The Galápagos Islands, situated 600 miles off the coast of Ecuador on the equator, are largely the product of volcanic and geologic forces that remain as active as ever.
The islands sit on the Nazca tectonic plate, which is moving toward mainland South America at about three inches per year. As they move, the islands pass over a geologic "hot spot." Those islands over the hot spot, such as Santiago, have had volcanic activity within the last century. The easternmost islands, however, have already passed over the hot spot and their volcanoes are now extinct. On many of the islands, the black lava that created them is still readily seen.
Except for the highlands of Santa Cruz, which gets considerable moisture from clouds and mist, most of the Galápagos Islands are dry and vegetation is sparse and desert-like. Several varieties of cactus are found, the most interesting of which is the opuntia cactus, which has evolved differently on different islands depending on the native animals. On islands where tortoises are found, the opuntia cactus has developed a thick bark-like outer covering that the tortoises find difficult to eat. On other islands, where land iguanas abound, the opuntia cactus has instead retained traditional cactus-like thorns to dissuade the lizards. On islands where there are no threats, the sharp thorns have turned into harmless, hair-like growths.
Other plants have also adapted in unique ways to the desert environment. For example, some of the bushes in the Galápagos orient their leaves vertically so as not to absorb too much of the harsh rays of the equatorial sun. Most plants also have very small leaves to prevent the loss of moisture, which no doubt contributed to Darwin's assessment of them as "wretched-looking little weeds."