Igneous RocksLecture 3

Types of Rock include Igneous, Sedimentary and Metamorphic

Marble demo, rock specimens, Petrographic Microscope, Olivine Porphyry or Gabbro

Characteristics of magma

• Igneous rocks form as molten rock cools and solidifies

• Characteristics of magmas (molten rock) depend on parent material and where they crystallize

• Where determines speed of crystallization

• At surface, fast cooling makes small crystals

Geothermal

Gradient Silica-rich rocks (with Quartz) melt at cooler temperatures. Melts are viscous

Silica-poor rocks (with Olivine, Pyroxene) melt at higher temperatures Melts are very fluid Hot

Cool

Characteristics of magma

• General Characteristics of molten rock• Forms from partial melting of rocks inside

the Earth• Rocks formed from lava at the surface are

classified as extrusive, or volcanic rocks• Rocks formed from magma that crystallizes

at depth are termed intrusive, or plutonic rocks

http://www.casdn.neu.edu/~geology/department/staff/colgan/iceland/usgs/

Igneous RocksFormed in Rift

Igneous RocksFormed AboveSinking Plate

Two Geologic Environments Where Igneous Rocks Form

Extrusive Igneous Rock - Lava (Hawaii)

Intrusive Igneous Rock (Granite) – This granite cooled 30 kilometers under the surface

Characteristics of magma

• The nature of magma• Consists of three components:

– A liquid portion, called melt, that is composed of mobile ions

– Solids, if any, are silicate minerals that have already crystallized from the melt

– Volatiles, which are gases dissolved in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

Characteristics of magma• Crystallization of magma

• Cooling of magma results in the systematic arrangement of ions into orderly patterns

• The silicate minerals resulting from crystallization form in a predictable order

Rock-forming minerals crystallize with increasing complexity as the magma cools. The most complex 3-D minerals crystallize last. The hottest magmas can only crystallize Olivine (Independent Tetrahedra), but as the magma cools, more complex minerals can form.

Crystallization of minerals in magma bodies

• Bowen’s reaction series and the composition of igneous rock

• Norman L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points

http://vgp.agu.org/bowen_paper/bowen_paper.html

Bowen’s Reaction SeriesMolten- VERY Hot

Molten- Not so hot

100% Solid

Fine crystalsNeed a microscope

Course crystalsEasily seen

Low silica, HOT, fluid High silica, warm, viscousIntermediate

GraniteHand Sample

GraniteThin Section

Order ofCrystallization

Bowens reactrion series says: as a granitic melt cools, Biotite Mica and Plagioclase Feldspar

crystallize out before Quartz

We can see the order of crystallization under the microscope

Zoned feldspar (plagioclase) showing change in composition with time in magma chamber (calcium-rich in core to sodium-rich at rim)

If the first formed crystals of Calcium-rich (Ca) Plagioclase are allowed to react with the melt, they will become more sodium-rich on their outer rims

Crystals can react with the melt if they touch it

However, if early crystals are removed the melt becomes richer in Silica

A rising melt may leave its mafic components behind, and the upper part may be granitic

Remove Fe, Mg, CaSome Si

Left withK and AlMost of Si

You can start with aMafic (silica-poor) magmaand end up with some Felsic (silica-rich)Granites. Marble Demo

Characteristics of magma

• Crystallization of magma

• Texture in igneous rocks is determined by the size and arrangement of mineral grains

• Igneous rocks are typically classified by both:– Texture– Mineral composition

Igneous textures

• Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals

• Most important is crystal size

• Factors affecting crystal size• Rate of cooling

– Slow rate promotes the growth of fewer but larger crystals

Igneous textures

• Factors affecting crystal size • Rate of cooling (continued)

– Fast rate forms many small crystals

– Very fast rate forms glass

• Amount of silica (SiO2) present

• Amount of dissolved gases

Types of Igneous textures

• Types of igneous textures• Aphanitic (fine-grained) texture

– Rapid rate of cooling of lava or magma– Microscopic crystals– May contain vesicles (holes from gas bubbles)

