Chapter 6 & 9: Igneous Rocks

Glassy texture

A texture formed when a rock cools so fast it doesn't have time to crystallize. Ex: Glassy texture in obsidian.

Balsaltic/Mafic/Ferromagnesian composition

Igneous rocks composed of dark-colored silicates, also known as mafic. -Mafic lava has low viscosity. -Silica content of ~ 50% -High concentrations of Fe, Mg and Ca -High temperature of molten magma-1000o to 1200oC -Major minerals: 1.Olivine 2.Pyroxene (Augite) 3.Amphibole (Hornblende) 4.Biotite

The Four Categories of Magma

*-Felsic/silicic magma (66-76 % silica)* -Intermediate magma (52-66 % silica) *-Mafic magma (45-52 % silica)* -Ultramafic magma (38-45 % silica)

Aphanitic

*Rapid cooling* at earth's surface results in tiny mineral crystals that can only be seen under a microscope. Ex: Rhyolite *Vesicles may be present*

Decompression

- The kind of melting that occurs when hot mantle rock rises to shallower depths in the Earth so that pressure decreases while the temperature remains unchanged. - Mantle convection moves deep mantle rocks upwards. - Such movement of magma occurs in mantle plumes, beneath rifts, and beneath mid-ocean ridges.

Composition

- When volatiles (substances such as water and carbon dioxide) mix with hot mantle rock, they help break chemical bonds. This process is called *flux melting*. Flux melting occurs where volatiles enter hot mantle; *this happens at subduction zones.*

Volatiles

Gaseous components of magma dissolved in the melt. Volatiles will readily vaporize (form a gas) at surface pressures. Ex: 1.Water 2.Carbon dioxide 3.Nitrogen 4.Hydrogen 5.Sulfur dioxide

Source Rocks

The composition of a melt reflects the compositon of the solid from which it was derived.

Phaneritic

-Coarse-grained -Equigranular, interlocking crystals -A relatively slow rate of cooling Ex: Granite. Pegmatite is an exception. It doesn't cool slowly even though crystals are pretty big. This is because they form from water-rich melts in which atoms can move around so rapidly that large crystals can grow very quickly.

Types of magmatic intrusions

-Dike -Sill -Laccolith -Batholith

Basaltic Eruptions

-Low Silica + High T = Low Viscosity Produce many types of features: -Lava Flows (Pahoehoe or Aa), Lava tubes, Pressure ridges, Columnar jointing, Lava bombs, Flood basalts, Fissure eruptions, Spatter cones, Cinder cones, Shield volcanoes, Pillow lavas, Calderas

Igneous Rock Bodies

-Magma crystallizes and produces igneous rocks that are a part of an "igneous body." *-Extrusive Igneous Rock Bodies* *-Intrusive Igneous Rock Bodies*

Composition of igneous rocks

-Oxygen plus major elements -Generally a silica (SiO2) melt -Silica content is used in classification: *1.Granitic (felsic/silicic magma)* *2.Basaltic (mafic magma)*

mantle plume

-Partial melting of rising plumes of solid mantle material -Distinctive basaltic magma is produced -Rising magma may produce: -Intraplate island chains -Flood basalt -Basalt plateaus and rhyolitic calderas

Pyroclastic

-Produced by explosive volcanic eruptions -May appear porphyritic with visible crystals --Crystals show breakage or distortion -*Matrix* may be dominated by glassy fragments --Fragments also show distortion --Hot fragments may "weld" together

Magma differentiation processes

-Source Rocks *-Partial Melting* *-Fractional Crystallization* -Assimilation -Magma Mixing

Igneous Rocks: Convergent-plate boundaries

-Subduction and partial melting of wet basalt, sediments and the surrounding mantle -Andesitic and rhyolitic magma generated through fractional crystallization -Ascending magma may assimilate lower crustal material

Fissure eruption

-The most common. It is in every diverging boundery. A volcanic eruption emanating from a large, nearly vertical crack in Earth's surface rather than a central vent.

