Figure 01: Igneous Rock. Intrusions are formed inside country rock which prevents the magma (=molten rock) intrusion from cooling quickly. As an example, a rock with the same composition could be called gabbro if it were plutonic, diabase if it were intrusive, or basalt if it were extrusive. Coarse grained. Basalt is a mafic extrusive igneous rock formed from the rapid cooling of lava rich in magnesium and iron exposed at or very near the surface of a terrestrial planet or a moon. Greater then 95% of Diabase is composed of 2 minerals. Diabase is composed of plagioclase feldspar (mostly labradorite) and pyroxene ().The crystals that make up dolerite are usually visible to the naked eye, but sometimes porphyritic rocks of basaltic composition with pyroxene and especially plagioclase phenocrysts are also named that way. Extrusive. (after Le Maitre, 2005) Once you have determined the mineral assemblage of a rock, you can use the appropriate (intrusive or extrusive) QAPF chart to name it (fig. They contain less than 45 percent silica, and are believed to originate from the mantle. Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals. 2 Extrusive igneous rocks. Preliminary QAPF classification of: a – intrusive rocks; b – extrusive rocks. Whereas plutonic rocks form continents, basalt lies in the crust underneath the oceans. Pyroxene, Amphibole, possibly Biotite and/or Olivine. Intrusive. Basaltic Glass. Figure 4. So, an extrusive igneous rock, is in the extrusive category. Extrusive rocks have very small, almost microscopic crystals. The Jurassic age York Haven Type Diabase is what forms the Monocacy Hill intnlsion. This rock's name (Diabase) is defined by it's medium grained size crystals. The bottom salt-and-pepper colored portion is a polished surface displaying the plagioclase (white) and pyroxene (black) minerals that make up this specimen of an intrusive igneous rock. Examples of intrusive igneous rocks are diorite, gabbro, granite, pegmatite, and peridotite. Quartz, Potassium-Felspar, Plagioclase Fledspar, Biotite Mica & Amphibole ... Diabase. Properties of Igneous Rocks. Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling that occurs there allows large crystals to form. can be intrusive or extrusive Can be fine or coarse grained* mafic. These types of rock include diabase, quartz-dolerite, micro-granite and diorite. Intrusive, or plutonic, igneous rock forms when magma remains inside the Earth's crust where it cools and … These rocks are in many ways similar to other intrusive rocks, but have intermediate grain sizes and textures, generally half way between those found in extrusive and other types of intrusive rocks. 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