igneous rocks (4) Flashcards
Magma
A molten rock found at depth, including any dissolved gases and crystals
Igneous Rock
Rock formed from the crystallization of magma
Lava
Magma that reaches Earth’s surface
Melt
The liquid portion of magma excluding the solid crystals
Volatiles
Gaseous components of magma dissolved in the melt. Volatiles will readily vaporize (form a gas) at surface pressures
Crystallization
The formation and growth of a crystalline solid from a liquid or gas
Intrusive Igneous Rock
Igneous rock that formed below Earth’s surface
Plutonic Rock
Igneous that form at depth. Named after Pluto, the god of the lower world in classical mythology
Extrusive Igneous Rock
Igneous rock formed when magma solidifies at Earth’s surface
List and describe the three components of magma.
Liquid- melt
Solid- crystals of silicate minerals
Gas- volatiles (H2O, CO2, SO2)
Igneous rocks are composed mainly of which group of minerals?
Silicate minerals
How do light-colored igneous rocks differ in composition from dark-colored igneous rocks?
Light-colored are felsic or granitic (lots of silica) (low temperature) (viscous)
Dark-colored are mafia or basaltic (dense)
List the four basic compositional groups of igneous rocks, in order from highest silica content to lowest silica content.
Felsic
Intermediate
Mafic
Ultramafic
Texture
The size, shape, and distribution of the particles that collectively constitute a rock
What are the three factors that influence the textures of igneous rocks?
The rate at which molten rock cools
The amount of silica present in the magma
The amount of dissolved gases in the magma
Aphanitic/Fine-Grained
A texture of igneous rocks in which the crystals are too small for individual minerals to be distinguished without the aid of a microscope
Porphyritic Texture
An igneous rock texture characterized by two distinctively different crystal sizes. The larger crystals are called phenocrysts, whereas the matrix of smaller crystals is termed the groundmass.
Phenocryst
A conspicuously crystal embedded in a matrix of finer-grained crystals
Groundmass
The matrix of smaller crystals within an igneous rock that has porphyritic texture
Porphyry
An igneous rock that has a porphyritic texture
Vesicular Texture
A term applied to aphanitic igneous rocks that contain many small cavities called vesicles
Glassy Texture
A term used to describe the texture of certain igneous rocks, such as obsidian, that contain no crystals
Fragmental/Pyroclastic Texture
An igneous rock texture resulting from the consolidation of individual rock fragments that are ejected during a violent volcanic eruption
Pegmatite
A very coarse-grained igneous rock (typically granite) commonly found as a dike associated with a large mass of plutonic rock that has smaller crystals. Crystallization in a water-rich environment is believed to be responsible for the very large crystals
Pegmatitic Texture
A texture of igneous rocks in which the interlocking crystals are all larger than one centimeter in diameter
How does the rate of cooling influence crystal size?
The slower the magma cools the bigger the crystal
What does a porphyritic texture indicate about the cooling history of an igneous rock?
2 stage cooling
Granite
An abundant, course-grained igneous rock composed of about 10-20% quartz and 50% potassium feldspar. Granite is used as a building material.
Rhyolite
The fine-grained equivalent of the igneous rock granite, composed primarily of the light-colored silicates
Obsidian
A volcanic glass of felsic composition
Pumice
A light-colored, glassy vesicular rock commonly having a granitic composition
Andesite
A gray, fine-grained igneous rock, primarily of volcanic origin and commonly exhibiting a porphyritic texture
Diorite
A coarse-grained, intrusive igneous rock primarily composed of plagioclase feldspar and amphibole minerals
Basalt
A fine-grained igneous rock of magic composition
Gabbro
A dark-green to black intrusive igneous rock composed of dark silicate minerals. Gabbro makes up a significant percentage of oceanic crust.
Pyroclastic Rocks
Rocks composed of consolidated rock fragments ejected during a volcanic eruption
Tuff
A rock composed mainly of tiny, ash-size fragments cemented together
Welded Tuff
A pyroclastic deposit composed of particles fused together by the combination of heat still contained in the deposit after it has come to rest and the weight of overlying material
List the two criteria by which igneous rocks are classified.
Texture and composition
How are granite and rhyolite different? In what way are they similar?
Granite is phaneritic
Rhyolite is aphanitic
Describe each of the following in terms of composition and texture: diorite, rhyolite, and basalt porphyry.
Diorite: intermediate, phaneritic
Rhyolite: felsic, aphanitic
Basalt porphyry: mafic, porphorytic
Geothermal Gradient
The gradual increase in temperature with depth in the crust. The average is 30 degrees Celsius per km in the upper crust
Decompression Melting
Melting that occurs as rock ascends due to a drop in confining pressure
What role does water play in the formation of magma?
Water lowers the melting temperature of the rock
What is one way that basaltic magma can generate felsic magma?
Subduction zone, when mantle melts next to continental crust, there is new magma
Crystal Settling
A process that occurs during the crystallization of magma, in which the earlier-formed minerals are denser than the liquid portion and settle to the bottom of the magma chamber
Magmatic Differentiation
The process of generating more than one rock type from a single magma
Assimilation
In igneous activity, the process of incorporating country rock into a magma body
Magma Mixing
The process of altering the composition of a magma through the mixing of material from another magma body
How does the crystallization and settling of the earlier formed minerals affect the composition of the remaining magma?
Earliest formed crystals are mafic, they settle and what is left behind is more felsic (more silica rich)
Partial Melting
The process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results
What is the process thought to generate most basaltic magmas?
Partial melting of the earth’s mantle
What is the process thought to generate most granitic magmas?
Transfer of heat from rising magma that is too dense to overtake the continental crust
Host Rock/Country Rock
Preexisting crustal rocks intruded by magma. Host rock may be displaced or assimilated by magmas
Intrusions/Plutons
A structure that results from the emplacement and crystallization of magma beneath the surface of Earth
Tabular
Describing a feature such as an igneous pluton that has tow dimensions that are much longer than the third
Massive
An igneous pluton that is not tabular in shape
Discordant
A term used to describe plutons that cut across existing rock structures, such as bedding planes
Concordant
A term used to describe intrusive igneous masses that form parallel to the bedding of the surrounding rock
Dikes
A tabular-shaped intrusive igneous feature that cuts through the surrounding rock
Sill
A tabular igneous body that was intruded parallel to the layering of preexisting rock
Columnar Jointing
A pattern of cracks the forms during cooling of molten rock to generate columns
Batholith
A large mass of igneous rock that formed when magma was emplaced at depth, crystallized and subsequently exposed by erosion
Stock
A pluton similar to but smaller than a batholith
Laccolith
A massive igneous body intruded between preexisting strata
Distinguish among batholiths, stocks, and laccoliths in terms of size and shape.
Batholiths-big
Stocks-smaller than batholiths
Laccoliths-mushroom shape
Describe dikes and sills using: massive, discordant, tabular, and concordant.
Dikes and sills are tabular
Dikes are discordant
Sills are concordant
