The National Oceanic and Atmospheric Administration—NOAA—is a leader in exploring the geologic and biologic mysteries of the deep-sea. Recently, NOAA sent an interdisciplinary exploration team of U.S. and Canadian scientists to witness the birth of new ocean crust off the coast of western North America.
The Explorer Ridge, part of the Pacific “Submarine Ring of Fire”, lies just 60 nautical miles from the northwest United States. It is one segment of the massive Mid-Ocean Ridge system—a 65,000 km long series of seafloor spreading centers where new earth is created. The exploration team on board the research vessel Thomas G. Thompson studied the Explorer Ridge—home to massive volcanoes, violent earthquakes, and bizarre life forms hidden deep beneath the sea.
NOAA’s “Submarine Ring of Fire Expedition,” has helped illuminate the dynamic forces at work along mid-ocean ridges. The nearly continuous, global mid-ocean ridge system snakes across the Earth’s surface like the seam on a baseball. It is clearly visible on this map of global topography above and below sea level. The ridge system forms the longest and largest mountain range on Earth. It winds its way between the continents and is more than 65,000 kilometers long and 1,500 km wide. A steep-sided, flat-bottomed rift valley sits atop the crest of most ridges. This series of mountains and valleys forms at divergent plate boundaries, where the Earth’s oceanic plates are torn apart.
Here, where two pieces of the Earth’s crust pull away from one another and new sea floor is created in the gap between them, the surface of our planet is renewed. Older crust is recycled back into the mantle elsewhere on the globe, where plates collide.
Divergence begins when tectonic activity pulls and stretches part of the oceanic crust, causing cracks along a narrow, linear, rift zone. As the plate thins, the underlying mantle starts to melt, and magma rises into the fracturing crust. Because the rift zone becomes less dense and more buoyant as it thins down and heats up, the crust rises to form a high ridge on the sea floor.
As cracks penetrate deeply into the crust, a rift valley drops between them in the center of the ridge. A tremendous amount of magma is created beneath the rift zones from partial melting of the upper mantle, making mid-ocean ridges the most volcanically active areas on Earth. The magma initially collects in a reservoir a few kilometers below the seafloor. The basaltic magma oozes upward into vertical fissures in the crust. Most of the magma cools and solidifies while still within the plate. These upright columns of rock, called basaltic dikes, form the bulk of the new oceanic crust.
Basalt dikes typically comprise the lower 7-9 km of oceanic crust. On top of the dikes lies a relatively thin (1-3 km) veneer of erupted material known as pillow basalt. Sea floor eruptions occur from long fissures and undersea volcanoes that line the sides and bottom of the rift valley. Lavas pour from the fissures and across the surface of the seafloor, adding a thin coat of new lava (typically <10 m thick) with each eruption. This video, shot off the coast of Hawaii, shows how hot lava responds to contact with cold seawater. The outer skin of the flow solidifies almost instantly while the interior remains molten and keeps moving. This inner turmoil distorts the surface of the basalt into irregular, lumpy pillows.
Along the rift valley, divergence leads to volcanism that creates new oceanic crust, crust that is in its turn broken and pushed apart by continued divergence and volcanism. By this perpetual process, oceans form and widen slowly but surely. An example occurs at the Juan de Fuca Ridge, where the spreading process creates an average width of (c) 6 m of new crust every 100 years.
The spreading rates of ridges vary. Some spread quite slowly, no more than 1-3 cm per year while others spread at rates approaching 10 - 20 cm per year. The rate of spreading influences the topography of the ridge. A slowly spreading ridge displays a steep irregular topography, and is relatively narrow. The Southwest Indian Ridge, curving south of Africa, is spreading at about 1.5 cm/year. In contrast, the Mid-Atlantic Ridge separating South America and Africa is spreading 3-4 cm/yr, producing a much wider profile and more gentle slopes.
Mid-oceanic ridges are laced with fractures and intruded by magma. Seawater seeps into the oceanic crust and becomes extremely hot, as much as 400°C. The superheated water dissolves minerals and carries them along as it circulates through the crust. Eventually the mineral-laden water rises back out of the crust, boiling into the bottom of the sea through ocean floor hot springs, called hydrothermal vents, scattered along the rift valley.
This heated water begins to cool quickly when it re-enters the ocean. Most of the dissolved minerals precipitated out onto the seafloor around the vents. Some of the hot mineral water nourishes a bizarre fauna, completely independent of solar energy that exists only at hydrothermal vents. Chemosynthetic bacteria living in the tissues of these giant red and white tube worms convert minerals to energy. The tube worms are eaten by fish and crabs. This ecosystem thrives in some of the harshest conditions ever discovered to support life. Concealed by the darkness and the depth of the sea, mid-oceanic ridges are nearly inaccessible. And yet the powerful forces unleashed along them have shaped our world and the history of life upon it.
The creation of oceanic crust is one of the most important geologic processes on Earth. Approximately seventy percent of the Earth’s surface is oceanic crust, all formed along mid-oceanic ridges in just the last 200 million years.
The mid-oceanic ridge system is the most significant topographic feature on the sea floor, and its activity has a profound impact on global sea level. When rapid sea floor spreading inflates the ridges, enough water is displaced to raise sea level by hundreds of meters. When divergence slows, the ridges shrink and the oceans draw away from the continental shelves.
Submarine hydrothermal systems are an integral component of mid-ocean ridges. Although venting develops only on a small portion of the seafloor, its effects influence the chemistry of the world ocean. Vent faunas, supported solely by the planet’s internal heat, comprise a unique biological community. Life on Earth may well have begun in the mineral-rich heat of a hydrothermal vent.
In addition, studying the distribution of vent species, which drift from vent to vent on undersea currents, help scientists better understand patterns of circulation and mixing of the deep and intermediate ocean waters.
While we understand many of the basic principles that govern the phenomena of seafloor spreading, we remain in the explorative stage of this research. The Submarine Ring of Fire Expedition provided a window through which to view mid-ocean ridges, and mapped new hydrothermal plumes to be explored in future expeditions. NOAA continues to draw the fascinating connections between volcanic, tectonic, hydrothermal, and biological systems in order to better understand the Earth’s remarkable, evolving geography.