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Grain size in igneous rocks results from cooling time so porphyritic rocks are created when the magma has two distinct phases of cooling. [18] 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. [citation needed]
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 ...
Igneous rocks include rocks that have solidified from a melt either below the surface of a planet or natural satellite as intrusives or on the surface as extrusives or volcanic eruptions. The main article for this category is Igneous rock .
Gabbro specimen Photomicrograph of a thin section of gabbro. Gabbro (/ ˈ ɡ æ b r oʊ / GAB-roh) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface.
The Canadian Shield (French: Bouclier canadien [buklje kanadjɛ̃]), also called the Laurentian Shield or the Laurentian Plateau, is a geologic shield, a large area of exposed Precambrian igneous and high-grade metamorphic rocks. It forms the North American Craton (or Laurentia), the ancient geologic core of the North American continent.
Mineralogical composition of igneous rocks with decreasing silica (SiO 2) content. The rock names for volcanic/extrusive and plutonic/intrusive rocks are written above. Notice that this is a rough way to determine a rock name: the exact difference between the given igneous rocks does not only depend on silica content. Date: March 2009: Source ...
Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth's surface. As magma cools slowly the minerals have time to grow and form large crystals. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye.
These structures are lighter colored than the host rock and typically range in diameter from 0.05mm to over 5 cm. [1] [2] [3] In 1648, Aldrovandi created the term variolite for aphanitic or fine-grained igneous rocks containing varioles. [4] [5] The weathering of varioles often cause variolites to have a pock-marked appearance.