Igneous Rocks

Igneous Rocks:

• Magma:

  • The liquid or molten rock is found beneath the Earth’s surface.

• Lava:

  • Molten rock (magma) that has erupted from a volcano or crack in the Earth’s surface in the form of liquid rock.

• Crystallization:

  • The process of taking material from its liquid or molten form and gradually freezes until the atoms and molecules are highly organized into a structure known as a crystal.

• Mineralogy:

  • The scientific study of minerals includes their chemical composition, crystal structure, physical properties, classification, and how they form and are distributed within the Earth.

• Silicate Minerals:

  • A large group of minerals primarily composed of silicon and oxygen atoms.

    • The largest class of rock-forming minerals.

    • Quartz, feldspar, mica and pyroxene.

• Nonsilicate Minerals:

  •  A group of minerals that do not contain silicon and oxygen in their chemical structure.

    • They lack “silicate tetrahedra”, which are the building blocks of most common minerals in the Earth’s crust.

    • Carbonates, oxides, sulfates and halides.

• Ferromagnesian Silicates:

  • Silicate minerals contain significant amounts of iron and magnesium in their chemical composition.

    • Gives them a dark color and higher density.

    • Olivine, puroxene, amphibole and biotite mica.

• Nonferromagnesian Silicates:

  • Silicate minerals that contain very little to no iron or magnesium; therefore are composed of elements like sodium, potassium, and aluminum.

    • Gives them a lighter color.

    • Quartz, muscovite mica and feldspar.

• Texture:

  • Refers to the size, shape, and arrangement of the mineral grains or crystals within a rock.

    • The physical appearance of a rock.

• Bowen’s Reaction Series:

  • A model/concept that describes the sequence in which minerals crystallize from cooling magma.

• Fractional Crystallization:

  • The process where different minerals within a cooling magma solidify and separate from the liquid melts at varying temperatures, resulting in a change in the overall composition of the magma and ultimately leading to the formation of different types of igneous rocks with distinct mineral composition.

• Assimilation:

  • The process where a body of magma incorporates surrounding rock materials moves through the Earth’s crust, leading to changes in the magma’s composition and characteristics; which absorbs elements from the surrounding rock.

• Xenolith:

  • A fragment of rock trapped/embedded in another type of rock.

    • An older rock is trapped within a larger igneous rock.

    • Usually, a rock is embedded in magma while the magma is cooling.

• Phaneritic Texture:

  • An igneous rock texture where the crystals are big enough to be seen with the naked eye.

    • Course-grained texture.

    • Ex: granite.

• Porphyritic Texture:

  • An igneous rock texture where large crystals (phenocrysts) are embedded within a finer-grained matrix or groundmass; indicating a distinct difference in crystal size due to varying cooling rates during magma formation.

    • Formed larger crystals first during slow cooling, and then magma rapidly cooled, creating the smaller matrix around them.

    • Fine-grained texture.

    • Ex: basalt.

• Phenocrysts:

  • A early forming, relatively large crystal that is visibly distinct from the smaller grains of the groundmass of an igneous rock.

• Groundmass:

  • The fine-grained, crystalline matrix within an igneous rock, where larger crystals (called phenocrysts) are embedded.

    • The base material in which the larger crystals are set, often appearing as a smaller, more densely packed texture compared to the phenocrysts.

• Rhyolite:

  • A fine-grained, extrusive igneous rock formed from magma rich in silica, typically appearing light in color and often glassy in texture, essentially the volcanic equivalent of granite, composed mainly of quartz and feldspar minerals.

    • Cools quickly on the Earth’s surface after erupting from a volcano.

• Granite:

  • A coarse-grained, intrusive igneous rock primarily composed of quartz, feldspar, and mica, formed when magma cools slowly deep within the Earth's crust, resulting in a visible crystalline texture.

• Diorite:

  • A coarse-grained, intrusive igneous rock that forms when magma cools slowly underground.

    • A dark colored rock with large visible crystal like a dark granite.

• Andesite:

  • A fine-grained igneous rock formed when magma erupts and crystallizes outside of the volcano.

    • Has phenocrysts and can also be porphyritic.

• Gabbro:

  • A course-grained, dark-colored, intrusive igneous rock that forms from the slow cooling of magma inside a volcano.

    • Found in oceanic crust.

• Basalt:

  • A type of fine-grained extrusive igneous rock taht gets its dark color from the composition of its minerals.

    • Are mafic from their mineral composition.

    • Rich in iron and magnesium.

• Glassy Texture:

  • A rock texture where there are no visible mineral crystals.

    • Looks like a block of colored glass.

• Obsidian:

  • A fine-grained extusive rock that is naturally occurring volcanic glass formed when lava rapidly cools with minimal rcystal growth which gives it a smooth glassy texture.

• Know the three major rock groups and their definitions.

  • Igneous:

    • Forms from the cooling and crystallization of molten rock.

  • Sedimentary:

    • Forms from lithification of sediment.

  • Metamorphic:

    • Form when previously existing rocks undergo tremendous heat and/or pressure (metamorphosed).

• The processes that form each of the three main rock groups.

  • Igneous:

    • Rocks formed by the cooling and crystallization of minerals from molten rock.

  • Sedimentary:

    • Rocks formed from the lithification of sediment.

  • Metamorphic:

    • Rocks formed by metamorphism (transforming of existing rocks under heat and pressure).

• How N.L. Bowen devised his Reaction Series to explain the mineralogy of igneous rocks.

  • By conducting laboratory experiments where he heated powdered igneous rock samples to melting point, then carefully cooled them at different temperatures, observing which minerals crystallized at each stage. He was able to identify the sequence of mineral formation based on temperature, revealing the relationships between different minerals and their crystallization order within a cooling magma.

• Known Bowen’s Reaction Series (the composition of igneous rocks).

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• Texture of plutonic and volcanic igneous rocks as it relates to their cooling history.

  • Plutonic igneous rocks, which cool slowly deep within the Earth's crust, typically have a coarse-grained texture due to the ample time for large crystals to form, while volcanic igneous rocks, cooling rapidly at the surface, have a fine-grained texture because crystals don't have enough time to grow large; essentially, the cooling rate directly impacts the texture of an igneous rock, with slower cooling leading to larger crystals and a coarser texture. 

• What are the controls on the violence of a volcanic eruption?

  • The magma's composition (particularly its silica content), its viscosity, and the amount of dissolved gases it contains.

    • Essentially, how easily trapped gases can escape from the magma, with higher gas content and higher viscosity leading to more explosive eruptions.