The More Local Galapagos Geology

The previous page looked a bit at the origin of the Galapagos archipelago. This page will look at some of the features that are approachable and intriguing. Volcanic debris is highly variable. the flowability (viscosity) of the material varies with chemistry and temperature. Some volcanoes explode others ooze. Some make solid mountains other collapse in one way or another. Volcanoes, like water and wind, are constantly reshaping the surface of the earth. Geology in action is an awesome thing.

In some cases the volcanic activity is more of a blurp that an explosion. This is a spatter cone or mud cone that forms (most likely) from gases venting through a semi-solid. The surrounding material is hard on your perhaps but it is not really igneous rock.
The image above and the three below show a lava tube or the remains of a collapsed lave tube. When hot lava flows it stays warmest in the center of the flow. Thus the outside can cool and harden and the inside might still flow. In some cases this happens and the result is a hollow tube formed by a hot flow of lava. A few of the larger lava tubes have been made available (usually through private landowners) for public visitation.
The large pit shown is one of two pits (Los Geminos, the twins) on the island of Santa Cruz on the cross-island road from the ferry to town. (Planes land on the small island of Baltra and many people are bussed to a ferry that takes them to Santa Cruz and then another coach to Puerto Aroyo where the tourist boats are often waiting.) The best conjecture now seems to say that there was a large lave tube here and the roof(s) collapsed creating open pits. They could have been some sort of lava bubble I suppose. I have always been interested in the layered look of the edges as I think of a lava tube being formed in a more homogeneous sort of situation rather than within a sedimentary matrix. 

Ash and volcanic debris can take many forms. A hard gray hillside with very limited vegetation and few animals is one form and the sedimentary layers below speak to many ash-laden eruptions.
But it is lava that we usually associate with volcanos and the Galapagos Islands are rich in lava as well. There are two types of lava, both given Hawaiian names. The smooth ropey-looking lava is called pahoehoe, The sharper, more brittle-looking lava is called ‘a’a. The viscosity of the lava is dependant on the mineral make-up of the stuff and the temperature is has reached.
There are several forms of lava that are dependent on the mineral makeup of the flow. Felsic lava is high in silica, aluminum, potassium, sodium, and calcium. Intermediate lava is usually richer in iron and magnesium but lower in aluminum and silica. Mafic lavas are hotter and also are rich in iron and magnesium. 

Cooling can cause crack is any direction. This pahoehoe probably cracked when cooling. This could have been amplified by the substrate shape over which the lava flowed. (There is almost always another possibility.) 
Many lava flows are recent, within the past few hundred thousand years that is. It takes a while for plants to become established and to start the chemical processes needed to break down the lava and to add organic material in order to make soil. This lava flow looked like this when Darwin was here and perhaps when the first finches arrived as well.
The lava flows often reach the sea. The Flightless Cormorant and the Galapagos Penguin often are creatures of old seaside lava flows. 
Some of the lava flows will form chunks or blocks instead of a smooth liquid-like flow. In many cases this sort of flow shows the blocks on the outer surface but the inner flow stay hotter and is actually quite liquid. The measure of a substances viscosity describes its “runnyiness”. A high viscosity will seem almost solid like tar or pitch. The lower viscosity fluids seem thinner. In the hot southwest autos will use a thicker (higher viscosity) motor oil that in the cooler northeast.

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