Lecture 4
Magmas and Igneous Rocks


Useful Web page -- UBC provides an excellent homepage for Igneous Rocks
Also, don't miss the Granite Page!

Earth Scientists study many different types of volcanoes and can make the observation that there are a wide variety of igneous rocks.

The qestion is why? Why are there so many different types of volcanic rock, where does magma come from, and why do volcanoes erupt?

First observations are about composition.

Composition and Appearance

volcanic	basalt		andesite	dacite		rhyolite
plutonic	gabbro		diorite		granodiorite	granite
SiO2		<53%		53-63		63-70		>70
description	mafic		intermediate	silicic		Silicic (high silica)
viscosity	low		intermediate	high		very high
Minerals	Ol, px, pl	Px, amph, pl	Amph, pl,	qtz, pl, mica
(phenocrysts)					+/-qtz		K-feldspar, amph,
Color (huge	darker		dark		light		lighter
generalization)											dark obsidian

See Fig 3.25, which demonstrates different mineral assemblages.

Note names based on SiO2 content and not mineralogy.

Plutonic = coarse grained -- cooled slowly beneath the surface
Volcanic = fine grained +/- phenocrysts -- cooled quickly above the surface.

Phenocryst= a larger mineral in a fine-grained groundmass of a volcanic rock

Observations concerning Volcanic Eruptions


Viscosity -- the resistance to flow -- determines the kind of eruption and volcanic deposit.

viscosity is dependent on two things:
1) temperature -- the higher the temperature the less viscous. This is because as the temperature of solids become higher the bonds become weaker.
2) composition -- In a melt, silicate tetrahedra become polymerized, that is they become joined together by sharing oxygens. The more polymerization, or the more shared oxygens, the more viscous the magma. Inreased silica ioons increase the degree of polymerization and thus the viscosity of the melt.

Gas content - nearly all magmas contain gas that exolves at lower pressures as the magma reaches the surface. The same thing happens to CO2 in soda when the cap is removed and pressure is released. The viscosity of the magma controls how easily this gas can be removed from the magma.

In fluid magmas, the gas can be released easily. Gas bubbles caught in a frozen magma (a volcanic rock) are called vesicles.

In a viscous magma (like a rhyolite) the gas cannot easily escape. The espanding gas sometimes tears apart the sticky, viscous magma and causes an explosive eruption where much ash and tephra are erupted. More about these later.

Types of eruptions and volcanoes
The USGS photo-archives are probably one of the best sources of information. Go there for photographs of various types of volcanoes. Their Mt. St. Helens slide set is excellent.

Lava Flows -- very fluid to very viscous

In fluid magma eruptions, gas can exolve and escape relatively easily. Fluid flows are usually basaltic. Difference between aa and pahoehoe is that aa is more viscous.

Eruptions can fountain if there is a lot of magma, but not really dangerously explode. Lava collects at the bottom of the cone and forms a flow.
Cinder cones are piles of vesicular material where the blobs of lava froze in the air before reaching the ground. Thus a flow was unable to materialize.

Rhyolitic lava flows are much more viscous and thus form larger, more steep-sided features.
A dome is a viscous dacitic to rhyolitic lava flow that was too viscous to
move very far.
Shield volcano a volcano with very gentle slopes composed almost entirely
of very fluid basaltic magma.
Pillow Basalt -- basalt erupted under water produces this shape.

Explosive Eruptions -- Caused by exolved gas trying to escape a viscous magma.

Pyroclastic is a general term for rocks and deposits generated by explosive eruptions. Pyro=fire, and clastic=composed of pieces or fragments.
Pumice is a rock composed of little more than the sides of tiny bubbles. It can be less dense than water.
Pyroclast is a general term for a fragment of of rock ejected during an explosive eruption. Pumice is an example of a pyroclast.
Tephra is a deposit of pyroclasts, that have not yet turned into a rock. Pumice is coarse tephra, ash is fine-grained tephra. Can be built into a tephra cone.
Ignimbrite -- a tephra deposit fused together by heat and pressure into a very hard rok.

Two types of explosive eruption:
Airfall -- tephra falls from the eruption cloud as it begins to drift with
atmospheric winds
Pyroclastic flow a mixture of ash, tephra, and other debris that flows down the mountainside.
Lahar -- hot mudflow. Common on stratovolcanoes where eruptions melt snow and cause landslides.

Stratovolcano -- a steep-sided volcano composed of both tephra and viscous lava flows. Usually andesitic to dacitic in composition. Fuji, shown in fig 3.13, however, is basaltic incomposition.

Caldera - a huge crater. Yellowstone National park is actually a huge caldera.

Why Volcanoes Erupt

Reason #1 -- liquid less dense than solid. Magma will migrate towards surface if allowed to
The density difference is not great, so stress regimes play a role.
Reason #2 -- volatiles become less soluble in magma as density decreases; -- cause explosions

Types of Plutonic Bodies

Pluton small body of plutonic rock. Example is Ben Lomond Qtz Diorite.
-- size of Ben Lomond Mountain.
Batholith- Large mass of crystaline rock. As big as the Sierras. Fig 3.31.
Dike sheeet like body that cuts across other rock layers

Origin of Magma

How are the huge variety of magmas generated?
What causes the SiO2, and therefore the viscosity, to increase and produce potentially dangerous eruptions?

Crystal Fractination
Basalt differentiates (changes chemical composition to andesite then dacite) by crystal fractionation, which is a process that removes crystals (perhaps by gravitational settling) from a magma and thereby changes the magma's composiiton.

Partial Melting of the mantle
Mantle is composed of peridotite
Melts to produce basalt -- Note that melt is not the same composition as the solid. Why?
-because some peridotite minerals (like olivine) have higher melting
temperatures than other peridotite minerals (like pyroxene), and don't melt in the same proportions.
--decompression dry
mid-ocean ridges; oceanic islands
No abnormal heat sources! -Plumes at oceanic islands
[draw P-T diagram]
-- wet in mantle wedge, or near source of water
convergent margins
andesites differentiates from wet basalt.
[draw P-T diagram]

Critical role of water -- much of it comes from the sea in altered oceanic crust or sediments. Water from the ocean makes Mt St Helen's erupt.

Origin of rhyolitic magma -- The Granite Problem
-FC -- where are the cumulates?
-Partial melting of crustal rocks -- where is the heat for large batholiths?