Petro

• Igneous Petrography is the description and classification of igneous rocks
• Field Geology gathers the structure, texture, and physical appearance of an igneous rock during fieldwork
• Plutonic rocks have coarse or medium grain sizes (>1mm) and are inferred to have crystallized deep in the crust
• IUGS classification is based on the modal amounts of common minerals, divided into five groups by the International Union of Geological Sciences
• Euhedral minerals contain complete crystal faces not impinged upon by other crystals
• Aphanitic is a major igneous texture that can be subdivided into microcrystalline and cryptocrystalline, with chemical composition reflecting the magma
• Pyroclastic is a term used for volcanic materials ejected from a volcanic vent
• Internal gas expansion from within a magma body produces Pyroclasts particles
• Volcaniclastic has a broader meaning and applies to clastic deposits with particles of volcanic composition regardless of origin
• Essential (Juvenile) particles such as crystal, lithic, & vitric are derived from new magma
• Igneous Petrogenesis studies the origin & evolution of igneous rocks
• Pressures & Temperatures are required for igneous rocks to form and evolve, reproduced in the laboratory
• Mafic rocks contain pyroxene, amphibole, olivine, and biotite
• Intermediate rocks have a weight percentage silica of 52-66%
• Anorthosite is a special name applied to rocks containing more than 90% plagioclase
• Texture of the rock depends largely on the cooling history of the magma
• Large crystals in a porphyritic texture are referred to as Phenocrysts
• Major igneous textures based on crystal size are Phaneritic, Aphanitic, Pegmatitic, and Porphyritic
• Criteria for classifying igneous rocks include minerals present, texture, color, and chemical composition
• Approaches in the study of igneous petrology include Field Geology, Laboratory Identification, Geochemistry, and Experimental Petrology
• Rocks are named based on the minerals that make them up and their chemical composition
• Plutonic rocks have coarse or medium grain sizes (>1 mm) while Volcanic & Hypabyssal rocks are fine-grained to glassy (<1mm)
• Volcanic rocks crystallize at the surface while hypabyssal rocks crystallize at shallow depths
• Classification of igneous rocks based on mineral composition includes Ultramafic, Mafic, Intermediate, and Felsic
• Holocrystalline rocks are wholly crystalline, Hypocrystalline are partially crystalline/partially glassy, and Holohyaline are wholly glassy
• Euhedral crystals contain complete crystal faces, Subhedral have partially complete crystal forms, and Anhedral lack observable crystal faces
• Porphyritic rocks consist of two distinctly different size crystals.
• Explosive activity can be caused by internal gas expansion or magma-water interactions
• Aphanitic rocks contain crystals less than 1mm in diameter, subdivided into microcrystalline and cryptocrystalline
• Spatter is nearly molten bombs, usually basaltic, that weld upon impact to form agglutinate
• Pyroclastic deposits and rocks can be classified based on grain size, mixture components, and name
• Sources of fragments include Essential (Juvenile), Accessory (Cognate), and Accidental
• Intermediate rocks are grayish to salt and pepper-colored, rich in plagioclase, amphibole, biotite, and quartz with 52-66% weight percent silica
• Accretionary Lapilli are lapillus-size particles formed by concentric accretion of ash
• Pumice is highly vesicular glass that usually floats, while scoria is less vesicular, sinks in water, and is more mafic
• Felsic and mafic rocks are classified in terms of three components, either QAP or FAP because rocks never contain both quartz and feldspathoids
• Pyroclastic particles form by the disintegration of magma as gases are released by decompression and ejected from a volcanic vent
• Color is not reliable in identifying rocks because it may be misleading when applied to rocks of the same composition but different grain size
• Autoclastic fragments form by mechanical friction during lava movement or breakage of cool brittle outer margins
• Hydroclasts form through magma-water interactions, either explosive fragmentation or nonexplosive thermal contraction and granulation