Plate Tectonic Setting of Igneous Rocks

1) Convergent-plate boundaries 2) Diverging-plate boundaries 3) Mantle plume

Origin of Magma: Causes of melting

1. A decrease in pressure (Decompression). 2. Addition of volatiles (Composition). 3. Heat transfer from rising magma (Temperature).

Concondant

A concordant igneous rock body runs parallel to the pre-existing bedrock. Ex: Laccoliths and sills

Volcanic cones

A cone of material created by successive eruptions of lava and pyroclastic material. Spatter cones: -A miniature volcanic cone on a crater floor or lava flow from which lava is ejected in drops or gobs. Cinder cone: -They consist of loose pyroclastic debris formed by explosive eruptions or lava fountains from a single, typically cylindrical, vent.

Discondant

A discordant igneous rock body cuts perpendicular to the pre-existing rock bed. Ex: Dikes

Pipe

A long tube through which magma moves from the magma chamber to Earth's surface

Shield volcano

A low, flat, gently sloping volcano built from many flows of fluid, low-viscosity basaltic lava.

Laccolith

A mass of igneous rock (lens-shaped) that has been intruded between rock strata causing uplift in the shape of a dome.

Xenolith

A piece of rock within an igneous rock that is not derived from the original magma but has been introduced from elsewhere, especially the surrounding country rock.

Ash-flow tuff

A pyroclastic rock formed when an ash flow solidifies.

Volcanic dome

A roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings

Sill

A tabular (tabletop-shaped) igneous body formed when magma is injected along sedimentary bedding surfaces

Dike

A tabular (wall-shaped) intrusion of rock that cuts across the layering of country rock.

Stratovolcano

A tall, conical volcano composed of one layer of hardened lava, tephra, and volcanic ash. These volcanoes are characterized by a steep profile and periodic, explosive eruptions. The lava that flows from them is highly viscous, and cools and hardens before spreading very far.

Columnar jointing

A type of fracturing that yields roughly hexagonal columns of basalt; columnar joints form when a dike, sill, or lava flow cools.

Lava bombs

A volcanic rock that forms from a glob of magma that is blown explosively into the air

Batholith

An immense mass of igneous rock created by the intrusion of numerous plutons in a region. This vast composite body may be several hundred kilometers long and over 100 km wide.

Pressure Ridge

Buckling of partially solidified lava as it continues to flow. Formation occurs when the outer edges and surfaces of the lava flow begin to harden. If the advancing lava underneath becomes restricted it may push up on the hardened crust, tilting it outward. Inflation also takes place and is a process where the plastic layer of lava underneath expands as it cools and small crystals form.

Caldera formation

Collapse of roof of a magma chamber. Ex: Mauna Loa & Yellowstone

Magma Viscosity

Controlled by silica, water content, and temperature: --The hotter, the less viscous. --The more volatile, not dry (volatile-free), it is, the less viscous. --Mafic melt is less viscous than felsic because more silicon-oxygen tetrahedra occur in felsic melt. -As magma cools, silica tetrahedron form links -Linkages increase viscosity -H2O and CO2 make up >90% of dissolved gases in magmas• The typical range of dissolved gases is 0.1 to 5% --Up to 15% is possible --High H2O content prevents silica linkages --High volatile content may cause explosive eruptions

Flood basalt

Flow of basaltic lava that issue from numerous cracks or fissures and commonly cover extensive areas to thicknesses of hundreds of meters. Ex: Columbia River Plateau in Oregon and Washington.

Intermediate & Silicic Eruptions

Higher Silica + Lower T = Higher Viscosity Produce: -Lava Domes, Composite or Stratovolcanos, Calderas, or Ash Flow

Igneous Textures

How Igneous Rocks are classified. No crystals texture: -Glassy Crystalline textures (Crystal growth requires time for ions to migrate. Thus, it forms minerals): -Aphanitic -Phaneritic -Porphyritic -Pyroclastic

Granitic/Sialic/Felsic/Non-ferromagnesian composition

Igneous rocks composed of light-colored silicates, also known as felsic. -Felsic lava is very viscous. -Silica content of 65-77% -High concentrations of Al, Na, and K -Lower temperature magmas-Less than 850oC -Major mineral: 1.Feldspars (Ca/Na/K-plagioclase): Higher T plagioclase is Ca-rich and darker, lower T plagioclase is Na-rich and lighter. 2.Muscovite 3.Biotite 4.Quartz: All mineral except Quartz contain aluminum.

