Ch. 4 - Igneous Rocks and Intrusive Activity

Class: GEOL-101

Notes:

4.1 Magma Parent Material of Igneous Rock

List and describe the three major components of magma.

Igneous rocks

Presentation content:

• Igneous rocks form as molten rock (magma) cools and solidifies
• General characteristics of magma:
  • Parent material of igneous rocks
  • Formed by partial melting in the Earth’s crust
  • Magma at surface is called lava
    • Lava may be emitted explosively or nonviolently

Notes from the lecture:

• As molten rock cools/crystalizes it forms an igneous rock
• Elements in the magma dictate what type of rock it forms
  • More potassium, sodium, etc...
• Think of it as what is available, continental crust will add a lot more elements
• When it reaches the surface, it can have a lot of different behaviors: pressure, mineralogy, etc.

Magma consists of three components

Presentation content:

• Liquid portion = melt
• Solids, if any, are crystals of silicate minerals
• Volatiles are dissolved gases in the melt that vaporize at surface pressure
  • Most common volatiles in magma:
    • Water vapor (H2O)
    • Carbon dioxide (CO2)
    • Sulfur dioxide (SO2)

From Magma to Crystalline Rock

Presentation content:

• Crystallization is the cooling of magma which results in the systematic arrangement of ions into orderly patterns.
• Silicon and oxygen atoms link together first to form silicon−oxygen tetrahedra.
• As heat loss continues, the tetrahedra join with each other and other ions to form crystal nuclei.
• Minerals that form earliest have space to grow and have better developed crystal faces than those that form later.

Notes from the lecture:

• Essentially, as the magma cools, the ions are systematically arranged into the most appropriate pattern
  • Largely dictated by their temperature
• Start to form that silica tetrahedra
• Granite is one of these rocks
  • quartz crystals (gray, glasy)
  • Plagioclase feldspar (white)
  • Amphibole (black)
  • Potassium feldspar (pink)

Igneous Processes

Presentation content:

• Magma that crystallizes at depth forms plutonic or intrusive igneous rocks
  • These rocks are observed at the surface following periods of uplifting and erosion of overlying rocks.
• The solidification of lava or volcanic debris forms volcanic or extrusive igneous rocks.

Notes from the lecture:

• Extrusive: cools very quickly (food in a plate)
• Intrusive: cools slowly (letting the food sit in the cook)

4.2 Igneous Compositions

Compare and contrast the four basic igneous compositions: felsic, intermediate, mafic, and ultramafic.

Composition of igneous rocks

Presentation content:

• Igneous rocks are composed mainly of silicate minerals.
  • Dark (ferromagnesian) silicates
    • Fe, Mg
    • Olivine, pyroxene (augite), amphibole (hornblende), and biotite mica
  • Light (nonferromagnesian) silicates
    • K, Na, Ca
    • Quartz, muscovite mica, and feldspars

Notes from the lecture:

• These are going to be abundant in specific igneous rocks
• We can put this on a spectrum
• Granitic
  • Mostly white looking
• Andesitic
  • Have a little bit of both Light and dark minerals
• Basaltic
  • Predominantly dark silicates
• Ultramafic
  • Olivine-rich
  • First material to cool is the ultramafic
  • Second material to cool is your basaltic component
  • Andesitic is next
  • The last thing to crystalize is the granitic component

Groups of igneous rocks

Presentation content:

• Igneous rocks are divided into two broad groups:

  • Granitic (Felsic) versus Basaltic (Mafic) Compositions

  • Granitic or felsic composition

    • Light-colored silicates
    • Composed almost entirely of quartz and potassium feldspar
    • Termed felsic (feldspar and silica) in composition
    • High silica (SiO2) content
    • Contain about 10% dark silicate minerals
    • Major constituent of continental crust

Notes from the lecture:

• Felsic = Feldspar and Silica rich

Basaltic or mafic composition

Presentation content:

• Contain at least 45% dark silicates and calcium-rich feldspar
• Contain no quartz!
• Termed mafic in composition (Fe; Mg)
• Higher density than granitic rocks
• Comprise the ocean floor and many volcanic islands
• Also forms extensive lava flows on the continents

Other Compositional Groups

Presentation content:

• Andesitic or intermediate composition
  • Contain 25% or more dark silicate minerals
  • Associated with volcanic activity on the seaward margins of the continents and volcanic island arcs.
• Ultramafic composition
  • Rare composition of mostly olivine and pyroxene
  • Composed almost entirely of ferromagnesian minerals
  • Peridotite is an example and the main constituent of the upper mantle.

