Located in the northern Gulf of California, Isla (island) San Lorenzo and Isla Las Animas—part of the Midriff Islands—record geologic processes involved in the creation of the Baja California peninsula over several hundred million years. If you were hiking southeast to northwest along the 17-kilometer-long (10.6-mile) central ridge of Isla San Lorenzo, you would first encounter granite rocks from the Cretaceous Period (146 to 65 million years ago); this light tan rock occupies the southeastern third of the island (image center left). In the central third of the island, you would see mainly older metamorphic rocks from the Paleozoic Era (543 to 248 million years ago); these rocks are brown (image center). At the end of the hike, at the northwestern third of Isla San Lorenzo (and much of adjacent Isla Las Animas), you would find much younger volcanic and marine sedimentary rocks (yellow-brown to light brown, image center right). These rocks were formed by volcanoes and fissure eruptions in and around basins in the growing Gulf of California between 5–8 million years ago. The islands themselves were formed as a result of uplift of crustal blocks along the southeastward-trending San Andreas Fault.
This astronaut photograph illustrates the largely pristine nature of these islands. The islands are located in the rain shadow of mountains on the Baja Peninsula to the west, and arid conditions prevail through much of the year. The scarcity of water has limited human presence on the islands, and allowed flora and fauna unique to each island to flourish, particularly reptiles. The islands are also home to colonies of seabirds and seals, both of which take advantage of deep, productive waters adjacent to the eastern Baja coast. Shallow waters and high levels of nutrients can also lead to blooms of green phytoplankton; two such blooms can be seen along the coastline of Isla Las Animas (image center right, in north- and west-facing bays). Winds and currents roughen the surface waters around the islands, and sunlight reflecting off the water makes the patterns visible (silver-gray regions). Regions of dark blue water indicate calm surface conditions, or the presence of oils and surfactants that decrease surface tension.
Featured astronaut photograph ISS015-E-7928 was acquired May 13, 2007, by the Expedition 15 crew with a Kodak 760C digital camera using a 400 mm lens. The image is provided by the ISS Crew Earth Observations experiment and the Image Science & Analysis Laboratory, Johnson Space Center. The image in this article has been cropped and enhanced to improve contrast. The International Space Station Program supports the laboratory to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth.