• Phaneritic (coarse-grained) texture– Slow cooling– Crystals can be identified without a microscope

Aphanitic texture

Fine grained because it cooled quickly at the surface

Phaneritic texture

Coarse crystals cooled slowly at great depth

Igneous textures• Types of igneous textures

• Porphyritic texture– Minerals form at different temperatures as

well as differing rates– Large crystals, called phenocrysts, are

embedded in a matrix of smaller crystals, called the groundmass

• Glassy texture– Very rapid cooling of molten rock– Resulting rock is called obsidian

Porphyritic texture

Granite

Glassy texture

Obsidian

More types of Igneous textures

• Types of igneous textures• Pyroclastic texture

– Various fragments ejected during a violent volcanic eruption

– Textures often appear to more similar to sedimentary rocks

Pyroclastic Rock - Superheated Flows

Naming igneous rocks – pyroclastic rocks

Composed of fragments ejected during a volcanic eruption

VarietiesTuff – ash-sized fragments

Volcanic breccia – particles larger than ash

Ash and pumice layers

Still more types of Igneous textures

• Types of igneous textures

• Pegmatitic texture

–Exceptionally coarse grained crystals

–Form in late stages of fractionation of magmas

–This is often what prospectors are looking for

A Pegmatite with Feldspar and Zircon Zircon is very good for obtaining radiometric ages

Igneous Compositions

• Igneous rocks are composed primarily of silicate minerals that include:

• dark (or ferromagnesian) colored silicates– Olivine– Pyroxene – Amphibole– Biotite mica

– And …

“MAFIC” Magnesium and Iron

Show tray of Mafic Minerals

Igneous Compositions

• Igneous rocks also contain light colored silicate minerals that include:

– Quartz

– Muscovite mica

– Feldspars

“FELSIC” Feldspar and Silica

Show tray of Felsic Minerals

Igneous Rock Classification- Bowen’s Reaction Series on its side

Felsic rocks crystallize from warm melts Mafic from hot melts

Note Minerals inNote Minerals in

Igneous compositions

Granitic composition– Composed of light-colored silicates

– Designated as being felsic (feldspar and silica) in composition

– Contains high amounts of silica (SiO2)

– Major constituents of continental crust

Igneous compositions• Naming igneous rocks – granitic (felsic)

rocks• Granite

– Phaneritic– Over 20 percent quartz, about 25 percent or

more feldspar (usually much more feldspars).– Plagioclase is Sodium-rich– Abundant and often associated with mountain

building– The term granite covers a wide range of mineral

compositions

Igneous compositions

• Naming igneous rocks – granitic (felsic) rocks

• Rhyolite– Extrusive equivalent of granite

– May contain glass fragments and vesicles

– Aphanitic texture (means fine grained minerals)

– Less common and less voluminous than granite

– Phenocrysts can include quartz and feldspar

Figure 3-7 continued

Rhyolitefine grained because extruded,

so crystallized quickly

Igneous compositions

• Basaltic composition can be fine or coarse– Composed of dark Olivine and Pyroxene and grey

calcium-rich plagioclase feldspar– No Potassium-rich feldspar (no K-spar

‘Microcline’)– Designated as being mafic (magnesium and ferrum, for iron) in composition

– Much denser than granitic rocks - sinks– Comprises the ocean floor as well as many volcanic

islands such as Hawaii. Also rift valley lavas

Igneous compositions• Naming igneous rocks – basaltic (mafic) rocks:

Fine-grained

•Basalt– Volcanic origin

– Aphanitic texture

– Composed mainly of pyroxene, some olivine and also calcium-rich plagioclase feldspar

– Most common extrusive igneous rock

Scoria type Basalt note Gas Bubbles

Igneous compositions

• Naming igneous rocks – basaltic (mafic) rocks: Coarse Grained

• Gabbro–Intrusive equivalent of basalt

–Phaneritic texture consisting of pyroxene and calcium-rich plagioclase (med. grain “diabase”)

–Makes up a significant percentage of the oceanic crust, beneath the basalt pillow lavas.

