Birth of an Ocean

By Annie Reisewitz

The Sea of Cortez, or Gulf of California as it is known in the United States, is rich in history, which began long before John Steinbeck and Ed Ricketts’ chronicled their seagoing accounts in the 1940’s Log from the Sea of Cortez.

Baja California was part of mainland Mexico until about 10 million years ago when the opposing forces created by the separation of the North American and Pacific tectonic plates caused the Baja peninsula to break away from mainland Mexico. This plate boundary, better known as the southern extension of the San Andreas Fault system, shifted, causing the crust to stretch thin and sink inside the continent.

“This whole evolution is happening very rapidly,” said Scripps geologist Peter Lonsdale who led several scientific expeditions to the gulf, including the most recent in May 2008. “It is in the final stages of becoming a full oceanic basin.” 

Spreading centers along the seafloor are widening the gulf at the same time as the shifting plates are pushing Baja California cities like Ensenada on a northwesterly route away from the Mexican mainland. The new ocean being born in the gulf will be added to what is currently a shrinking Pacific Ocean.

Geologists from Scripps Institution of Oceanography at UC San Diego are using remotely-operated vehicles to excavate rocks from deep sea canyons to map the expanding ocean bottom and document the birth of the next ocean basin.

The process that is taking place in the gulf today is similar to what occurred 130 million years ago when Africa split away from South America and formed the Atlantic Ocean.

Researchers from around the world are taking notes as the continental crust rifts, or breaks apart. As the seafloor spreads over the next million years it will widen the oceanic crust creating a fully-connected ocean basin. By studying the process underway in the gulf today, researchers hope to gain new insight into the history of how ocean basins around the world were formed.

“It’s an ideal place to study the birth of an ocean,” said Pat Castillo, a geologist at Scripps who participated in a 2008 expedition to the region. The only other place in the world where this geological phenomenon is happening is in the Red Sea.

Charting New Oceans

Roughly eight million years ago the elongated depression left by the continent rifting apart was flooded by the Pacific Ocean, creating the subtropical marine conditions that exist in the Gulf of California today. Then about three million years ago, new oceanic crust began rapidly emerging from the earth below, which marked the transition into a new ocean.

The seafloor below the gulf is made up of two types of crust – continental and oceanic. The new seafloor that is forming below supports a unique environment where marine life, such as Vaquita porpoises, migrating whales and whale sharks thrive. The elongated bay is its own underwater wonder with 1,219-meter (4,000-foot) deep submarine canyons, massive underwater mountains and hydrothermal vents teeming with new life.

Scripps geologists discovered these hydrothermal vents more than three decades ago and, more recently, during a late-2008 expedition to the gulf, Scripps biologists have documented new species and marine animals previously never seen alive. (See: Expeditions Reveal Gulf of California's Deep Sea Secrets, As Well as Human Imprints)

Lonsdale has been studying the sea’s rocky history for nearly four decades and has uncovered valuable clues into the geological processes that shape the past and future of this fertile marine environment, which is rich in economically-important resources such as oil and natural gas deposits flanked by active underwater earthquake and volcanoes.

The majority of scientific studies undertaken in the gulf have focused largely on understanding the region’s land geology, yet according to Lonsdale, “the real history of the region’s transition from rifting to spreading is underwater.” the large continental mass, created when the land rifted and submerged over time, comprises much of the seafloor along both sides of the peninsula’s coasts.

In 2003, Scripps research vessel Roger Revelle took scientists on a preliminary dredging expedition to examine the submerged continental crust that comprises much of the gulf’s seafloor as well as the new oceanic crust that is emerging. To collect rocks by dredging, the Scripps team positioned the ship near cliffs and rock outcroppings, and then lowered a large metal-chain bucket off the side, collecting a random sample of rocks and mud that lie below.

According to Lonsdale, early dredging operations were akin to “flying a hot-air balloon over the Himalayas and chucking a rope with a bucket at the end over the side.  Wherever the balloon ends up is where you collect your samples,” said Lonsdale.

The technique is much more refined these days, but obtaining good rock samples by dredging is still challenging. Dredgers more often than not pick up mud and loose stones that have already fallen to the base of the cliffs.  Collecting a mixture of various rock types from all different ages provides only modest insight into the region’s rich seafloor history.

To analyze the region’s transformation from a sea to an ocean, researchers required direct access to the rocks below. The remotely operated vehicle Jason offered them the unprecedented access to the deep sea they were after.

Aboard R/V Atlantis and with ROV Jason in tow, Lonsdale led a 28-day expedition to the region in May 2008, accompanied by a 25-person science team, which included Castillo, two Scripps graduate students and geologists and students from the University of Mexico, CICESE Oceanographic Institute in Ensenada, Mexico and San Diego State University. 

Rock Picking

During the month-long expedition, ROV Jason roamed for 12 hours at a time, collecting large chunks of rocks from deep-sea cliffs and outcrops in 26 different locations along the seafloor.

With the scientists’ trained eyes and Jason’s cameras rolling, the vehicle plunged hundreds of feet to the muddy seafloor where researchers immediately picked up a few rocks to fill the crates fastened to the craft before they paced themselves for the remainder of the overnight dive.

The team witnessed hydrothermal vents expelling super-heated water —in regions where they have never before been documented — and got up close and personal with stingrays and squid.

Scripps graduate student Jared Kluesner is studying magma intrusion in the gulf that is forming the new ocean floor. As the magma rises, it heats the surrounding sediments, generating super-heated fluids and gases that are rapidly expelled from the seafloor through hydrothermal vents.  This process, according to Kluesner, provides evidence that these vent systems are directly caused by intrusions.

The team returned to Scripps with four tons of Gulf of California seafloor in burlap bags. At first glance, the rocks looked alike until researchers cleaned them up for closer geochemical analysis.

Early analyses so far have offered scientists a better view of the ocean processes taking place as new seafloor emerges.  They have already revealed that a sharp boundary separates the continental and oceanic crusts. This new interpretation contradicts the currently held belief from studying older margins, such as the Mid-Atlantic Ridge, that a fusion of both oceanic and continental crusts creates a “transition zone.”

Lonsdale suggests that the idea of a transition zone was a “psychological problem created by low-resolution tools.” These new detailed analysis in the gulf could change scientists’ view of how other margins around the world developed.

Interpreting geophysical data from older plate margins, such as the Mid-Atlantic Ridge, which took tens of million years to transition from rifting to spreading, is no easy task for marine geologists.  The deeper scientists look into the earth with seismic imaging tools, the less resolution they have to interpret what is happening.

Everything happens fast on the West Coast, geologically speaking.  The rapid evolution from rifting to spreading in just 10 million years could offer an explanation as to why a transition zone didn’t develop in the gulf.

Lonsdale and Kluesner hope to collect more clues into this newly emerging ocean during future trips to the region.  In late June 2009, the research team plans to perform seismic profiling to obtain three-dimensional pictures of the layers of rock beneath the seafloor and outcrop from cliffs. In October 2009 they will continue sampling rocks along the cliffs to get a more-complete view of the new ocean basin.

Geologists cannot predict what this new ocean will looking like. It will take several millions of year before it fully forms, if not interrupted by unknown tectonic forces before then. Meanwhile, Scripps geologists are hoping to obtain more detailed pictures of how these movements are shaping the planet’s future.