Igneous Rocks

How igneous rocks form and are organized: Formation Process: Magma (molten rock underground) or lava (molten rock at surface) cools and minerals crystallize. The rate of cooling determines texture - slow cooling creates large crystals, fast cooling creates small crystals or glass. Intrusive (Plutonic): Formed from magma that cools slowly underground. Coarse-grained texture with visible crystals. Examples: granite, gabbro, diorite. Often exposed at surface through erosion. Extrusive (Volcanic): Formed from lava that cools quickly at surface. Fine-grained or glassy texture. Examples: basalt, rhyolite, obsidian, pumice. Form volcanic features. Composition Classification: - Felsic: High silica, light-colored (granite, rhyolite) - Intermediate: Medium silica (diorite, andesite) - Mafic: Low silica, dark-colored (gabbro, basalt) - Ultramafic: Very low silica, very dark (peridotite, komatiite) Texture Classification: - Phaneritic: Coarse-grained, visible crystals (granite) - Aphanitic: Fine-grained, crystals not visible (basalt) - Porphyritic: Large crystals (phenocrysts) in fine matrix - Glassy: No crystals, like glass (obsidian) - Vesicular: Contains gas bubbles (pumice, scoria) - Pyroclastic: Formed from volcanic fragments (tuff, breccia) Rockhounding Value: Igneous rocks, especially pegmatites and some volcanic rocks, are excellent sources of well-formed crystals and gem materials.

Light-colored, silica-rich rocks: Granite: Coarse-grained, intrusive. Composed of quartz, feldspar, and mica. Most common intrusive rock. Forms large bodies (batholiths) and smaller intrusions. Often contains interesting minerals and can be source of gem materials. Rhyolite: Fine-grained, extrusive equivalent of granite. Often porphyritic with quartz and feldspar phenocrysts. Can contain obsidian and pumice. Less common than granite. Composition: High silica (65-75%), rich in quartz and feldspars. Light colors (white, pink, gray). Low density. Formation: Form from felsic magmas, which are viscous (thick) and don't flow easily. This affects how they erupt and what features they create. Mineral Content: Primarily quartz, potassium feldspar, plagioclase feldspar, and micas. May contain accessory minerals like tourmaline, beryl, and topaz, especially in pegmatites. Occurrence: Granite forms large parts of continental crust. Rhyolite is less common, found in volcanic areas. Both are important building stones. Pegmatites: Very coarse-grained felsic rocks, often with extremely large crystals. Source of many gem-quality minerals (tourmaline, beryl, topaz, spodumene). Highly valued by collectors.

Dark-colored, silica-poor rocks: Basalt: Fine-grained, extrusive. Most common extrusive rock. Dark gray to black. Forms most of ocean floor and many volcanic features. Can contain interesting minerals in vesicles (gas bubbles). Gabbro: Coarse-grained, intrusive equivalent of basalt. Dark, dense. Less common than basalt but important. Forms large bodies in some areas. Composition: Low silica (45-55%), rich in iron and magnesium. Dark colors (black, dark gray, dark green). High density. Formation: Form from mafic magmas, which are fluid and flow easily. This creates different volcanic features than felsic magmas. Mineral Content: Primarily pyroxene, plagioclase feldspar, and olivine. May contain magnetite and other dark minerals. Occurrence: Basalt forms most of ocean floor and many continental volcanic areas. Gabbro is less common but forms large bodies in some locations. Volcanic Features: Basalt creates shield volcanoes, lava flows, and other features. Often contains vesicles (gas bubbles) that can be filled with minerals (agate, zeolites). Rockhounding Value: Basalt can contain interesting minerals in vesicles and fractures. Some areas have excellent collecting opportunities.

Rocks with intermediate and unusual compositions: Intermediate Rocks: - Diorite: Coarse-grained, intrusive. Intermediate composition between granite and gabbro. - Andesite: Fine-grained, extrusive equivalent of diorite. Common in volcanic arcs. Ultramafic Rocks: - Peridotite: Very low silica, very dark. Source rock for basalt magmas. Rare at surface but important in mantle. - Komatiite: Very high temperature lava, now rare (mostly ancient). Unusual texture. Special Textures: - Obsidian: Volcanic glass. Forms when lava cools too quickly for crystals to form. Can be very sharp. Often black but can be other colors. - Pumice: Very vesicular (full of gas bubbles), very light (can float on water). Forms from explosive eruptions. - Scoria: Vesicular like pumice but darker and denser. Forms from mafic magmas. - Tuff: Rock made from volcanic ash. Can contain interesting minerals and fossils. Pegmatites: Extremely coarse-grained, often with very large crystals. Can contain rare elements and gem-quality minerals. Highly valued by collectors. Xenoliths: Foreign rock fragments in igneous rocks. Can be interesting and provide information about deep Earth materials.

Physical features of igneous rocks: Intrusive Structures: - Batholiths: Very large intrusive bodies (hundreds of square kilometers) - Stocks: Smaller intrusions (less than 100 km²) - Dikes: Sheet-like intrusions cutting across existing rocks - Sills: Sheet-like intrusions parallel to existing rock layers - Laccoliths: Mushroom-shaped intrusions - Plugs: Cylindrical intrusions filling volcanic vents Extrusive Features: - Lava Flows: Sheets of solidified lava - Volcanic Cones: Built from erupted material - Calderas: Large collapse features - Lava Domes: Bulbous masses of viscous lava Textures and Structures: - Columnar Jointing: Hexagonal columns in cooling lava (like Devil's Tower) - Flow Banding: Layers in lava flows - Vesicles: Gas bubbles, can be filled with minerals (amygdules) - Phenocrysts: Large crystals in porphyritic rocks Rockhounding Implications: Different structures expose different rocks and minerals. Dikes and sills can contain interesting minerals. Vesicles in volcanic rocks can contain beautiful minerals.

The sequence of mineral crystallization: Concept: As magma cools, minerals crystallize in a specific order. This sequence, discovered by N.L. Bowen, explains mineral associations and rock compositions. Discontinuous Series: Minerals that react with remaining magma to form new minerals: 1. Olivine (high temperature) 2. Pyroxene (forms from olivine) 3. Amphibole (forms from pyroxene) 4. Biotite mica (forms from amphibole) Continuous Series: Plagioclase feldspar, which changes composition continuously: - High temperature: Calcium-rich (anorthite) - Low temperature: Sodium-rich (albite) - Composition changes gradually as magma cools Final Crystallization: After discontinuous and continuous series complete: - Potassium feldspar - Muscovite mica - Quartz (last to crystallize) Implications: Explains why certain minerals are found together. Early-formed minerals can react with remaining magma. Explains rock compositions and mineral associations. Fractional Crystallization: If early-formed crystals are removed (by settling or other processes), the remaining magma becomes more felsic. This explains how different rock types can form from same magma. For Rockhounds: Understanding Bowen's series helps predict what minerals might be found in different igneous rocks and explains mineral associations.