Igneous Rocks
Igneous Rocks
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Igneous Rocks
Igneous Rocks
The Rock Cycle
Characteristics of magma
Igneous rocks form from molten rock (magma)
Characteristics of magma
Protolith of igneous rocks
Forms from partial melting of rocks
Magma at the surface is called lava
Characteristics of magma
Rocks formed from lava are classified as extrusive, or volcanic rocks
Rocks formed from magma are termed intrusive, or plutonic rocks
The nature of magma
Consists of three components:
A liquid portion, called melt, made of mobile ions
Solids, if any, are crystallized silicate minerals
Volatiles, (dissolved gases), mostly water (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)
Crystallization of magma
Texture = the size and arrangement of mineral grains
Igneous rocks are classified by
Texture
Mineral composition
Igneous textures
Texture describes the size, shape, and arrangement of interlocking minerals
Factors affecting crystal size
Rate of cooling
Slow rate promotes fewer and larger crystals
Fast rate forms many small crystals
Very fast rate forms glass
Types of igneous textures
Aphanitic (fine-grained) texture
Rapid rate of cooling
Microscopic crystals
May contain vesicles
Phaneritic (coarse-grained) texture
Slow cooling
Crystals can be seen
Aphanitic texture
Phaneritic texture
Types of igneous textures
Porphyritic texture
Minerals form at different temperatures as well as differing rates
Large crystals (phenocrysts) embedded in a matrix (groundmass)
Glassy texture
Very rapid cooling
Results in obsidian
Porphyritic texture
Andesite porphyry
Porphyritic Texture
Glassy (vitreous) texture
An obsidian flow in Oregon
Types of igneous textures
Pyroclastic texture
Fragments ejected from violent volcanic eruption
Textures similar to sedimentary rocks
Pegmatitic texture
Exceptionally coarse grained
Late crystallization stages of granitic magmas
Igneous Compositions
Igneous rocks are primarily silicate minerals
Dark (ferromagnesian or mafic) silicates
Olivine
Pyroxene
Amphibole
Biotite mica
Igneous rocks are primarily silicate minerals
Light (felsic) silicates
Feldspars
Muscovite mica
Quartz
Granitic versus basaltic compositions
Granitic composition
Composed of light-colored silicates
Felsic composition
High silica (SiO2)
Major constituents of continental crust
Granitic versus basaltic compositions
Basaltic composition
Composed of dark silicates and calcium-rich feldspar
Mafic composition
More dense than granitic rocks
Comprise the ocean floor as well as many volcanic islands – the most common volcanic rock
Gabbro
Basalt
Other compositional groups
Intermediate (or andesitic) composition
At least 25% dark silicate minerals
Associated with explosive volcanic activity
Ultramafic composition
Rare composition, very high in magnesium and iron
Composed entirely of ferromagnesian silicates
Mineralogy of common igneous rocks
Silica content influences a magma’s behavior
Granitic magma (high silica)
Extremely viscous
Higher temperatures Molten as low as 700oC
Basaltic magma (low silica)
Fluid-like behavior
Pyroclastic rocks
Eruptive fragments
Varieties
Tuff – ash-sized fragments
Welded tuff – ejected hot, “welds” on landing
Volcanic breccia – particles larger than ash (similar to sedimentary breccia)
Ash and pumice layers
Classification of igneous rocks
Origin of Magma
Several Factors Involved
Generating magma from solid rock
Partial melting crust and upper mantle
Role of heat
Temperature increases with depth in the upper crust (called the geothermal gradient, ~ 20oC to 30oC per km)
Estimated temperatures in the crust and mantle
Role of heat
Rocks in lower crust and upper mantle near melting points
Additional heat (from descending rocks or rising heat from the mantle) may induce melting
Role of pressure
Increase in confining pressure increases rock melting temperature and reducing the pressure lowers the melting temperature
When confining pressures drop, de-compression melting occurs
Heat and Pressure Affect Melting
Decompression melting
Role of volatiles
Volatiles (primarily water) lower rock melting temperatures
Particularly important with descending oceanic lithosphere
Evolution of magmas
A single volcano may extrude lavas exhibiting very different compositions
Bowen’s reaction series and the composition of igneous rocks
N.L. Bowen demonstrated that as a magma cools, minerals crystallize in order based on their melting points
Bowen’s reaction series
During crystallization, the composition of the liquid portion of the magma continually changes
Composition changes due to removal of elements by earlier-forming minerals
The silica component of the melt becomes enriched as crystallization proceeds
Minerals in the melt can chemically react and change
Processes responsible for changing a magma’s composition
Magmatic differentiation
Separation of crystals from melt forms a different magma composition
Assimilation
Incorporation of foreign matter (surrounding rock bodies)
Magma mixing
