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 3 and 4 of Tarbuck and Lutgens.
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 material from last week's online notes. You can go back to the syllabus and look up the notes for chemical bonds and properties of the rock-forming minerals.
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 3 is concerned with the evolution of magmas and igneous rocks in general, focusing on classification using textural and compositional characteristics. There is also a discussion of how magma evolves and changes during the heating and cooling processes below the earth's surface, and some discussion of the how it all relates to plate tectonics. Chapter 4 takes up the subject of the actual processes involved in volcanic eruptions and emplacement of intrusive bodies or plutons. 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 3
| TYPE OF VOLCANIC FEATURE | TYPE OF ERUPTION | TYPE OF MAGMA | PLATE-TECTONIC SETTING |
| Shield volcano, e.g. Mauna Loa, Kilauea | non-explosive, flowing lava, may have lava fountains | Basaltic; pahoehoe or aa lava | Hot spot, ocean floor |
| Flood basalt or plateau basalt, e.g. Iceland, Columbia Plateau | fissure eruptions on land, lava flowing over broad areas | Basalt | Hot spot (early stage of development) |
| Cinder cone, e.g. Paricutin, Mexico; Sunset Crater, Arizona; Cerro Negro, Nicaragua | pyroclastic but not necessarily explosive; often occur in groups or "swarms"; may be associated with larger volcanoes but cinder cones are smaller than other volcanoes | Basalt to andesite; even if basaltic, magma is cooler and more viscous than examples listed above | Generally on land, various settings; may be associated with areas of incipient continental rifting and uplift, or with last phase of activity in a region of basaltic flows |
| Pillow basalt | underwater eruption | Basalt | mid-ocean ridge or hot spot |
| Stratovolcano or composite volcano, e.g. Pinatubo, Mt. St. Helens, Krakatoa, Katmai | explosive, with large amounts of ash, pyroclastic flows, some lava flows | Andesite to dacite | convergent boundary, created by partial melting of oceanic lithosphere in subduction zone |
| Caldera, e.g. Taal, Crater Lake, Kilauea and Mauna Loa summit calderas | varies, but generally occurs by collapse of surface above depleted magma chamber | Basalt, andesite or rhyolite; the largest ones are associated with rhyolitic magmas | various |
| Large rhyolite caldera complexes, e.g. Yellowstone (click here for more); Long Valley Caldera, California (click here for more) | highly explosive, huge volumes of pyroclastic debris - world's larges eruptions (none in recorded human history, luckily for us!) | Rhyolite or dacite, forming ignimbrites and welded tuffs | Continental location above hot spot, rift zone or subduction zone, caused by partial melting of continental crust in contact with basaltic or andesitic magma rising from the mantle |