Igneous Rocks

Extrusive vs. Intrusive realm

Extrusive: above surface (ex. lava) → cools on surface

Intrusive: below surface (ex. magma) → cools below surface

What is classifying igneous rocks based on?

• Texture

  • Fine or coarse (depending on how quickly magma cooled)

• Mineral content

  • Depends on the origin & chemical evolution of the magma

Reflects the chemical & cooling history of the rocks

Texture: extrusive vs. intrusive

Extrusive: can’t see individual minerals (cooled at surface)

Intrusive: can see each individual mineral (bad at moving heat below surface)

6 minerals that are >95% of igneous rocks

Mafics (Mg-Fe): olivine, pyroxene, hornblende (amphibole)

Aluminosilicates (Si-Al-O): feldspar (plagioclase & K-spar), quartz, micas

3-4 of these minerals are common in each type of igneous rock

Silicate tetrahedron structure

SiO₄⁴⁻

Silicate minerals

• All have silicate tetrahedra as part of structure

• Some have many metal ions attached to the tetrahedral

  • Others are made of 3-dimensional framework of tetrahedral

Broad classification of igneous rocks

Felsic - Si + O

Intermediate - between the 2

Mafic - Mg + Fe

Fine vs. Coarse grained

Mafic rocks

Gabbro:

• Intrusive

• Cooled slowly

• Coarse-grained

Basalt:

• Extrusive

• Cooled quickly

• Fine-grained

Same chemical & mineralogical composition as olivine, feldspar, & pyroxene

Intermediate rocks

Diorite:

• Intrusive

• Cooled slowly

• Coarse-grained

Andesite:

• Extrusive

• Cooled quickly

• Fine-grained

Same chemical & mineralogical composition as feldspar, hornblende, & mica

Felsic rocks

Granite:

• Intrusive

• Cooled slowly

• Coarse-grained

Rhyolite:

• Extrusive

• Cooled quickly

• Fine-grained

Same chemical & mineralogical composition as mica, feldspar, & quartz

Rock families (3)

Basalt-Gabbro

• Mafic rocks

• Make up the ocean crust (ex. Hawaiian islands, hotspots, basalt plateaus)

Andesite

• Intermediate rocks

• Volcanic island arcs, active continental margins like the Andes, subduction zones

Granite-Rhyolite

• Felsic rocks rich in quartz, feldspar, few mafic minerals

• Granite & granodiorite most common form of continental crust

• Occurs primarily on the continents

Textures of Igneous rocks

Phaneritic: intrusive & can see many minerals in a row

• Porphyritic: different mineral sizes in a rock with larger K-feldspar “phenocrysts”

Aphanitic: extrusive & can’t see minerals (came out of volcano & cooled really quickly)

• Porphyritic: larger amphibole phenocrysts

They have the same chemical/mineralogical composition but different cooling rates

Bowen’s Reaction Series

Describes the specific sequence in which minerals crystallize from cooling magma. Different minerals form at distinct temperatures, from high-temperature - iron-rich, mafic minerals - (e.g., olivine) to lower-temperature - silica-rich, felsic minerals - (e.g., quartz).

Partial Melting

Makes the melt silica-enriched because felsic minerals melt first

Always makes a more felsic magma than the rocks you start with

Different types of magma

• Partial melting of upper mantle (felsic) rocks make basaltic (mafic) magma

• Sedimentary rock + basaltic oceanic crust make andesitic (intermediate) magmas at subduction zones (ex. Andes)

• Melt of sedimentary, igneous, & metamorphic crustal rocks make granitic (felsic) magma - only found on the continents (subduction zones)

Lava Viscosity

Mafic → Felsic with low → high viscosity

Viscosity ∝ silica content
Silica tetrahedra link to each other, causing higher viscosity
25% increase in silica (basalt to rhyolite) causes 10^5 viscosity increase

Shield Volcano

Low silica content, low viscosity, runny lava placid eruptions

Mafic volcanoes which are the biggest on earth due to the lava flowing down, forming a small hillC

Composite Volcano

High silica content, stiff, viscous lava, highly explosive

Felsic volcanoes

Violent eruptions from gas buildup due to stiff, viscous, high-silica magmas (rhyolite)

“Cone” shape is made from debris (“ash”: small rock pieces) that falls out near volcano vent

Tectonic setting of volcanoes (2)

Felsic volcanoes: ocean-ocean/continent subduction with barely an age difference; partial melting makes felsic & intermediate rocks; either makes island arcs or continental volcanic belt

Mafic volcanoes: formed by the eruption of low-viscosity, Mg and Fe rich magma from the partial melting of the mantle at hot spots or divergent plate boundaries. These hot, runny, and gas-poor lavas erupt effusively, flowing easily to build broad, gentle-sloped shield volcanoes or cinder cones.

Tectonic setting of igneous activity

Mantle flood basalts (mafic) rise through continental crusts

Mantle hot-spots under the continental crust cause melting of Si-Al crust = rhyolite (felsic)

What does volcanism look like at MORs (divergent boundaries)?

Whole different ecosystem that doesn’t need light with microbes that depend on sulfite (& other minerals) from the magma underneath that’s “superheating” the water

Types of basaltic lava

1. Pahoehoe: ropy/smooth & flows easily with top solid surface that cools while the underneath still melts

2. aa lava: thicker, forms blocks that tumbles over itself
   Hazards: vog (volcanic smog), forest fires, asphalt (toxic when melted) on road melts

Mount St. Helens

• Convergent boundary volcano

• Right before: Gas pressure in magma chamber pushed out against the volcano, causing a bulge on the north flank

  • After: mini volcano (volcanic plug) formed in the middle due to cooled lava with the side of the mountain blown out

How high can eruption plumes get?

~ 2 km

2 types of volcanic flows

Pyroclastic: deadly, fast-moving clouds of volcanic ash & gas

• speed > 200 km/h

• temp > 800 degrees C

• feature of rhyolitic (felsic) volcanoes

Lahars: mix of ash & water from glacier volcanoes which melts the ice immediately when erupted

Which flow is faster & travels the farthest?

Lahars travel slower but travel the farthest

Yellowstone rhyolite cliffs

formed by explosive volcanic eruptions and lava flows

How do you check the status of a volcano?

• GPS size & shape

• Earthquakes

• Gas pressure

  • Temperature

Pumice blocks

From volcanic eruptions with high volcanic activity & made of solidified, highly porous (many holes/gas bubbles) volcanic foam (often rhyolite or dacite).

How are minerals in an igneous setting from a melt related by cooling speed & size?

They are large if formed slowly (vice versa)

Large Igneous Provinces relevance

Are basalt
Associated with mass extinctions
Gas released affects the climate, etc.

Basaltic Lava - Flood basalts

Very fluid that erupts on flat terrain & spreads out in sheets
Can be thick units covering huge area as in Columbia plateau

Intrusion types

Concordant (sills): within layers (horizontal)

Discordant (dikes): cuts across (vertical)

Batholiths

Huge features (made of granite) at the crustal scale