Involves two magmas intruding one another
Two distinct magmas may produce a composition quite different the originals
Assimilation and magmatic differentiation
Partial melting and magma formation
Incomplete melting of rocks is known as partial melting (silica rich minerals melt first)
Formation of basaltic magmas
Most originate from partial melting of ultramafic rock in the mantle
Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones
Decompression melting
Formation of basaltic magmas
As basaltic magmas migrate upward, confining pressure decreases which reduces the melting temperature
Large outpourings of basaltic magma are common at Earth’s surface
Formation of andesitic magmas
Interactions between mantle-derived basaltic magmas and more silica-rich rocks in the crust generate magma of andesitic composition (assimilation)
Andesitic magma may also evolve by magmatic differentiation (loss of early olivine and pyroxene)
Partial melting and magma formation
Formation of granitic magmas
Most likely the end product of an andesitic magma
Granitic magmas are higher in silica and therefore more viscous
Because of viscosity, mobility lost before reaching surface (rhyolite rarer than basalt)
Tend to produce large plutonic structures
Magmatic Differentiation
Mineral resources and igneous processes
Many important sources of metals are produced by igneous processes
Igneous mineral resources can form from
Magmatic segregation – separation of heavy minerals in a magma chamber
Hydrothermal solutions - Originate from hot, metal-rich fluids that are remnants of the late-stage magmatic process
Origin of hydrothermal deposits
The Nature of Volcanic Eruptions
Factors determining the “violence” or explosiveness of a volcanic eruption
Composition of the magma
Temperature of the magma
Dissolved gases in the magma
The above three factors actually control the viscosity of a given magma which in turn controls the nature of an eruption
Viscosity is a measure of a material’s resistance to flow (e.g., Higher viscosity materials flow with great difficulty)
Factors affecting viscosity
Temperature - Hotter magmas are less viscous
Composition - Silica (SiO2) content
Higher silica content = higher viscosity
(e.g., felsic lava such as rhyolite)
Factors affecting viscosity continued
Lower silica content = lower viscosity or more fluid-like behavior (e.g., mafic lava such as basalt)
Dissolved Gases
Gas content affects magma mobility
Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure
The violence of an eruption is related to how easily gases escape from magma
Factors affecting viscosity continued
In Summary
Fluid basaltic lavas generally produce quiet eruptions
Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions
Materials extruded from a volcano
Lava Flows
Basaltic lavas are much more fluid
Types of basaltic flows
Pahoehoe lava (resembles a twisted or ropey texture)
Aa lava (rough, jagged blocky texture)
Dissolved Gases
One to six percent of a magma by weight
Mainly water vapor and carbon dioxide
A Pahoehoe lava flow
A typical aa flow
Pyroclastic materials – “Fire fragments”
Types of pyroclastic debris
Ash and dust - fine, glassy fragments (<2 mm)
Pumice - porous rock from “frothy” lava
Lapilli - walnut-sized material (2-64 mm)
Cinders - pea-sized material
Particles larger than lapilli (>64 mm)
Blocks - hardened or cooled lava
Bombs - ejected as hot lava
A volcanic bomb
Global Volcanic Effects
Volcanoes
General Features
Opening at the summit of a volcano
Crater - steep-walled depression at the summit, generally less than 1 km in diameter
Caldera - a summit depression typically greater than 1 km in diameter, produced by collapse following a massive eruption
Vent – opening connected to the magma chamber via a pipe
Types of Volcanoes
Shield volcano
Broad, slightly domed-shaped
Composed primarily of basaltic lava
Generally cover large areas
Produced by mild eruptions of large volumes of lava
Mauna Loa on Hawaii is a good example
Shield Volcano
Types of Volcanoes continued
Cinder cone
Built from ejected lava (mainly cinder-sized) fragments
Steep slope angle
Rather small size
Frequently occur in groups
Sunset Crater – a cinder cone near Flagstaff, Arizona
Paricutin
Types of volcanoes continued
Composite cone (Stratovolcano)
Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)
Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base)
Composed of interbedded lava flows and layers of pyroclastic debris
A composite volcano
Mt. St. Helens – a typical composite volcano
Mt. St. Helens following the 1980 eruption
Composite cones continued
Most violent type of activity (e.g., Mt. Vesuvius)
Often produce a nueé ardente
Fiery pyroclastic flow made of hot gases infused with ash and other debris
Move down the slopes of a volcano at speeds up to 200 km per hour
May produce a lahar, which is a volcanic mudflow
A size comparison of the three types of volcanoes
A nueé ardente on Mt. St. Helens
St. Pierre after Pelee’s Eruption
Other volcanic landforms
