GEOL388
6-2-04

Introduction to Galápagos Geology:
The Global setting


John Merck

Galápagos Geography: Like it or not, it is essential that you have the basic features of Galápagos geography in your head. Here's a map of the islands and a list of names and features that you need to be able to locate a/o identify on a map.

Islands: Learn all of the major islands. Today, people typically refer to them by their Spanish names, but in some cases, you need to know the English names as well. We've indicated where you need to learn these. The names in boldface are the ones we'll routinely use:

Isabela volcanoes:

Towns:

Submarine features:

Trust us. Soon, all of these names will be meaningful.

Now for Geology. Althought the Galápagos haven't been the catalyst for any revolution in Geology as they were for Biology, they nevertheless nicely illustrate the 20th century revolution of Plate Tectonics. But first, some basics:

Rocks:

What is a rock? A purely descriptive definition is that a rock is - A naturally occurring aggregate of minerals and other solid material. - Usually, there are several minerals in the aggregate, though some rocks may have only one. The other materials may include natural glasses, organic material, or fossils.

Geologists usually think of rocks in a second important way, however. Please memorize this and recite it like a mantra:

* A rock is a record of the environment in which it formed. *

Rock types:

In Lyell's day, people recognized that rocks fell into broad catagories.

The Rock Cycle: Consider the three basic rock types and how they form:

The material that makes up any rock might have a complex history.

Geologists describe this range of possible histories as the Rock Cycle. As the schematic shows, it actually encompasses many possible cycles.

Additional information

Plate Tectonics

In Darwin and Lyell's time, people expected that the topography and composition of the ocean's floor should resemble that on land. The first practical test of that hypothesis occurred in 1872 when the British government sponsored the first interdisciplinary research expidition to expore the world's oceans - the four-year voyage of the H. M. S. Challenger. The deep oceans defied expecataions:

Clearly the geology of the oceans was unlike that of the continents. Geologists regarded continental and oceanic crust as very different beasts. WTF?

The beginnings of an answer came from an improbably source. The German meteorologist Alfred Wegener (1880-1930) performed field work in Greenland, covered by a continental ice sheet. There, he had ample opportunity to observe the behavior of glaciers. He observed that ice, when greatly compressed, flowed plasticly, allowing the ice sheet to glide slowly across the underlying rock, and apparently began wondering if rock did the same thing on a larger scale. He noticed the following patterns:

Continental Drift: To explain this, Wegener proposed the hypothesis of continental drift: i.e. that the location of continents was not fixed, and that they had "drifted" across the globe. Note: Wegener thought that the continental crust slid over the oceanic crust like glacial ice sliding over bedrock. Problem: While Wegener was a genius at making observations and recognizing patterns, he was not able to provide a theory to explain and predict the movements of continents, i.e. to say how it happened. Of course, no one else could, either, but that didn't really matter. The Geological profession didn't like amateurs claiming to solve puzzles that had defied them for 40 years, so....

From 1929 until 1960, no US textbook mentioned continental drift. In 1930 Wegener died in a freak storm while doing field work in Greenland.

New evidence and reconsideration

Paleomagnetism After WWII, several lines of evidence from the study of the intrinsic magnetic fields of igneous rocks began to come together.

Harry Hess puts the pieces together: Hess was an igneous rock geologist (a "hard-rock" man) who had participated in sea floor geologic surveys in the 1950s and early 60s. At the beginning of the sixties, he finally put the pieces together: Wegener had been right to say that the continents moved, but for the wrong reasons. Hess's view is the foundation of the theory of Plate Tectonics, which has become the unifying theory of modern Geology. Between 1960 and 1970, the academic community was won over to it. Here is its essence: Plate tectonics' explanatory power:

The Galápagos plate setting: The Galápagos, like Hawaii, are primarily oceanic hot spot volcanoes. We see a pattern similar to that of Hawaii as a result:

In some ways, however, the Galápagos are distinct. The big headache comes from the proximity of the Galápagos Rift 50 km. north of the the islands. The rift and the plume are separate and distinct sources of magma, but they are close enough to one another that they interact.

The age of the islands

Knowing that the Galápagos are hot spot volcanoes allows us to unravel some enigmas. The oldest island, Española, is about 5 million years old. DNA analyses of some Galápagos critters, however, suggests that their ancestors must have been on the island at least 8 million years ago. It is now plain that although these particular islands have not been there for eight million years, islands have been present above the Galápagos hot spot for at least that long. Indeed, on the Carnegie Ridge, there is an 8 million year old seamount (underwater volcano) with rounded boulders at its summit. These were formed when rock was eroded in the surf.