Plutons

Irregular intrusions that range in size from tens of meters across to tens of kilometers across.

Pillow lava

Lava that cools underwater, taking on a distinctive pillow-like shape as it hardens.

Lava

Magma that reaches Earth's surface. - Forms extrusive igneous rock.

Magma

Molten rock beneath the earth's surface. - Forms intrusive igneous rock once it solidifies.

Vesicles

Open holes in igneous rock formed by the preservation of bubbles in magma as the magma cools into solid rock.

Igneous Rocks: Divergent-plate boundaries

Partial melting of mantle produces basaltic magma

Plutonic rocks

Plutonic rocks are rocks formed when *magma* cools and solidifies *below the earth's surface.*

Intrusive igneous rock

Rock formed from the cooling and solidification of magma beneath Earth's surface. -Less dense magmas rise through the crust -Rising magmas slowly cool -Intrusions form as magma solidifies beneath the surface -Intrusions are classified by their size, shape, and relative age -Size: large vs. small -Shape: concordant vs. discordant

Porphyritic

Rock texture characterized by large, well-formed crystals surrounded by fine-grained crystals of the same mineral. -Well-formed crystals *(phenocrysts)* -Fine-grained matrix *(groundmass)* Complex cooling history: Initial stage of slow cooling (Intrusive) -Large, well-formed crystals form Later stage of rapid cooling (Extrusive) -Remaining magma crystallizes more rapidly.

Extrusive igneous rock

Rock that forms from the cooling and solidification of lava at Earth's surface. -A form of extrusive bodies influenced by magma properties Composition: -Silica content Viscosity: -Volatile content -Temperature

Igneous rock

Rock that forms when hot molten rock (magma or lava) cools and freezes solid.

Sill vs Flood basalt (Solidified lava flow)

Sills: partial melting and incorporation of the surrounding country rock in both, upper and lower side experienced metamorphism. Lava flows: Metamorphism only visible on lower side of the flow. In addition, lava flows will typically show evidence of vesicles. Lava flows will show weathering on their upper surface, whereas sills, if still covered by country rock, typically do not.

Composite volcano

Stereotypical volcano. It can have ash eruption.

Partial melting

The melting in a rock of the minerals with the lowest melting temperatures, while other minerals remain solid. -The first-formed melt will be richer in silica than the original rock. As melting continues, magma becomes increasingly mafic. -Individual minerals within source rock melt at different temperatures. -Magma composition changes. -Resulting magma is less dense than source rock. Thus, magma is less dense than solid rock.

Fractional crystallization

The process by which a magma becomes progressively more silicic as it cools, because early-formed crystals settle out. -Bowen's Reaction Series: Individual minerals crystallize at different temperatures. -Magma composition changes. -Remaining magma may migrate.

Assimilation

The process of magma contamination in which blocks of wall rock fall into a magma chamber and dissolve.

Magma mixing

The process whereby magmas of different composition mix together to yield a modified version of the parent magmas.

Bowen's Reaction Series

The sequence in which different silicate minerals crystallize during the progressive cooling of a melt.

Lava flow

The spread of lava as it pours out of a vent. Pahoehoe: Less viscous/ looks smooth Aa: More viscous/ broke in chunks

Volcanic Rocks

Volcanic rocks are rocks formed when *lava* cools and solidifies *on the earth's surface.*

Temperature

When rising magma brings heat up with it, it melts overlaying or surrounding rock. This process is called *heat transfer melting.* This happens in rifts, along convergent-plate boundaries, and at hot spots. (The cooler magma is felsic and the hotter magma is mafic.)