GEOL388
6-3-04
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:
- Volcanism
- Erosion
- Faulting.
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 throught he solidification of magma, but how do magmas form? Factors that influence melting point:
- Temperature. Every mineral has a distinct melting point. All other things being equal, the hotter you make a rock, the more likely it is to melt. (Duh)
- Pressure: Material at high pressure "wants" to stay solid so it s molecules can be closely packed. All other things being equal, the greater the pressure, the less likely materials are to melt. (This explains why the asthenosphere is limited to a shallow region of the mantle. Go deeper and the pressure is too great for melting.) When rocks experience decompression without losing their heat, they can experience decompression melting.
- Volatile substances: Generally, the addition of substances like water or CO2 to a mineral lowers its melting point. Take a really hot rock and add water, and you are likely to get magma. BTW, these volatiles would be in solution, just like CO2 in a soda.
Composition: So far we've assumed that all minerals have the same melting point (assuming they are "dry" - i.e. w/o volatiles i nsolution). 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?
- Mid-ocean ridges: Rising rocks in mantle convection cell bring heat near the surface, transfering heat to overlying rocks. At the same time, the hot rising mantle rocks experience decompression melting.
- Subduction zones: As oceanic crust sits at bottom of ocean, it becomes charged with sea water. Subducting slab, although relatively cold, dives into hot surrounding rock. The slab acts as conveyors drawing water into the hotter, drier asthenosphere. When the water percolates into the surrounding hot rocks, melting due to the infusion of volatiles occurs.
- Mantle plumes: Those enigmatic localized upwellings of hot mantle rock from very deep in the mantle, expressed on the surface as mantle hot spots. As in mid-ocean ridges, mantle plume rocks transfer heat to overlying rocks and experience decompression as they come up.
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.
- Emplacement process
- Texture
- composition
Process differences in igneous rocks: We considered this briefly yesterday. Intrustive rocks cool slowly and form big crystals. Extrusive/volcanics 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 called scouria contain with vesicles representing "frozen" gas bubbles.
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.
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 extruxive/volcanic rock types.
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, we will probably 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 whihc it erupts and the kind of volcanoes is constructs:
- Felsic magmas are viscous, often have large quantities of water vapor, and tend to erupt explosively, yielding volcanic ash that falls tot he ground to form welded tuff. When they erupt as lava, the lava, being viscous, doesn't flow readily. The effect is more like that of very stiff toothpaste being squeezed out of a tube. Think of a "constipated volcano."
- Felsic magmas are most common in continental settings and predominate in subduction zone volcanic arcs.
Some specialties of felsic eruptions: The following phenomena are unheard of in the Galápagos and other oceanic hot-spot volcanoes, but are very common in felsic subduction zone volcanoes, such as Cotopaxi.
- Pyroclastic flows: Occur when a cloud of ash and hot gasses traps a layer of beneath it and slides downhill at speeds up to 200 Km/hr.
- Plinian eruption: Occurs when the heat of the eruption creates a rising column of tephra laden air. Such a column may distribute pyroclasts to high altitude winds, resulting in widespread ash fall, or may collapse, resulting in large pyroclastic flows.
- Lahars: Mudflows caused when new volcanic deposits meet water (rain, fallen snow, or glaciers.)
- Ediface collapse: The sudden structural failure of a volcanic cone, such as St. Helens, 1980. This results in the rapid decompression of the magma chamber and a truly spectacular explosion. (St. Helens before and after)
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, they are stuck photographing these because the eruptions of felsic magmas are too dangerous to view up close.
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.)
- Peculiarities of mafic lavas: Because these tend to have less volatility and to be more fluid, mafic lavas retain textures indicative of flow, including:
- Pahoehoe: Ropy lava - formed when a lava flow skins over while flowing very slowly.
- Aa: Jagged lava formed by the shattering of a solid skin that forms over a rapidly moving flow.
- Vesicular basalt: Even basalt often contains some volatile substances. When these form bubbles, the result can be vesicular basalt.
- Cinders: Acorn sized fragments of basalt resulting from fountain-like eruptions. These, of course, are what make cinder cones.
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:
- Crater: Pit or depression at summit of most volcanoes.
- Caldera: Large basin resulting from collapse of volcano upon withdrawal of magma. Resurgent caldera - a caldera showing renewed volcanism. Calderas occur on a wide range of scales.
- Diamond Head on Oahu, HI is relatively small.
- Valles Caldera in Jemez mountains of NM encompasses over 200 km2. Is large enough that it is difficult to visualize on the ground.
Galápagos calderas are small, as calderas go. Most of the young volcanoes of Isabela and Fernandina have small calderas at their summits, where tourists can't hope to see them. One visible caldera is Darwin Bay on Genovesa, which has been breached and flooded by the ocean.
- Ash flow deposits The product of large explosive eruptions. Example: A filled in valley near the Valles Caldera.
- Lava tubes: Tubes formed when an active lava flow skins over then the liquid lava flows out, leaving the heardened outer surface roofing a tube. In the Galápagos, we will see both small and large versions.
- Spatter cones: Cones formed when erupting gasses bubble from a single spot, splashing lava into a cone or cylinder.
- Dike: A small intrusion in which magma was injected into a crack, resulting in a sheet of igneous rock that cross cuts adjacent layers. Dikes frequently occur as swarms near volcanoes. We are likely to see them on the tour.
- Sill: A small intrusion in which magma was injected between two preexisting rock layers. E.g.: The Palisades of New Jersy and New York. We may see these on the mainland.
An island oddity: We've hammered the idea that mafic magmas erupt non-explosively as fluid lava. Oddly, though, one finds in the Galápagos and Hawaii a good bit of welded tuff made from basaltic ash. It seems that basalt can erupt explosively after all. This happens because of Phreatic explosions. These occur when steam is generated by sea water or ground water abruptly encountering magma. In an island setting, there are plenty of opportunities for this. Welded tuff produced in this way has a special name - palagonite. We will see Palagonite cones caused by phreatic explosions. Example Kicker Rock near San Cristóbal.
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:
- Effect of prevailing winds on land forms: As Darwin noted, when lwo lying cinder or palaginite cones are breached by waves, they tend to be breached from the South because it is from that quarter that the prevailing winds blow.
- On some islands, when one strolls on the beach, one notices surf rounded boulders, cobbles, and pebbles of native rock (usually basalt). On others, one doesn't. lava or weldd tuff simply marches into teh sea. This difference help one distinguish volcanically active islands from extinct ones. On the oldest island of the archipelago - Española, for instance, much of the island is covered with such rounded boulders - evidence that it hasn't been resurfaced since the Holocene highstand about 5000 years ago.
- The most subtle effect concerns what's often missing. If the Galápagos volcanoes are erupting buckets of basalt, one would expect the breakdown products of the weathering of these basalt islands to be black basalt sand, and yet such black sand beaches are absent there and rare even in places like Hawaii. Instead, beaches are made of quartz sand and the ground up remains of sea critters.
WTF?
This reflects an interesting attribute of minerals. They tend to crystallize from magma under conditions of temperature and pressure at which they are stable. Most of the mineral components of basalt crystalize at high temperature, while quartz, the major component of beach sand, is one of the last to solidify, crystallizing at temperatures and pressures closer to those of the Earth's surface. You could say that quartz is "more nearly at home" at the surface, and therefore resists weathering processes that quickly break down the dark minerals of basalt. The result is that even thought he oceans are floored with basalt, most of the worlds beaches are paved with quartz sand.
