Physical geology lecture outline - Igneous Rocks

Lecture index:

Image to right: lava fountain from Kilauea basalt flow eruption. Source USGS - .

Types of rocks

rock -> a naturally occurring aggregate of mineral matter.

Major divisions of rock types:

Sketch of part of Yosemite National Park, deep in the heart of an igneous world, the Sierra Nevada Batholith. Image Source:

How do we tell an igneous rock from a sedimentary one from a metamorphic one?

Igneous rocks overall

Igneous rocks and the process of crystallization (cooling and phase change from liquid to solid):

In what different ways can the crystallization process be studied and observed?

Bowen's reaction series - a schema for the order of mineral crystallization from silicate melts. This is a very useful framework for understanding igneous rocks.

Grain size texture:

This rock is from a very shallow intrusion in Death Valley. It shows the larger phenocrysts (the larger white and rectangular crystal of feldspar) in the finer grained matrix. Much smaller feldspars also exist.

Volcanic rocks

How does one identify a rock as volcanic?

Volcanic rock types from poorer in SiO2 to richer in SiO2: komatiite (rare) --> basalt --> andesite/dacite --> rhyolite. For each, Bowen's Reaction Series predicts what mineral type of phenocrysts should be present (if phenocrysts are present).

3 tectonic settings for volcanism:

How do you melt source rocks to create magma?

Figure of phase space for silicate melt. The space makes intuitive space. The hotter the more likely it will be molten. The higher the pressure the more likely it will be solid. Note that material can move from the solid to the melt field in at least three different manners: reducing pressure while keeping temperature constant, increasing temperature, and by shifting the boundary .
Types of volcanic eruptions?

shield volcanoes versus strato- or composite volcanoes.

flood basalts and volcanic plateaus and large igneous provinces (LIPs):

What is the magmatic plumbing beneath volcanic activity?

Schematic diagram showing how volcanic rocks and features at the surface are connected to shallow and deeper plutonic rocks and features at depth. Diagram from USGS site:

Plutonic rocks

Map and/or cross section relationships of plutonic rocks give insight into their genesis.

What are intrusion geometries:

What are plutonic rock types from ultramafic to mafic to sialic (remember Bowen's reaction series as a framework for different compositions of igneous rocks):

So a gabbro magma chamber may have fed basalt eruptions above and a granite magma chamber may have fed rhyolite eruptions above.

How and why do magmas ascend through the crust?

Image of a sheet like (tabular) intrusion cutting the Madison limestone in Spearfish canyon in the Black Hills. This body is part of a suite of intrusions of Tertiary age found concentrated in the N part of the Black Hills uplift. Devil's Tower is a part of the suite.

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