Geography 111 - Principles of Geology

Notes for the third and fourth weeks of class

The material on the syllabus for this week includes magmas, igneous rocks, and volcanic activity. These topics are covered in chapters 4 and 5 of Press and Siever.

The outline at the bottom of this set of notes follows the sequence of topics presented in the text. Before working through this outline, a few basic points are in order:

First, igneous rocks are all derived from cooling and crystallization of magma, molten rock upwelling from the earth's interior. All magmas are composed almost entirely of materials that crystallize to form silicate minerals, and therefore we need to start with an understanding of the major groups of silicate minerals and how they are different. Therefore it is useful to review some basic information about the silicate minerals. Recall that there are several basic structural groups, all based on the silica tetrahedron as a building block, with different degrees of coordination among tetrahedra within the crystal structure.  (While you are at it, you might also review some of the basic physical properties of minerals.)

The basic structural groups, with the name of a prominent group of minerals belonging to each, are as follows:

• Isolated tetrahedra, also known as nesosilicates      (olivines are the most important group for our discussion)
• Ring structures, also known as cyclosilicates (no dominant group will be discussed in class)
• Single and double chains, also known as inosilicates (we will discuss pyroxenes as single-chain and amphiboles as double-chain)
• Sheet structures, also known as phyllosilicates (we are primarily concerned with the micas and the clay minerals)
• Framework silicates or tectosilicates (we are primarily concerned with the feldspars and quartz)

With the increasing degree of coordination among tetrahedra as you move from the top to the bottom of this list, there is an increase in the relative importance of silicon compared to other elements. We will also find that the olivines, pyroxenes, amphiboles, and some of the micas (e.g. biotite) contain a higher proportion of iron and magnesium, and to some extent calcium as well; whereas the framework silicates and some of the micas (e.g. muscovite) contain more silicon, aluminum, potassium,  and sodium. You can see this if you look at some of the typical chemical formulas (each group actually contains many different minerals with some variation in the chemical formulas):

Not all magmas are the same, and igneous rocks take on many different textures and compositions depending on the origin and evolution of the magma and its cooling history. We typically differentiate igneous rocks based on TEXTURAL and COMPOSITIONAL characteristics. Each compositional type is associated with a characteristic suite of silicate minerals, and each textural type has characteristics (coarse-grained vs. fine-grained, glassy, vesicular, etc.) that are diagnostic of the environment in which it cooled and the rapidity with which cooling and crystallization took place. Therefore you can get a coarse-grained intrusive igneous rock with a particular suite of minerals; and there may be a fine-grained extrusive or volcanic rock with the same composition and the same suite of minerals. These will be two different rock types, distinguished on the basis of where and how the magma cooled.

It is important to understand that magmas are complex mixtures, and that as they cool, different minerals crystallize from the melt at different temperatures. As crystallization occurs, the chemical composition of the remaining magma changes as well. Furthermore, the crystals themselves may react with the melt and recrystallize as different minerals. The same is true in reverse: when you melt a rock, different mineral constituents will melt at different temperatures, and therefore if you don't melt it completely, you will get a magma of a different composition than the composition of the original rock. Igneous petrology is the branch of geology concerned with these topics, and we will delve only into the most important aspects of this field.

There is also a correspondence between the types of magmas we see and the plate-tectonic environments in which they are found. We will explore this correspondence in class.

Chapter 4 is concerned with the evolution of magmas and igneous rocks in general, but it also includes a section devoted specifically to intrusive bodies of igneous rock and their typical shapes and sizes. Chapter 5 takes up the subject of extrusive rocks and volcanism. In discussing this topic we will consider the correspondence between magma composition, the physical properties of different kinds of magma, how these are related to the style of eruption, and how that in turn is related to the types of volcanic landforms and deposits that will form as a result of the eruption. All of these can be related back to the plate-tectonic context. There are also natural hazards associated with volcanic activity, and these also can be related to plate tectonics in a general way.

Chapter 4

• Igneous rocks:
• Intrusive vs. extrusive
• coarse-grained vs. fine-grained
• other textural characteristics
• Chemical composition
• Ultramafic-mafic-intermediate-felsic
• Relation of chemical composition to crystallization temperature
• Importance of silica content, effect on viscosity
• Major igneous rock types associated with different combinations of chemical composition and texture (e.g. Table 4.2, Fig. 4.6)
• How magmas form
• partial melting
• the influence of water and pressure on melting-point temperature
• Magma chambers and their importance
• Where magmas form
• plate-tectonic environments for magma formation
• mid-ocean ridges
• subduction zones
• mantle plumes (hot spots)
• Magmatic differentiation and the evolution of magmas
• Bowen's continuous and discontinuous reaction series
• fractional crystallization, partial melting, and the granite problem
• the Palisades sill: an example of fractional crystallization in an igneous intrusion
• Forms of intrusion, both discordant and concordant:
• plutons, batholiths, dikes, sills, laccoliths
• veins and hydrothermal processes

Chapter 5

• Volcanism and volcanic deposits
• composition and texture of volcanic deposits
• compositional spectrum: basaltic to andesitic to rhyolitic (mafic to intermediate to felsic)
• textural spectrum
• lavas: smooth and fluid to viscous and blocky
• pyroclastic textures: ash, volcanic glass, vesicular textures; tuffs, breccias, pyroclastic flow deposits
•  Types of volcanoes and their eruptive styles
• shield volcanoes, cinder cones, and composite or stratovolcanoes
• associated features: volcanic domes, craters, calderas and how they form
• explosive eruptions and their relation to volcano type, magma composition, and plate-tectonic environment
• fissure eruptions and flood basalts; relation to divergent plate boundaries or hot spots
• shield volcanoes and hot spots
• global distribution of volcanoes in relation to plate boundaries and hot spots
• hazards associated with volcanoes: pyroclastic flows, lahars, tsunami, climatic effects

Volcano World discussion of different types of volcanoes