Post on 28-Jan-2016
Chapter 4Rocks & Igneous Rocks
Rock DefinitionA naturally occurring, solid
Composed of one or more minerals
Exceptions to the Definition
Some rocks are not formed of minerals
Noncrystalline, glassy volcanic rocks such as obsidian and pumice
Coal which is formed from compacted plant remains
Appearance of a Rock
The appearance of a rock is determined by two major factors
Mineralogy
Texture
Appearance of a RockMineralogy refers to the relative proportions of the constituent minerals
Texture is determined by the size, shape and arrangement of the mineral crystals
Appearance of a Rock
Mineralogy and texture are related to how and where a rock was formed and what has happened to the rock since it was created
Three Families of Rocks
Igneous- from magma/lava
Sedimentary-compaction/cementing of sediments
Metamorphic- changed by heat and pressure
Three Types of Rocks
Igneous Rocks
Rocks formed by the solidification of magma (molten rock)
Granite
Two Types of Igneous Rocks
Extrusive igneous rocks form when magma erupts at the surface (i.e., above ground) and rapidly cools
Intrusive igneous rocks form when magma intrudes into bedrock and slowly cools (i.e., below ground)
Igneous RocksIgneous Rocks
Most Igneous Rock are Silicates
QuartzFeldspar
MicaPyroxene Amphibole
OlivineBasalt
Granite
Sedimentary Rocks
Rocks formed by consolidation of sediments
Classified by the type of sediments
Three Major Types of Sediments
Clastic Sediments are physically deposited particles derived from weathered rocks
Chemical Sediments include minerals carried in solution such as calcite and halite
Organic Sediments are composed of living materials (plant/animal material and fossils/shells)
From Sediments to Solid RocksLithification is the process of converting sediments into solid rocks
(will cover in detail in another lecture)
The Two Types of Lithification
Compaction – sediments are squeezed together by the weight of overlying sediments into a solid mass
Cementation – minerals precipitate around the sediments and bind them into a solid rock
Sedimentary Rocks
Sedimentary Rocks
5 % by volume of the upper crust
75 % by exposed surface area of continents
Sedimentary RocksSedimentary rocks commonly contain fossils
In fact, some sedimentary rocks are almost entirely composed of fossils
Can exhibit extensive horizontal layers called bedding
Composition
Sedimentary rocks can be composed of sediments created from any of three great families of rock (igneous, sedimentary, metamorphic)
Chemical cementation commonly includes calcite, gypsum and halite
Metamorphic Rocks
Metamorphic rocks are formed by the transformation of previously-existing rocks in the solid state due to increased temperature and pressure
Metamorphic RocksMetamorphism can change the mineralogy, texture and/or the chemical composition of a “parent rock” while maintaining its solid form
Note that the rock does not melt (temperature range 250 to 700 C)
Regional and ContactMetamorphism
Regional metamorphism occurs where high temperature and pressures occur over large region (plate tectonics)
Contact metamorphism is limited to smaller areas such as around a magma intrusion into bedrock
Regional Metamorphism
Contact Metamorphism
Other Forms of MetamorphismUltra-high pressure metamorphism occurs deep in the continental and oceanic crust
High-pressure, low-temperature metamorphism occurs where oceanic crust subducts under a continental plate
Shock metamorphism at impact sites
Metamorphic MineralizationSilicates are the most common mineral in metamorphic rocks
Minerals can tell you the “grade”of metamorphism
Certain minerals are uniquely characteristic of metamorphic rocks (kyanite, staurolite, some garnets)
Contact Ultra-high pressure Regional High-press, low-temp
Metamorphic Mineralization
Proportions of the Rock Types
A Few More Facts…Igneous processes create new rock
Metamorphic and sedimentary processes re-work old rock into a new form
Silicate mineral are the most common minerals found in all types of rocks
Common Minerals Found in Rocks
Five Very Different Locations
Rock Cycle
Rock Cycle
Subduction of an oceanic plate beneath a continental plate uplifts a volcanic mountain range
Magma rises from melting plate and intrudes or extrudes in the crust
Rock Cycle
Magmas cool to make igneous rock
Basalt versus granite
Rock Cycle
Weathering and erosion creates sediments
Rock Cycle
Sediments are carried to the oceans and lakes
Lithification
Rock Cycle
Tectonic plates interact
Metamorphism of subducted rock
Rock Cycle
Entire process starts over as plates interact
Rock Cycle
Igneous Rocks
Igneous Rocks
How do igneous rocks differ from one another?
Where do they form?
How do rocks solidify from a melt (magma)?
Where do rock melt?