Notes from the lecture:

• You may have oceanic crust sub-ducting, as it melts, the oceanic curst is mixing with the magma from the continental crust.
• Mixing as they melt, giving us different compositions

Silica Content

Presentation content:

• Silica Content as an Indicator of Composition

  • Crustal rocks exhibit a considerable range of silica content (40% in ultramafic to 70% in felsic)

  • Silica content influences magma behavior

    • Granitic magmas have high silica content, are viscous (thick), and erupt at a lower temperature (1200 deg. F).
    • Basaltic magmas have much lower silica content, more fluid like behavior, and erupt at a higher temperature (1920-2280 deg. F).

Notes from the lecture:

• If you increase your iron and magnesium composition of a crust, that crust is heavier, so it sits lower
• Silica is an interesting component to magma because it makes it more viscous and with more explosive reactions
  • Silica rich magma traps more oxygen, more water vapor, so when it comes to the surface that pressure trapped gas comes to explosion
• Magma tends to flow, this is because the differences in viscosity, and the materials composed in magma

4.3 Igneous Textures: What Can They Tell Us?

Identify and describe the six major igneous textures.

Texture

Presentation content:

• Texture:
  • Describes the size, shape, and arrangement of mineral grains.
  • Reveals a great deal about the environment in which the rock formed
• Factors influencing texture:
  • Rate of cooling
    • Slow rate = fewer but larger crystals
    • Fast rate = many small crystals
  • Amount of silica
  • Amount of dissolved gases

Notes from the lecture:

• If you can actually see the crystals easily, you consider those phaneritic
  • Very slowly cooling and crystalizing
• Aphanitic textures are more homogenous, you cannot see the crystals in there easily
  • You cannot recognize individual crystals
  • They have the same composition as its phaneritic opposites but they are cooling very fast

Types of Igneous Textures

Presentation content:

• Aphanitic (fine-grained)
  • Rapid rate of cooling
  • Microscopic crystals
• Phaneritic (coarse-grained)
  • Slow cooling; large
  • Visible crystals
• Porphyritic texture
  • Large crystals (phenocrysts) embedded in a matrix of smaller crystals (groundmass)
• Vesicular texture
  • Rocks contain voids left by gas bubbles in the lava
  • Common feature of an extrusive igneous rock
  • Vesicular texture forms when volatiles (gases such as H₂O, CO₂, SO₂) escape from magma or lava as it cools and solidifies.
• Glassy texture
  • Very rapid cooling
  • Ions are frozen in place before they can unite in an orderly crystalline structure
• Pyroclastic (fragmental) texture
  • Forms from the consolidation of individual rock fragments ejected during explosive eruptions
• Pegmatitic texture
  • Exceptionally coarse-grained; form in late stages of crystallization of magmas (rocks are called pegmatites)

Notes from the lecture:

• Porphyritic
  • Large crystals
  • Can be very easily misidentified as pyroclastic
  • Large pieces that are visible surrounded by a not visible matrix
  • Consists of minerals that form at different cooling rates
    • Starts to cool very rapidly as it ascends
  • Can be garnets
• Pyroplastic
  • Ejected pieces, more like a sedimentary rock
  • More like ash
  • Also include sedimentary parts of the volcano that get ejected
• Vesicular
  • Not as viscous as silica rich compositions
  • Gas bubbles escaping, so you end up with a very porous texture
• Glassy texture
  • Very rapid cooling rate
  • Basically those ions are locked in to place
• Pegmatitic texture
  • Very very large crystals around a centimeter in size

Igneous Rocks Classification

Presentation content:

• Based on texture and mineral composition
• Mineralogy is influenced by the chemical composition of the parent magma, texture results from cooling history.
• Rocks with the same composition but different texture are given different names.

Notes from the lecture:

• Based on mineralogy and crystal size

4.4 Naming Igneous Rocks

Distinguish among the common igneous rocks based on texture and mineral composition.