Pyroclastic flows
Associated with felsic & intermediate magma
Consists of ash, pumice, and other fragmental debris
Material is ejected a high velocity
e.g., Yellowstone Plateau
Calderas
Steep walled, roughly circular, depressions at the summit of a volcano
Size generally exceeds 1 km in diameter
Formed by the collapse of the structure
Caldera Formation
Type 1 Empty magma chamber under volcano leads to collapse (like Crater Lake)
Type 2 Magma chamber empties laterally through lava tubes (like Mauna Loa)
Type3 Magma chamber produces ring fractures and empties with explosive eruption producing silica-rich deposits and very large caldera (like Yellowstone, and Long Valley)
Crater Lake, Oregon is a good example of a caldera
Long Valley Caldera
Eruptions from 3.8 to 0.8 Ma (from basalt to rhyolite) ended with Glass Mountain eruption (0.76 Ma) that formed ring-structure caldera and produced the Bishop Tuff
More recent Mono-Inyo Crater system (0.4 to present, also from basalt to rhyolite) includes Mammoth Mountain (made of a series of lava domes and flows)
Currently active, probably reflecting smaller magma bodies
Long Valley Caldera
Bishop Tuff
Mono-Inyo Craters Activity
Explanation of Mono-Inyo Craters Trend
Fissure eruptions and lava plateaus
Fluid basaltic lava extruded from crustal fractures called fissures
May travel as much as 90 km from source
e.g., Columbia River Plateau individual flows accumulate to more than a km thick
Fissure Eruptions
The Columbia River basalts
Lava Domes
Bulbous mass of congealed lava formed post eruption
Most are associated with explosive eruptions of gas-rich magma
Volcanic pipes and necks
Pipes are conduits that connect a magma chamber to the surface
A lava dome on Mt. St. Helens
Kimberlite Pipe
Volcanic pipes and necks continued
Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone
Formation of a volcanic neck
Shiprock, New Mexico – a volcanic neck
Volcanic Neck
Plutonic igneous activity
Most magma is emplaced at depth in the Earth
An underground igneous body, once cooled and solidified, is called a pluton
Classification of plutons
Shape
Tabular (sheetlike)
Massive (massive refers to shape, not size)
Classification of plutons continued
Orientation with respect to the host (surrounding) rock
Discordant – cuts across sedimentary rock units
Concordant – parallel to sedimentary rock units
Types of intrusive igneous features
Dike – a tabular, discordant pluton
Sill – a tabular, concordant pluton (e.g., Palisades Sill in New York), uplifts overlying rock, so must be a shallow feature
Laccolith
Similar to a sill
Lens or mushroom-shaped mass
Arches overlying strata upward
A sill in the Salt River Canyon, Arizona
Intrusive igneous features continued
Batholith
Largest intrusive body
Discordant and massive
Surface exposure of 100+ square kilometers (smaller bodies are termed stocks)
Frequently form the cores of mountains like our Sierra Nevada Batholith
Intrusive Igneous Structures
Batholith Distribution
Plate tectonics and igneous activity
Global distribution of igneous activity is not random
Most volcanoes are located within or near ocean basins (e.g. circum-Pacific “Ring of Fire”)
Basaltic rocks are common in both oceanic and continental settings, whereas granitic rocks are rarely found in the oceans
Distribution of some of the world’s major volcanoes
Igneous activity along plate margins
Spreading centers
The greatest volume of volcanic rock is produced along the oceanic ridge system
Mechanism of spreading
Lithosphere pulls apart
Less pressure on underlying rocks
Results in partial melting of mantle
Large quantities of basaltic magma are produced
Igneous activity along plate margins
Subduction zones
Occur in conjunction with deep oceanic trenches
Descending plate partially melts (fluxed with water)
Magma slowly moves upward
Rising magma can form either
An island arc if in the ocean
A volcanic arc if on a continental margin
Subduction zones
Associated with the Pacific Ocean Basin
Region around the margin is known as the “Ring of Fire”
Most of the world’s explosive volcanoes are found here
Intraplate volcanism
Activity within a tectonic plate
Intraplate volcanism continued
Associated with plumes of heat in the mantle (perhaps from the core-mantle boundary, de-compression melting after rise)
Form localized volcanic regions in the overriding plate called a hot spot
Produces basaltic magma sources in oceanic crust (e.g., Hawaii)
Volcanic Activity
Volcanic Activity
A hot spot affecting a tectonic plate
Key Terms Chapter 6
Volcano (shield, stratovolcano, composite cone, cinder cone)
Lava
Magma
Pyroclastic
Pyroclastic flows
Viscosity
Fractional melt, fractionation, fractional crystallalization
Crystallization
Volcanic and plutonic rocks
Pluton (batholith, stock, laccolith, sill, dike)
Source : http://www2.bakersfieldcollege.edu/moldershaw/Ch%206%20Web%20Notes.doc
Web site link: http://www2.bakersfieldcollege.edu/moldershaw/
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