Dispatch from the Deep: Shaping the Ocean Floor at the Mid-Ocean Ridges

Part of the Deep Sea Vents Curriculum Collection.

Map Ridge
Circles on map indicate hydrothermal vent sites along the Juan de Fuca ridge. Stars indicate destination of recent expeditions to research vent site. Photo © University of Washington, American Museum of Natural History, and Pennsylvania State University.

Greetings from the deck of the RV Atlantis! It's just after sunrise, and I'm looking out over the vast Pacific Ocean; in the east, the sky is now light. The ocean seems to stretch on forever!

Even though it looks that way from here, I know that the ocean doesn't go on forever. The coast of Washington State is about 300 miles away; and then North America stretches east for about another 3,000 miles until the Atlantic Ocean. But out here, I can get a sense of how much of our planet's surface lies below this blue blanket of water. There's a lot of space to fill!

From approximately 2,300 meters (~1.5 miles) above the mid-ocean ridge at the Juan de Fuca Ridge, it's hard to imagine the shape of the sea floor below, but each night I've been hearing reports from the researchers who have gone down in the ALVIN, so I'm starting to be able to imagine it. Of course, I always knew it wasn't flat! In fact, if all the water in the ocean were drained away somehow, we'd be able to see that the ocean basins have canyons, mountains, valleys, and long flat plains that are similar to those you see on land. We're here in this part of the ocean so that we can study part of the vast volcanic mountain chain that circles the globe like the seams on a baseball; this chain is called the mid-ocean ridge system. These ridges, formed as the Earth's plates separate from each other, rise from the deep sea floor as volcanic mountains. In some places, such as Iceland, the peaks of the mountains rise above the surface of the ocean as islands.

We often think of the Earth as a rock-hard, unchanging sphere, but in truth the Earth is truly dynamic, and the changes that take place can be pretty dramatic! To understand this, imagine that the Earth is a giant egg. The hard shell, the Earth's crust, is pretty thin; and when it cracks a bit, the insides ooze out. The big pieces of the eggshell are the "plates" of our Earth's crust; the edges of each plate are the cracks in the shell.

But why does the Earth's crust crack and move along plate boundaries? To crack an egg, you'd need to tap on it from the outside, but the Earth's cracks form as a result of the intense heat that's inside the Earth—the Earth's internal energy. This heat comes mainly from two sources: the radioactive decay of unstable elements in the Earth's mantle and the energy left over from the Earth's formation.

The Earth's internal heat energy isn't evenly distributed. Some regions in the Earth's mantle are hotter than others; and, like most other substances on Earth, hot mantle rocks are less dense—and thus lighter—than colder mantle rocks. It's all relative; where a hot mantle region is in contact with a colder mantle region, the hotter one "wants" to rise. In other words, the hotter mantle region is more buoyant than the colder region, just as a hot air balloon is more buoyant than the air around it. What does a hot air balloon do? It rises! The same thing applies to hotter mantle rock, which rises just as colder mantle rock sinks. So we get large currents of slowly moving solid rock, hotter rock rising and colder rock sinking, in a process called mantle convection. The convecting (or churning) mantle causes the Earth's crust—the shell of the egg—to crack and the Earth's plates—the pieces of the shell—to move. This process is happening right now—this very minute! At mid-ocean ridges like the Juan de Fuca (and in places like Hawaii), hot mantle is rising. In subduction zones, like the Mariana Trenchs, colder mantle is, in general, sinking.

Plate Spreading
This large fissure observed on the crest of the Endeavor Segment of the Juan de Fuca Ridge (47°57' N,129°6' W) demonstrates that cracking processes are taking place in response to the global tectonic forces that cause the oceanic plates to separate along midocean ridges. Photo © WHOI-J. R. Delaney, principal investigator, University of Washington. Pictures taken by cameras mounted on hull of the submersible ALVIN.

Submarine volcanoes form at mid-ocean ridges because, as the hot mantle rises, some of it melts. This molten rock, or magma, oozes up from the Earth's mantle to fill the gap formed by plates separating from each other, or "spreading apart." Mid-ocean ridges are not just volcanic, though. Like all the plate boundaries, they are also fault zones. In the case of mid-ocean ridges, the Earth's plates are separating because of tension along their boundaries, the cracks in the egg. This tension causes rock to crack, creating earthquakes. The tension also creates the gaps that allow magma to flow up from the mantle below to form the oceanic crust.

We don't usually think of volcanoes as cracks that let molten rock ooze out, and in fact, most of the submarine volcanoes along the mid-ocean ridges don't look like any volcanoes you'd see on land because they are so far below the ocean's surface. The intense pressure and cold water of the deep sea prevent the rivers of lava that you'd see on Hawaii and the exploding eruptions we've seen at volcanoes like Mount St. Helens. Lava flows form cone-shaped structures on land, but submarine volcanoes are built up of jumbled, rounded forms called pillow lavas. These bulbous-shaped volcanic rocks form interconnected tubes. Hot magma flows along the sea floor inside these tubes; and the walls of the tubes insulate the magma from the frigid water. Sometimes submarine volcanoes also form smooth, flat sheet flows of lava; these sheets seem to spill out of open cracks in the sea floor called fissures. So far, I've only seen pictures and videos of the incredible evidence of submarine volcanoes; bulbous or flat, they are always foreign and dramatic. Maybe I'll get to see some "in person"—or at least through the porthole of the ALVIN—if I get picked to go down!

The research teams aboard this expedition are studying just one small piece of the global system of underwater volcanic ridges; the ridge system is over 70,000 kilometers long! And most folks are not even aware that 80% to 90% of Earth's eruptive activity takes place along these submarine ridges. This eruptive activity creates new oceanic crust at a rate as fast as your fingernails grow. But you have to cut your fingernails to keep them from growing too long; what keeps the Earth's surface from expanding as all this new ocean crust is created? Remember those subduction zones I told you about? Those are the fingernail clippers! At subduction zones, old, cold, and dense oceanic crust is actually sinking, slowly but surely—and continuously—down into the Earth's mantle. This is all part of the endless cycle of creation and destruction of crust by the processes of plate tectonics.

In fact, far below me on the deck of the Atlantis, new crust is forming on the ocean floor at the deep sea ridge right this second! And I can't feel a thing! Except the ocean swells—and butterflies in my stomach. Tomorrow we draw straws; one REVEL teacher will pick the shortest straw and get to go down on the ALVIN to see the deep sea vents up close. Keep your fingers crossed for me!