Chapter 4 Rocks & Igneous Rocks

Rock Definition A 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 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 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 Rocks

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

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 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

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)

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

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

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

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 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 IntermediateMafic GraniteGranodioriteDioriteGabbro Basalt AndesiteDaciteRhyolite

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

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 melting

Less dense magma The Formation of Magma Chambers

Partial melting Less dense magma Magma rises The Formation of Magma Chambers

Partial melting Less dense magma Magma rises Magma pools in magma chamber The Formation of Magma Chambers

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

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

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

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

x

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