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History and Geology of Guaymas Basin

The Gulf of California, also called the Sea of Cortez, is the body of water separating mainland Mexico from the peninsula of Baja California. The first European to sail into the Gulf was the Spanish explorer Hernan Cortez in 1532. Cortez, however, mistook Baja California for a large island. Seven years later, another Spanish explorer, Francisco de Ulloa, discovered that Baja California was in fact part of North America.

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Geological History
Five million years ago, the Gulf of California did not exist. How did it form? The answer lies in the piece of the Mid-Ocean Ridge called the East Pacific Rise. The northernmost part of the East Pacific Rise runs along the Mexican coast. About five million years ago, this segment of the East Pacific Rise began to spread. As a result, a thin sliver of continental crust in western Mexico started to rift apart. The shallow sea that formed is now the Gulf of California.

The Gulf of California seafloor is spreading at a rate of five to six centimeters every year. Seafloor spreading in the Gulf of California is unique. In most places, seafloor spreading occurs along a continuous ridge. In the Gulf of California, it takes place along short ridge segments. These segments are separated by transform faults. One of these short spreading centers is in the Guaymas Basin.

Why There Are No Eruptions in Guaymas Basin
If you dived down to most parts of the Mid-Ocean Ridge, you would see volcanoes and evidence of big eruptions. However, you won’t see these things in Guaymas Basin. That is because a huge layer of sediment smothers the seafloor. This sediment layer is made of sand and mud that have washed into the water from the shore at a rate of 1 to 3 meters every thousand years. This layer of sediment is so thick (up to a kilometer) that magma cannot penetrate it. Instead, magma moves part way up into this sediment layers and forms a layer of volcanic rock that is a 1 to 20 meter thick. Think of it as a layer cake. The sediment is the cake. The volcanic rock (also called basalt) is the icing between the layers.

What’s Different About Guaymas Basin Vents
Hydrothermal vents are common in Guaymas Basin. However, there are important differences between the chemistry of these vents and the chemistry of vents found on most other parts of the Mid-Ocean Ridge. These differences are caused by the sediment layer. The heated seawater must move up through hundreds of meters of sediment before reaching the seafloor and the lush communities of creatures that thrive there. Over millions of years, this scalding hydrothermal fluid has converted the organic matter in these sediments into petroleum or oil products. This oil is toxic to almost all living things on earth. Therefore, organisms that flourish at these vents not only must withstand the heat, pressure, and toxic chemicals normally found in vents, they must survive the oil as well.