Gabbro - a maficigneous rock. A large-grain version of Basalt

Igneous compositions

• Other compositional groups

• Intermediate (or andesitic) composition– Contain at least 25 percent dark silicate minerals– Associated with explosive volcanic activity– Often gray

Igneous compositions

• Intermediate rocks• Andesite

– Volcanic origin

– Aphanitic texture

– Often resembles rhyolite

– Intermediate silica content

– Frequent composition in volcanoes above subduction zones, e.g. in Andes Mountains

Igneous compositions

• Extrusive products can include:• Pumice

– Volcanic

– Glassy texture, very light weight, mostly air

– Frothy appearance with numerous voids (extrusive foam)

– Forms when lavas have a lot of water and other volatiles

Common with intermediate compositions

Igneous compositions

• Intermediate rocks• Diorite

– Plutonic equivalent of andesite

– Coarse grained

– Intrusive

– Composed mainly of intermediate feldspar and amphibole

Igneous compositions• Silica content influences a magma’s

behavior• Granitic magma

– High silica content

– Extremely viscous

– Liquid exists at temperatures as low as 700oC

– Huge explosion if it erupts (Yellowstone, Toba)

Plutonic“Granite”

Volcanic“Rhyolite”

Igneous compositions

• Silica content influences a magma’s behavior

• Basaltic magma– Much lower silica content– Fluid-like behavior

– Crystallizes at higher temperatures– Gurgles when it erupts (Hawaii)

Origin of Magma

• Generating magma from solid rock• Produced from partial melting of rocks in the

crust and upper mantle• Role of heat

– Temperature increases within Earth’s upper crust with increasing depth. The geothermal gradient averages between 20oC to 30oC per kilometer

Origin of Magma

• Role of heat–Rocks in the lower crust and upper

mantle are near their melting points

–Any additional heat (from rocks descending into the mantle or rising heat from the mantle) may induce melting

Origin of Magma

• Role of Pressure– Reducing the pressure lowers the melting

temperature – the rock probably melts– RIDGE: When confining pressures drop,

decompression melting occurs

Decompression meltingAt a Mid-Ocean Ridge

Origin of Magma

•Role of volatiles - WATER–Volatiles (primarily water) cause rocks to melt at lower temperatures

–This is particularly important where oceanic lithosphere descends into the mantle in a subduction zone

The role of water Pillow Lavas

Water promotes melting despite high pressures

Evolution of magmas

• Processes responsible for changing a magma’s composition

• Assimilation– Changing a magma’s composition by the incorporation

of foreign matter (surrounding rock bodies) into a magma

– The rising magma melts near surface rock, changing the composition of the melt.

• Magmatic differentiation = Fractionation– Separation of a melt from earlier formed crystals causes

a different composition in the remaining magma

Evolution of magmas

• Processes responsible for changing a magma’s composition

• Magma mixing–Involves two bodies of magma

intruding one another

–Two chemically distinct magmas may produce a composition quite different from either original magma

Assimilation and magmatic differentiation

Show Samples

Evolution of magmas

• Partial melting and magma formation• Incomplete melting of rocks is known as partial

melting• Formation of basaltic magmas

– Most originate from partial melting of ultramafic rock in the mantle

– Basaltic magmas form at mid-ocean ridges by decompression melting of rising mantle material

– also by partial melting of ocean crust + sediments+ water in subduction zones

Basalts forming in rifts

Decompression Melting:Magma under lithosphere heats and cracks it. Mantle rock is exposed to low pressures – it partially melts

Origin of Andesite & Diorite: intermediate silica content

Good diagram for the Andes Mountains

Small blobs, not much heat in themAssimilate some crust, fractionate

62

Plate Tectonics- Andesite Line

Andesites form above the deep portions of a subduction zone

Andes

Origin of Granitic Rocks

Can also get granites from deep rock burial

Huge blobs w/ low temps but lots of magma, fractionation & assimilation => Granite Batholiths

End of Lecture 3