Granitic (Felsic) Igneous Rocks

Presentation content:

• Granite
  • Course-grained (phaneritic)
  • One of the best known and most abundant igneous rocks
  • 10−20% quartz, roughly 50% potassium feldspar
  • Small amounts of (<10%) dark silicates
  • Some granites have a porphyritic texture
    • These contain elongated feldspar crystals a few centimeters long
• Rhyolite
  • Extrusive (fine-grained/aphanitic) equivalent of granite
  • Composed essentially of light-colored silicates
  • Typically buff to pink or light gray in color
  • Less common and less voluminous than granite
• Obsidian
  • Dark-colored, glassy rock
  • Forms when silica-rich lava cools quickly at Earth’s surface
  • Usually black to reddish-brown in color
  • Similar chemical composition to granite
  • Dark color is the result of small amounts of metallic ions in an otherwise clear, glassy substance
• Pumice
  • Glassy textured rock with vesicular texture that forms when large amounts of gas escape from the lava
  • Voids are quite noticeable and matrix resembles fine shards of intertwined glass
  • Typically found in deposits with obsidian
  • Will float when placed in water

Notes from the lecture:

• Granite
  • White-ish
  • Compositionally similar to rhyolite but they look very different
• Rhyolite
  • Cool inside the earth, inside of a magma chamber, very slowly
• Obsidian
  • Mostly consisted of silica
  • It does have some iron and magnesium but not volumetric significant
  • You think of it exactly like a glass
• Pumice
  • Very pourus, it has a vesicular texture
  • It floats
    • Lighter than water
  • Extrusive igneous rock

Andesitic (Intermediate) Igneous Rocks

Presentation content:

• Andesite
  • Medium-gray, fine-grained rock
  • Volcanic origin
  • Commonly exhibits a porphyritic texture
• Diorite
  • Intrusive equivalent of andesite
  • Coarse-grained rock
  • Looks like gray granite, but lacks visible quartz crystals
  • Can have a salt-and-pepper appearance

Notes from the lecture:

• Largely the result of chemical differentiation or mixing of two magma bodies
• Andesite
  • Formed extrusively
  • very rapid cooling and you cannot see the crystals
• Diorite
  • Large crystals
  • Slight difference in the proportion of dark vs. light silicate minerals
• Some minerals cool at higher temperatures
  • Different minerals start to crystalize out

Basaltic (Mafic) Igneous Rocks

Presentation content:

• Basalt
  • Very dark green to black, fined-grained rock
  • Composed mostly of pyroxene and calcium-rich plagioclase feldspar
  • When porphyritic, contains small, light-colored feldspar or olivine phenocrysts
  • Most common extrusive igneous rock
  • Upper layers of oceanic crust, Hawaiian Islands, and Iceland are composed of basalt
• Gabbro
  • Intrusive equivalent of basalt
  • Very dark green to black, phaneritic rock
  • Composed mostly of pyroxene and calcium-rich plagioclase feldspar
  • Uncommon on the continental crust but makes up a significant portion of the oceanic crust

Notes from the lecture:

• Think of the island of Hawaii, floded in basalt
• Basalt
  • Cools very very quickly
  • Mixing with cold sea water
  • Can also have porphyritic textures
• Gabbro
  • You can actually see the crystals it is composed of, they are large

Pyroclastic Rocks

Presentation content:

• Composed of fragments ejected during a volcanic eruption

• Names do not imply mineral composition and are identified with a modifier. Example: rhyolite tuff

• Tuff

  • Most common pyroclastic rock
  • Composed of ash-sized fragments cemented together

• Welded tuff

  • Ash particles are hot enough to fuse together
  • Can contain walnut-sized pieces of pumice and other rock fragments
  • Covers vast portion of previous volcanically active areas of the western United States

• Volcanic breccia

  • Composed of particles larger than ash
  • Includes streamlined lava blobs, broken blocks of vent walls, ash, and glass fragments

Notes from the lecture:

• pyro = fire
• clastic = sediment
• Basically a volcanic eruption ejecting them
• Tuff
  • Tiny microscopic ash
  • Welded together in the form of a rock
• Welded Tuff
  • Hot gases cool almost instantaneously when upwelling in the atmosphere
  • Chunks being deposited

4.5 Origin of Magma

Summarize the major processes that generate magma from solid rock.

Generating Magma from Solid Rock

Presentation content:

• Geothermal gradient: temperatures in the upper crust increase about 25°C per kilometer
• Rocks in the lower crust and upper mantle are near their melting points
  • Tectonic processes trigger melting by reducing the melting point
    • Decrease in pressure
    • Addition of water
    • Increase in temperature of crustal rocks

Notes from the lecture:

• As you move from the exterior of the earth to the interior
  • You have an increase in pressure and temperature
• Reflecting changes of pressure, changes of water volume in the rock, and the material available (composition change)
  • Certain minerals melt at different temperatures

Decompression Melting

Presentation content:

