Geology 125 Mineralogy and Petrology
Igneous Rocks Major Concepts

Goals

After completing this section of the course you should be able to

1) recognize the prominent characteristics of  igneous rocks,
2) recognize and classify the major igneous rock types,
3) write a professional description of the major igneous rock types.
4) have general knowledge with respect to how igneous rocks form.
Igneous Rock Description

What are the major rock characteristics that can be used to adequately describe a rock?

Phanerites (plutonic igneous rocks)

Felsic Phaneritic (coarse grained felsic plutonic rocks) are classified on the basis of the modal abundance of four key minerals:

The color index of the rock is the volume percentage of the rest of the minerals that comprise the rock (e.g., biotite, amphibole etc.) and the following terms can be used to describe the color index of igneous rocks


This classification scheme is presented in Figure 4-8 & 4-9 of your text and was presented as a handout. This classification scheme was also discussed in lab. Given the modal abundance of the minerals that comprise a rock you should be able to correctly classify the rock.
 

Ultramafic Phaneritic (coarse grained mafic plutonic rocks) are classified on the basis of the modal abundance of five key minerals:

An example of one of these classification diagrams is presented in Figure 4-8 & 4-20 of your text, and as a handout in class. Given the modal abundance of the minerals that comprise an ultramafic rock you should be able to correctly classify the rock.

Aphanites and Aphanite Porphyries (shallow plutonic and volcanic igneous rocks)

We discuss the inherent problems of modal classification schemes for fine- to glassy (vitric) rock types. A chemical classification scheme based on the chemical composition of the rock using the abundance of Total Alkalis vs Silica was presented in class. The agreement between this classification scheme and the modal classification scheme is quite good. Thus, it is still worthwhile to be familiar with the modal scheme which was presented as a handout in class.

Tables 4-4 and 4-5 are both helpful in classifying these types of igneous rocks.

Pyroclasts, Tephra and Pyroclastic Rocks (volcanic igneous rocks)

We discussed the origin of tephra and pyroclastic rocks and their classification. Classification schemes are presented in Figures 4-34 & 4-35 and in Table 4-6 of your text. This material was also distributed in class.

These rocks form from material violently injected into the atmosphere from erupting from volcanoes. The material then returns to the Earth surface to be deposited, and thus shares many characteristics of sedimentary rocks (e.g., sorting).

The material is comprised of glass shards, crystal fragments, and rock fragments, and can also be classified with respect to the size of this material (bombs, lapilli) and the physical state of the material during eruption (magma, rock).

Important modes of deposition are from nuee ardentes, lahars, ash-fall, and ash-flow tuffs. The rocks can also be described in terms of the extent of welding.

Petrogenesis of Igneous Rocks

Igneous rocks crystallize from magma. How can we explain the diversity in the composition of igneous rocks? Do we need a distinct magma composition to produce each igneous rock?

Igneous rocks represent a past thermal anomaly, and thus require a heat source to induce melting.

Possible heat sources include:

The composition of the Source Material (gotta have something to melt) has a significant effect on the composition of the melt produced. The composition of magma can be changed after it segregates form the source region by "magmatic processes" The tectonic setting (compressional vs extensional) can impart a distinct geochemical signature on the resulting igneous rock. Movie "In the Path of A Killer Volcano" Employee's Pick!