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Chapter 4 Rocks & Igneous Rocks
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Rock Definition A naturally occurring consolidated mixture of one or more minerals
e.g, marble, granite, sandstone, limestone
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Rock Definition Must naturally occur in nature, cannot be man-made
Is a solid, not gas, ice or liquid
Composed of one or more minerals
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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
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Appearance of a Rock The appearance of a rock is determined by two major factors
Mineralogy
Texture
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Appearance of a Rock Mineralogy refers to the relative proportions of the constituent minerals
Texture is determined by the size, shape and arrangement of the mineral crystals
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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
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Three Great Families of Rocks
Igneous Sedimentary Metamorphic
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Three Types of Rocks
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Igneous Rocks Rocks formed by the solidification of magma (molten rock)
Granite
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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)
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Igneous Rocks
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Igneous Rock Texture Intrusive rocks such as granite are coarse grained (can easily see the grains with the unaided eye)
Extrusive igneous rocks such as basalt are glassy or fine grained
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Most Igneous Rock are Silicates
Quartz Feldspar
Mica Pyroxene
Amphibole Olivine Basalt
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Granite
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Sedimentary Rocks
Rocks formed by consolidation of sediments
Classified by the type of sediments
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Two Major Types of Sediments Clastic sediments are physically deposited particles derived from weathered rocks
Chemical and biochemical sediments include minerals carried in solution such as calcite and halite
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From Sediments to Solid Rocks Lithification is the process of converting sediments into solid rocks (will cover in detail in another lecture)
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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
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Sedimentary Rocks
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Sedimentary Rocks 5 % by volume of the upper crust
75 % by exposed surface area of continents
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Sedimentary Rocks Sedimentary rocks commonly contain fossils In fact, some sedimentary rocks are almost entirely composed of fossils Can exhibit extensive horizontal layers called bedding
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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
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Metamorphic Rocks
Metamorphic rocks are formed by the transformation of previously-existing rocks in the solid state due to increased temperature and pressure
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Metamorphic Rocks Metamorphism 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)
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Regional and Contact Metamorphism
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
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Regional Metamorphism
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Contact Metamorphism
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Other Forms of Metamorphism Ultra-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
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Metamorphic Mineralization Silicates 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)
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Contact Ultra-high pressure Regional High-press, low-temp
Metamorphic Mineralization
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Proportions of the Rock Types
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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
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Common Minerals Found in Rocks
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Five Very Different Locations
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Rock Cycle
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Rock Cycle
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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
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Magmas cool to make igneous rock
Basalt versus granite
Rock Cycle
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Weathering and erosion creates sediments
Rock Cycle
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Sediments are carried to the oceans and lakes
Lithification
Rock Cycle
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Tectonic plates interact
Metamorphism of subducted rock
Rock Cycle
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Entire process starts over as plates interact
Rock Cycle
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Igneous Rocks
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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?
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We Classify Igneous Rocks by Appearance
Mineralogy (Chemistry)
Texture
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Igneous Rocks
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We Classify Igneous Rocks by Genetics (Origin)
There are two classes of igneous rocks Intrusive Extrusive
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Genetic Classification of Igneous Rocks
Intrusive igneous rocks crystallized from slowly cooling magma intruded within the Earth’s crust, such as granite and gabbro
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Granite Intrusions
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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
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Extrusive Igneous Rocks Rocks 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
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We Can Also Classify Igneous Rocks by Composition
Chemistry Mineralogy
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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”
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Mineralogy
There are four major divisions of igneous rocks based upon the content of elements in the minerals:
Felsic Intermediate Mafic Ultramafic
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Rich (high) in minerals containing silica Poor (low) in iron and magnesium They include: Granite Rhyolite
Felsic Igneous Rocks
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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 Dacite Diorite Andesite
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Mafic Igneous Rocks
Poor (low) in minerals containing silica
Rich (high) in iron and magnesium They include: Gabbro Basalt
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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
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Felsic Intermediate Mafic Granite Granodiorite Diorite Gabbro
Basalt Andesite Dacite Rhyolite
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Common Minerals of Igneous Rocks
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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
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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?
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Magma Differentiation
The process by which rocks of various compositions can arise from a
uniform parent magma
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The Formation of
Magma Chambers
Par tial melting
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Par tial melting
Less dense magma
The Formation of
Magma Chambers
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Par tial melting
Less dense magma
Magma r ises The
Formation of Magma
Chambers
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Par tial melting
Less dense magma
Magma r ises
Magma pools in magma chamber
The Formation of
Magma Chambers
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Magma Differentiation Occurs because different minerals crystallize (solidify) at different temperatures
In other words, as the magma cools some minerals form first, some form last
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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
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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
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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
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Bowen’s Reaction Series
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Evidence of Fractional Crystallization in the Palisades Sill
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Evidence of Fractional Crystallization in the Palisades Sill
first olivine next pyroxene pyroxene finally plagioclase & plag. Feldpar finishes
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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
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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
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Rising Magma Wedges Open and Fractures Overlying Country Rock
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Overlying rocks may bow up
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Magma melts surrounding rock
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…changing the composition of the magma
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Blocks of the Overlying Country Rocks (Xenoliths) May Break Off and
Sink into the Magma
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Plutons Large igneous bodies formed at depth in the
Earth’s crust
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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
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Types of intrusive and extrusive igneous structures
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Sill
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Dike
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Where Do Most Magmas Occur? Divergent Plate Margins Convergent Plate Margins Mantle Plumes/Hot Spots
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Island Arc Plate Subduction
(Japanese Islands)
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Continental Plate Subduction
(Mount St. Helens)
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Hot Spot Volcanism (Hawaii)
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Generation of Magmas at Convergent Plate Margins
Subduction drags oceanic lithosphere (including a veneer of “ wet” sediments) beneath the adjacent plate
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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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x
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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
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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
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Chapter 5 Volcanoes