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.

• Felsic magmas are viscous, often have large quantities of water vapor, and tend to erupt explosively, yielding volcanic ash that falls to the ground to form welded tuff. When they erupt as lava, the lava, being viscous, doesn't flow readily, instead forming lava domes. 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. Why?

  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.

  • Lahars: Mudflows caused when new volcanic deposits meet water (rain, fallen snow, or glaciers.)

  • Edifice 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). For and Andean analogy, consider Pasochoa which experienced a St. Helens'style ediface collapse just prior to the Spanish conquest in 1533.

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.
  • 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.

• Shield volcanoes: Scale - very large - Large, with gentle slopes, formed of copious fluid non-explosive eruptions. Large examples on Earth may be 10 km high from base to summit and 120 km in diameter. Generally associated with hot spots. The Galápagos volcanoes are mostly of this type. Example - Pinzon. Many of the younger Galápagos volcanoes have a slightly odd profile called the inverted salad bowl, caused by their having a large crater and slightly inflated flanks. Example, Fernandina, right, Mauna Loa, HI. To see real monster shield volcanoes, you have to go to Mars. Why should that be?
  

  

• Cinder-cone volcanoes: Scale - Small, consisting of cinders (acorn sized chunks of scouria) in unconsolidated mounds. Forms higher angle slopes that are a function of the angle of repose of the cinders. Typically mafic magmas. Example - Haleakala Valley, Maui, HI. Many Galápagos volcanoes are studded with small cinder cones. E. G. Santiago. (Right: Cinder cone in Haleakala Valley, HI.)
  

  

• Composite volcano: (also "stratovolcano") Scale - medium. Volcano built of alternating lava and pyroclastic deposits. Often erupt explosively (E.G. Mt. St. Helens, 1980.) Large but frequently steep sided. Example - Cotopaxi, Ecuador. Usually built from felsic to intermediate magmas. (Right: Cotopaxi, Ecuador.)
  

Associated structures:

• Crater: Pit or depression at summit of most volcanoes. (Right: Crater of Cotopaxi, Ecuador.)
  

  

• 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. (Oh no! Try not to think of the earworm!)
  • 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. A couple of exceptions:

    • Darwin Bay on Genovesa, a caldera that has been breached and flooded by the ocean.
    • Volcan Ecuador at the northwestern tip of Isabela has been cut in half by a fault. Thus from the ocean one views the interior of the caldera.
  (Right: Crater Lake, OR.)
  

  

• Ash flow deposits The product of large explosive eruptions. Example: A filled in valley near the Valles Caldera. (Right: A filled in valley near the Valles Caldera, near San Ysidro, NM.)
  

  

• 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. (Right: Collapsed lava tubes on hillside, Bartolomé.)
  

  

• Spatter cones: Cones formed when erupting gasses bubble from a single spot, splashing lava into a cone or cylinder. (Right: Spatter cones, Bartolomé)
  

  

• Driblet cones: (AKA "hornitos" - "little ovens") Like spatter cones on a smaller scale. (Right: Driblet cone, Santiago<.)
  

  

• 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. (Right: Dikes near Volcan Ecuador, Isabela.)
  

  

• 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. (Right: Sill, Svalbard.)
  

  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. (Right: Mafic tuff, Santiago.)
  

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

• Beachrock: Beach sand that has been cemented by the precipitation of minerals from solution.
• Reef rock: Limestone made of the stranded remains of marine organisms. Examples:
  • North shore of Baltra: The light layer is a stranded reef that formed about 5000 years ago when sea level was higher.
  • Urbina Bay: An earthquake in 1954 uplifted over a square kilometer of ocean floor stranding corals.

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 low lying cinder or palagonite cones are breached by waves, they tend to be breached from the South because it is from that quarter that the prevailing winds blow. (Right: Darwin Bay, Genovesa.)
  

  

• Rounding of boulders: 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 welded tuff simply marches into the 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 sea-level highstand about 5000 years ago. (Right: Española.)

Faults

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

• Fault block names: A fault separates adjacent rock into two blocks. Unless the fault is exactly vertical, we distinguish two kinds of blocks. To make this easy, imagine that you are actually stading on the fault plane, as miners sometimes do:
  • Hanging wall block: The block above your head
  • Footwall block: The block on which you are standing.
  
  

  

• Kinds of faults: Distinguished based on the orientation of the fault plane and the sense of movement.
  • Strike-slip faults: Faults in which neither block is downthrown but the sense of motion is parallel the the fault plane.
  • Dip-slip faults:
    Normal faults: Faults in which the hanging wall is downthrown (i.e. has moved down) with respect to the footwall.
    • Reverse faults: High angle faults (i.e. their dip is at least 30 deg.) in which the hanging wall has moved up with respect to the footwall.
    • Thrust faults: Low angle faults (i.e. their dip is at less than 30 deg.) in which the hanging wall has moved up with respect to the footwall.

  
  

  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:
    • Itabaca Channel between Baltra and Santa Cruz.
    • Academy Bay at Puerto Ayora.