We Classify Igneous Rocksby Appearance
Mineralogy (Chemistry)
Texture
Igneous Rocks
We Classify Igneous Rocks by Genetics (Origin)
There are two classes of igneous rocks
Intrusive
Extrusive
Genetic Classification of Igneous Rocks
Intrusive igneous rocks crystallized from slowly cooling magma intruded within the Earth’s crust, such as granite and gabbro
Granite Intrusions
Genetic Classification of Igneous Rocks
Extrusive igneous rocks crystallized from rapidly cooling magma extruded on the surface of the Earth as lava or erupted as pyroclastic material, such as basalt
Extrusive Igneous RocksRocks formed from the cooling of lavas extruded onto the Earth’s surface or onto ocean floors
Rocks formed by the cooling of pyroclastic material, such as fragmented pieces of magma and material erupted into the air
We Can Also Classify Igneous Rocks by Composition
Chemistry
Mineralogy
Chemistry
Modern classification of igneous rock is based upon the silica (SiO2) content
The silica content is determined by the silicate minerals that occur in the rock (i.e., the minerals contain SiO2)
The percentage of silica ranges from about 40% to about 70%
Rocks are referred to as “silica rich” or “silica poor”
Mineralogy
There are four major divisions of igneous rocks based upon the content of elements in the minerals:
Felsic
Intermediate
Mafic
Ultramafic
Rich (high) in minerals containing silica
Poor (low) in iron and magnesium
They include: Granite Rhyolite
Felsic Igneous Rocks
Intermediate Igneous Rocks
Intermediate in composition between felsic and mafic igneous rocks
Less silica, more Fe & Mg than felsic
More silica, less Fe & Mg than mafic
They include: Granodiorite DaciteDioriteAndesite
Mafic Igneous Rocks
Poor (low) in minerals containing silica
Rich (high) in iron and magnesium
They include: Gabbro Basalt
Ultramafic Igneous Rocks
Very uncommon on the Earth’s surface
Very poor (lower) in minerals containing silica
Consist primarily of mafic minerals (olivine, pyroxene)
The most common ultramafic rock is: Peridotite
Felsic Intermediate Mafic
Granite Granodiorite Diorite Gabbro
BasaltAndesiteDaciteRhyolite
Common Minerals of Igneous Rocks
When Do Rocks Melt?Melting starts at ~700o C
When the temperature exceeds the melting point of the rock or some minerals within the rock
Minerals melt at different temperatures
Three Factors Affecting Melting of Rocks
Pressure: Increased pressures raises melting points
Water Content: Increased water content lowers melting points
Composition: Felsic minerals melt at lower temperatures than mafic minerals
When Do Rocks Melt?
Magma Differentiation
The process by which rocks of various compositions can arise from a
uniform parent magma
The Formation of
Magma Chambers
Partial meltingPartial melting
Partial meltingPartial melting
Less dense magmaLess dense magma
The Formation of
Magma Chambers
Partial meltingPartial melting
Less dense magmaLess dense magma
Magma risesMagma risesThe
Formation of Magma
Chambers
Partial meltingPartial melting
Less dense magmaLess dense magma
Magma risesMagma rises
Magma pools in Magma pools in magma chambermagma chamber
The Formation of
Magma Chambers
Magma DifferentiationOccurs because different minerals crystallize (solidify) at different temperatures
In other words, as the magma cools some minerals form first, some form last
The process by which crystals forming in a cooling magma are segregated from the remaining liquid
In a simple scenario, the crystal settle to the floor or adhere to the walls of the magma chamber
Fractional Crystallization
Fractional Crystallization
The elements (such as Fe and Mg) used to create the newly formed crystals are now no longer available for creating new minerals
Therefore the chemical composition of the magma slowly changes as new minerals are continually formed and the available matter is selectively used up
Bowen’s Reaction Series
Experiments that determined the sequence of crystallization of minerals from a gradually cooling mafic (basaltic) magma
Conducted by Norman L. Bowen prior to 1916
These experiments totally rewrote our understanding of igneous rock formation
Bowen’s Reaction Series
Evidence of Fractional Crystallization in the Palisades Sill
Evidence of Fractional Crystallization in the Palisades Sill
first olivine next pyroxene pyroxene finally plagioclase & plag. Feldpar finishes
Partial melting of upper mantle: e.g. at divergent spreading centers
Partial melting of continental crustal rocks
Mafic
Magmas
Felsic Magmas
Partial Melting and the Origin of Magmas
Magmatic Stoping: Making Room for the Intrusion of Magma
Wedging open overlying rock
Breaking off large blocks of rock (remnants of which are called xenoliths)
Melting of surrounding country rock
Rising Magma Wedges Open and Fractures Overlying Country Rock
Overlying rocks may bow up
Magma melts surrounding rock
…changing the composition of the magma
Blocks of the Overlying Country Rocks (Xenoliths) May Break Off and
Sink into the Magma
PlutonsLarge igneous bodies formed at depth in the
Earth’s crust
Types of Plutons
Batholith: Massive, discordant intrusive body covering at least 100 km2
Stock: Massive, discordant intrusive body covering less than 100 km2
Dike: Tabular, discordant intrusive body
Sill: Tabular, concordant intrusive body
Types of intrusive and extrusive igneous structures
Sill
DikeDike
Where Do Most Magmas Occur?Divergent Plate Margins
Convergent Plate Margins
Mantle Plumes/Hot Spots
Island Arc Plate Subduction
(Japanese Islands)
ContinentalPlate Subduction
(Mount St. Helens)
Hot SpotVolcanism
(Hawaii)
Generation of Magmas at Convergent Plate Margins
Subduction drags oceanic lithosphere (including a veneer of “wet” sediments) beneath the adjacent plate
Generation of Magmas at Convergent Plate Margins
The release of volatiles lowers the melting point of the adjacent mantle, causing fluid-induced melting to form a mafic magma, which becomes more intermediate in composition as it rises through the overlying crust
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Ophiolite Suites
Unusual assemblages of rocks found on land that had characteristics of seafloor
Composed of deep-sea sediments, basaltic lavas and mafic igneous intrusions
Fragments of ocean crust moved onto land by plate tectonics
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Idealized Section of an Ophiolite
Suite
Generation of Igneous Rocks at Divergent Plate Margins
Some of the mafic magma cools in massive magma chambers to form massive gabbros
Some of the magma is intruded as sheeted dikes
These dikes are feeder for basaltic lava flows which form pillows as they extrude beneath the ocean