and the rock looks like a dull mass. A rock with crystals that are too small to see with the unaided eye has an aphanitic or fine-grained texture. Table 7. What this means is that two igneous rocks comprised of exactly the same minerals, and in the exactly the same proportions, can have different names.
A rock of intermediate composition is diorite if it is course-grained, and andesite if it is fine-grained. A mafic rock is gabbro if it is course-grained, and basalt if fine-grained. The course-grained version of an ultramafic rock is peridotite , and the fine-grained version is komatiite. It makes sense to use different names because rocks of different grain sizes form in different ways and in different geological settings. Something interesting happens when there is a change in the rate at which melted rock is cooling.
If magma is cooling in a magma chamber, some minerals will begin to crystallize before others do. If cooling is slow enough, those crystals can become quite large. Now imagine the magma is suddenly heaved out of the magma chamber and erupted from a volcano. The larger crystals will flow out with the lava. The lava will then cool rapidly, and the larger crystals will be surrounded by much smaller ones.
An igneous rock with crystals of distinctly different size Figure 7. View some examples of igneous rocks from the Australian Museum's Mineralogy Collection. Grain size reflects the depth at which molten rocks form within the Earth. Classification of igneous rocks can be based on their predominant grain size. Three types of rock can be identified:. Silica SiO2 content also controls the minerals that crystallise and is used to further classify igneous rocks as follows:.
This is similar to silica content, but allows distinction between feldspathoid-bearing and feldspathoid-free rocks. Feldspathoids belong to a group of rock-forming minerals chemically similar to the feldspars but containing less silica.
This scheme classifies igneous rocks into three categories:. The International Union of Geological Sciences IUGS classification scheme is the standard scheme for igneous rocks and uses the minerals present to classify them. It splitsrocks into feldspathoid-bearing i. Then it takes the relative percentages of quartz Q , plagioclase P and alkali feldspars A.
The values of Q, P, and A determines a given field for each rock type. Similar schemes also classify ultramafic rocks but using olivine, orthopyroxene and clinopyroxene and gabbroic rocks. Le Maitre, R. A classification of igneous rocks and glossary of terms. Blackwell Scientific Publications, p. The Australian Museum respects and acknowledges the Gadigal people as the First Peoples and Traditional Custodians of the land and waterways on which the Museum stands.
Image credit: gadigal yilimung shield made by Uncle Charles Chicka Madden. This website uses cookies to ensure you get the best experience on our website. Learn more. Skip to main content Skip to acknowledgement of country Skip to footer Scanning Electron Microscope image of Pumice, an igneous rock.
Igneous rocks. Basaltic scoria 13 cm x 9 cm , Anakie Eastern Hill, Victoria. Scoria is a pyroclastic volcanic rock containing abundant empty cavities or vesicles and is usually of basaltic composition.
Aa lava 14 cm x 12 cm. Kilauea, Hawaii.
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