Chapter 4 Rocks & Igneous Rocks
Rock Definition A naturally occurring consolidated mixture of one or more minerals e.g, marble, granite, sandstone, limestone
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
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 Rock Mineralogy 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 Great Families of Rocks Igneous Sedimentary Metamorphic
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 Rocks
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
Most Igneous Rock are Silicates Quartz Feldspar Mica Pyroxene Amphibole Olivine Basalt
Granite
Sedimentary Rocks Rocks formed by consolidation of sediments Classified by the type of sediments
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
From Sediments to Solid Rocks Lithification 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 Rocks Sedimentary 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 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)
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
Regional Metamorphism
Contact Metamorphism
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
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)
Metamorphic Mineralization Contact Ultra-high pressure Regional High-press, low-temp
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 Rock Cycle Magma rises from melting plate and intrudes or extrudes in the crust
Magmas cool to make igneous rock Basalt versus granite Rock Cycle
Weathering and erosion creates sediments Rock Cycle
Rock Cycle Sediments are carried to the oceans and lakes Lithification
Rock Cycle Metamorphism of subducted rock Tectonic plates interact
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 Rocks by 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 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
We Can Also Classify Igneous Rocks by Composition Chemistry Mineralogy
Chemistry Modern classification of igneous rock is based upon the silica (SiO 2 ) content The silica content is determined by the silicate minerals that occur in the rock (i.e., the minerals contain SiO 2 ) 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
Felsic Igneous Rocks Rich (high) in minerals containing silica Poor (low) in iron and magnesium They include: Granite Rhyolite
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
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 Rhyolite Dacite Andesite Basalt
Common Minerals of Igneous Rocks
When Do Rocks Melt? Melting starts at ~700 o C When the temperature exceeds the melting point of the rock or some minerals within the rock Minerals melt at different temperatures
When Do Rocks Melt? 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
Magma Differentiation The process by which rocks of various compositions can arise from a uniform parent magma
The Formation of Magma Chambers Partial melting
Less dense magma The Formation of Magma Chambers Partial melting
Magma rises Less dense magma The Formation of Magma Chambers Partial melting
Magma pools in magma chamber Magma rises Less dense magma The Formation of Magma Chambers Partial melting
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
Fractional Crystallization 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 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 and the Origin of Magmas Partial melting of upper mantle: e.g. at divergent spreading centers Mafic Magmas Partial melting of continental crustal rocks Felsic 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
Plutons Large igneous bodies formed at depth in the Earth s crust
Types of Plutons Batholith: Massive, discordant intrusive body covering at least 100 km 2 Stock: Massive, discordant intrusive body covering less than 100 km 2 Dike: Tabular, discordant intrusive body Sill: Tabular, concordant intrusive body
Types of intrusive and extrusive igneous structures
Sill
Dike
Where Do Most Magmas Occur? Divergent Plate Margins Convergent Plate Margins Mantle Plumes/Hot Spots
Island Arc Plate Subduction (Japanese Islands)
Continental Plate Subduction (Mount St. Helens)
Hot Spot Volcanism (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
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
Chapter 5 Volcanoes