• Melting occurs at higher temperatures with increasing depth (and increasing confining pressure).
• Reducing confining pressure lowers the melting temperature = decompression melting
• Solid, hot mantle rocks will ascend to regions of lower pressure, inducing melting.
  • Divergent plate boundaries
  • Mantle plumes at hot spots

Notes from the lecture:

• Divergent plate boundary
  • push and pull concept
  • combination of both
• What we see is that as magma chamber rises, it starts to incorporate the material in that depth
  • As we go into a shallower depth we start to lower temperature and include things like water, affects the process
  • Pressure changing with depth and incorporating in rocks

Addition of Water

Presentation content:

• Water and other volatiles act as salt does to melt ice
  • Causes rock to melt at lower temperatures
• Occurs mainly at subduction zones
• As an oceanic plate sinks, heat and pressure drive water from the crust and overlying sediments
• Fluids migrate into the overlying wedge of mantle
• The addition of water lowers the melting temperature of the mantle rocks to trigger partial melting.

Notes from the lecture:

• Subducting oceanic crust here
• As it starts to melt, additional water is lowering that melting temperature
  • The more water the lower the temperature needed to melt (lowers the molting point)
  • Now they don't have to be heated as much to get to magma again

Melting Crustal Rocks

Presentation content:

• Mantle-derived basaltic magma buoyantly rises toward the surface
  • There is “ponds” beneath the less dense crustal rocks
• Heat from these magma sources can melt the surrounding crustal rocks.
• Crustal rocks can also melt from heat generated during the continental collisions that result in the formation of large mountain belts.

Notes from the lecture:

• Using all the iron and magnesium rich material
• Then you start to look at things like sodium, potassium -> other minerals forming
• By the end they reach surface they are more silicate rich, it is not gonna have that much iron and magnesium
• As magma chambers cool, they are evolving.

Bowen's Reaction Series

• Temperature dependent, but it also depends on what your magma starts with

4.6 How Magmas Evolve

Describe how magmatic differentiation can generate a magma body that has a different chemical composition from its parent magma.

Magmatic Differentiation and Crystal Settling

Presentation content:

• Crystal settling: earlier-formed minerals are denser than the magma and sink to the base of the magma chamber.
• When the remaining magma solidifies, the mineralogy will be different from the parent magma.
• Magmatic differentiation: forming one or more secondary magmas from a single parent magma.

Notes from the lecture:

• You start to precipitate out dark silicate minerals
• As it cools or crystalizes your magma is evolving, losing iron and magnesium for other minerals
  • You are left with silicate, sodium, potassium
• Silica rich magma is more viscous, it traps more gas.

Assimilation and Magma Mixing

Presentation content:

• Assimilation
  • As magma migrates through the crust, it may incorporate some of the surrounding rock into the chamber, melting and changing the chemical composition.
• Magma mixing
  • During the ascent of two chemically different magma bodies, the more buoyant mass may overtake the slower-rising body, merging them, and their melts mixing by convective flow.

4.7 Partial Melting and Magma Composition

Explain how partial melting of the mantle rock peridotite can generate a mafic (basaltic) magma.

Incomplete melting of rocks is known as partial melting.

Presentation content:

• This process produces most magmas
• During partial melting, the melt is enriched in ions from minerals with the lowest melting temperature.
  • Ultramafic rocks yields mafic magmas
  • Mafic rocks yields intermediate magmas
  • Intermediate rocks yields felsic magmas

Notes from the lecture:

• If we take an ultramafic rock and we reheat it some of those minerals are going to be left behind
• As a result we end up with a mafic magma, it again gets reheated, which yields intermediate magmas
  • Things like Mica, Quartz, etc.
• It depends on what minerals get incorporated when heating.

Formation of Magmas

Presentation content:

• Formation of Basaltic Magmas
  • Most magma that erupts is basaltic (mafic) magma
  • Most originate from partial melting of mantle rocks at oceanic ridges
    • These melts are called primary or primitive magmas because they have not yet evolved.
• Formation of Andesitic and Granitic Magmas
  • Andesitic magma can form in two ways
    • Magmatic differentiation of mantle-derived basaltic magma
    • Basaltic magmas assimilating crustal rocks
  • Granitic magmas
    • Most form when basaltic magma ponds beneath the continental crust, heating and melting the much-lower melting temperature felsic minerals.
    • Can also form from magmatic differentiation of andesitic magma

• Cooling as its rises
• Changing composition
• More dense than continental crust
• Magnetic differentiation
• Evolving in terms of composition

4.8 Intrusive Igneous Activity

Compare and contrast these intrusive igneous structures: dikes, sills, batholiths, stocks, and laccoliths.

