| Isla San Lorenzo and Isla Las Animas, Gulf of California:
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. |
