GEOL388
6-11-08

Volcanic landscapes


John Merck

Intro: Yesterday we discussed the global geological setting of the Galápagos. Today, we'll consider how that geological setting is expressed as surface features. The islands' land forms are primarily the result of three processes:

Of these, volcanism is by far the most important, so we begin with a brief intro to volcanoes in general.

Definition review: Igneous rocks are rocks that form through the solidification of magma, but how do magmas form? Factors that influence melting point:

Composition: So far we've assumed that all minerals have the same melting point (assuming they are "dry" - i.e. w/o volatiles in solution). In fact, a rock's composition greatly influences its melting point. The higher a rock's silicate content, the sooner it melts. Because rocks are usually aggregates of different minerals, we get partial melting, in which the more silicon rich minerals melt first, leaving the more iron-magnesium rich minerals as solids. Likewise, when a magma starts to freeze, the iron-magnesium rich minerals crystallize out first and leave the remaining magma more silicon-rich.

Where does magma form?

How does magma behave? When melting first occurs, it happens mineral grain by grain, yielding tiny pockets of magma. Being liquid, magma tends to be lighter than surrounding material from which it has melted and percolates upward by any available means. As this happens, droplets coalesce, eventually forming large magma chambers.

Rocks from magma

How igneous rocks differ from one another.

Process differences in igneous rocks: We considered this briefly yesterday. Plutonic/intrusive rocks cool slowly and form big crystals. Volcanic/extrusive rocks cool quickly and have small ones. For this trip, we will focus on volcanics.

Textural differences in volcanic rocks: There are two parameters to keep track of here: presence of volatiles and composition.

  • Lavas: Rocks formed from the cooling of magma erupted as a flowing or oozing liquid. In some cases, lavas contain vesicles representing "frozen" gas bubbles.


  • Vesicular basalt at Sunset Crater, AZ

  • Pyroclastic rocks: Rocks formed when magma erupts as an aerosol of fine particles. The particles in this aerosol of molten rock quickly solidify to form volcanic ash. Often, ash fragments are still slightly sticky when they fall, sticking together to form welded tuff.


  • Welded tuff in Chiricahua Mountains, AZ

    Chemical and Mineral composition: I use the chart below in GEOL 100. It shows the important mineral components of common igneous rocks. Its x axis shows the percentage of silica (SiO2) in the rock, the y axis shows the relative abundance of different minerals in the rock. For GEOL 388 we need only worry about the extrusive/volcanic rock types, but note that each volcanic rock type has its plutonic equivalent with identical composition and differing textures - E.G. granite and rhyolite.

    The take-home message is that magma chemistry forms a continuum from silicon-rich magmas to iron-magnesium rich magmas, and that the type of rock you get from a volcano depends on where its magma resides on this continuum.

    Silicon-rich rocks are termed felsic while iron-magnesium rich rocks are termed mafic. In the Galápagos, we will probably see only basalt a mafic rock. On the South American mainland, one would see plenty of rhyolite which is felsic, and andesite which is intermediate.


  • Eruptions: So why do we care about all of this? Because magma composition drastically effects the manner in which it erupts and the kind of volcanoes it constructs: Mafic magmas are less viscous and usually have less water and tend to flow as a liquid after eruptions. These are the eruptions that yield the rivers of red-hot lava that nature cinematographers so love. (Of course, how many get near the explosive eruptions of felsic magmas and live to tell the tale?)

    Mafic magmas are most common in oceanic settings - mid ocean ridges (where they create new sea floor) and hot spot volcanoes. They can occur on continents, however, but there they share the billing with felsics and intermediates. (If time permits, I will explain this.)

    Volcano types: Since magmas of different compositions erupt and flow so differently, it stands to reason that the volcanoes they shape should look different.

    Associated structures:

    Other rock types

    Are all Galápagos rocks volcanic? Almost. There are a few places where you can see sedimentary rocks. These take two forms:

    Erosion

    The Galápagos are so frequently resurfaced by volcanism that one doesn't typically think of the effects of weathering and erosion there. Nevertheless, these are apparant in subtle ways:

    Faults

    Faults: Fractures in solid rock along which movement has occurred.

    Identifying faults: In the field, faults never appear as nice clear block diagrams. In fact, they are inconspicuous and reveal their presence only indirectly. Some keys:

    • Fault planes usually have a characteristic polished striated texture called slickensides.

    • Sometimes, rocks are caught between moving fault blocks and broken into angular fragments called fault breccia.

    • Faulting creates zones of weakness that are attacked by agents of weathering and erosion, so linear stream beds, canyons, and lakes often mark them.

    • The dead give-away is when you happen to notice that a stratum that ought to line up in adjacent places doesn't.

    In the Galápagos, you will probably not see faults directly, but movement along them has influenced the lansdscape. Particularly note:

    • Fault scarps: Cliffs formed by the hanging wall of normal faults. Example: The cliffs on the southeastern shore of Punta Suarez on Española.

    • Graben Valleys: Valleys formed by the downthrown block between two parallel normal faults. Examples:

    It's not usually this easy