Most magma is emplaced at depth in Earth

Presentation content:

• Intrusive Igneous Bodies
• A pluton is cooled, emplaced magma into preexisting rocks
• Classification of plutons
  • Plutons are classified by their shape and their orientation relative to the surrounding rock:
  • Tabular– table-like
    • Discordant– cut across existing structures (e.g., Dike)
    • Concordant– are parallel to features like sedimentary strata (e.g., Sill)
  • Massive– blob shaped

Tabular Intrusive Bodies

Presentation content:

• Dike—a tabular, discordant pluton
  • Magma was forcibly injected into fractures cutting across bedding planes
  • Transport magma upward
  • Parallel groups are called dike swarms
  • Can also radiate from a volcanic neck like spokes on a wheel
• Sill—a tabular, concordant pluton
  • Nearly horizontal; magma exploits weaknesses along bedding planes
  • Tend to accumulate magma and increase in thickness
  • Closely resembles buried lava flows
  • May exhibit columnar jointing
    • Occurs when igneous rocks cool and develop shrinkage fractures that produce elongated, pillar-like columns that often have six sides.

Notes from the lecture:

• Sills - basically extend across that region
• You can actually trace that sill
• Sills sometimes exhibit columni joints

Massive Intrusive Bodies

Presentation content:

• Batholith
  • Largest intrusive body
  • Occur as linear structures several hundred kilometers long
  • Surface exposure of 100+ square kilometers (smaller bodies are termed stocks)
  • While expansive, most are less than 10 km thick
  • Typically composed of felsic to intermediate rock types
• Emplacement of Batholiths
  • Magma at depth is much less dense than the surrounding rock.
  • In the mantle, the more buoyant magma pushes aside the host rock and rises in Earth through a process called shouldering.
  • As it encounters more cool and brittle rock, blocks of this rock are dislodged and sink into the magma – this is called stoping.
  • Evidence of this is seen as blocks of country rock, called xenoliths, encased in plutons.
• Laccoliths
  • Forcibly injected between sedimentary strata
  • Causes the overlying strata to arch upward
  • Overinflated sills

Notes from the lecture:

• Western US actually have quite a few
• A convergent plate boundary
• There is still a lot of volcanism going on today
  • West US has very large volcanic bodies
  • ~10km think
• Xenoliths:
  • Country rock that is being incorporated
    • Preexisting rock: rock that is already available, (it is either incorporated or passed by)
  • You can actually see the much lighter color magma and then differentiate it form the mafic part.
• Laccoliths
  • pimpled shape strata
  • Doesn't form volcanic pipe, but deforms strata above

End of Chapter 4 - Concept Checks

4.1 Magma: Parent Material of Igneous Rock

1. What is magma? How does magma differ from lava?
2. List and describe the three components of magma.
3. Compare and contrast extrusive and intrusive igneous rocks.

4.2 Igneous Compositions

1. Igneous rocks are composed mainly of which group of minerals?
2. How do light-colored igneous rocks differ in composition from dark-colored igneous rocks?
3. List the four basic compositional groups of igneous rocks, in order from highest silica content to lowest silica content.

4.3 Igneous Textures: What Can They Tell Us?

1. How does the rate of cooling influence crystal size? What other factors influence the texture of igneous rocks?
2. List the six major igneous rock textures.
3. What does a porphyritic texture indicate about the cooling history of an igneous rock?

4.4 Naming Igneous Rocks

1. List the two criteria by which igneous rocks are classified.
2. How are granite and rhyolite different? In what way are they similar?
3. Describe each of the following in terms of composition and texture: diorite, rhyolite, and basalt porphyry.

4.5 Origin of Magma

1. Explain the process of decompression melting.
2. What role does water play in the formation of magma?
3. Briefly explain one way that basaltic magma can generate felsic magma.

4.6 How Magmas Evolve

1. Define magmatic differentiation.
2. How does the crystallization and settling of the earliest formed minerals affect the composition of the remaining magma?
3. Describe the processes of assimilation.

4.7 Partial Melting and Magma Composition

1. Briefly describe why partial melting results in a magma having a composition different from the rock from which it was derived.
2. What is the process thought to generate most basaltic magmas? Most granitic magmas?

4.8 Intrusive Igneous Activity

1. What is meant by the term country rock?
2. Describe dikes and sills, using the appropriate terms from the following list: massive, discordant, tabular, and concordant.
3. Distinguish among batholiths, stocks, and laccoliths in terms of size and shape.