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Intrusive igneous rocks are classified separately from extrusive igneous rocks, generally on the basis of their mineral content. The relative amounts of quartz, alkali feldspar, plagioclase, and feldspathoid is particularly important in classifying intrusive igneous rocks. [9] [10] Intrusions must displace existing country rock to make room for ...
QAPF diagram for the classification of plutonic rocks Devils Tower, United States, an igneous intrusion exposed when the surrounding softer rock eroded away. Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form intrusions, such as batholiths, dikes, sills, laccoliths, and volcanic necks.
Trachyte – Extrusive igneous rock – A silica-undersaturated volcanic rock; essentially a feldspathoid-bearing rhyolite; Troctolite – Igneous rock – A plutonic ultramafic rock containing olivine, pyroxene and plagioclase; Trondhjemite – Light-colored intrusive igneous rock – A form of tonalite where plagioclase-group feldspar is ...
Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are usually much finer-grained, often resembling volcanic rock. [8] Hypabyssal rocks are less common than plutonic or volcanic rocks and often form dikes, sills, laccoliths, lopoliths , or phacoliths .
These rock types can include: Mangerite: a pyroxene-bearing monzonite intrusive igneous rock; Charnockite: an orthopyroxene-bearing quartz-feldspar rock, once thought to be intrusive igneous, now recognized as metamorphic; Iron-rich felsic rocks, including monzonite and rapakivi granite; Iron-rich diorite, gabbro, and norite
Quartzolite or silexite is an intrusive igneous rock, in which the mineral quartz is more than 90% of the rock's felsic mineral content, with feldspar at up to 10%. [1]: 135 [2] Typically, quartz forms more than 60% of the rock, [3] the rest being mostly feldspar although minor amounts of mica or amphibole may also be present. [2]
Monzonite is an igneous intrusive rock, formed by slow cooling of underground magma that has a moderate silica content and is enriched in alkali metal oxides. Monzonite is composed mostly of plagioclase and alkali feldspar. Syenodiorite is an obsolescent term for monzonite [1] or for monzodiorite. [2] Larvikite is a particular form of monzonite ...
Shonkinite is an intrusive igneous rock found in few places in the world. It is unique in having low silica, feldspathoid minerals, and large blocky crystals of black augite. It makes up much of the hard dark grey rock comprising certain mountains and buttes in Montana that are remnants of laccoliths and stocks, such as the Highwood mountains.