Exploring Atlantic Canyons & Seamounts





Clues to a Marine Geologic Puzzle

Lava flows can have different morphologies based on how quickly they were extruded. Pillow lavas are a common and very striking type of flow seen at seamounts.

Lava flows can have different morphologies based on how quickly they were extruded. Pillow lavas are a common and very striking type of flow seen at seamounts. Image courtesy of Submarine Ring of Fire 2002, NOAA/OER. Download high-resolution version (570 kb).

Susan Schnur
PhD Student
Oregon State University

Geologists are scientific storytellers. They study the history of rocks in order to tell evidence-based stories about how the Earth formed in the past, how it is evolving now, and what it might look like in the future. To reconstruct geologic history, we need to measure time, or at least be able to put geologic events in chronological order.

On ROV dives, we get a very limited picture of the seafloor. We see only what is at the surface instead of getting a slice through the layer-cake of geologic history. However, there are still many visual cues that can tell us whether the rocks and sediments we are seeing are old or young and whether they have been moved or modified by seafloor processes.

Strong currents can flow around seamounts, leaving ripple marks on sandy slopes.

Strong currents can flow around seamounts, leaving ripple marks on sandy slopes. Click image for credit and larger view.

Here are some of the things we look for during a dive:

Manganese crust is a type of deposit that crystallizes out of seawater. In this picture a rock has first been altered by contact with seawater (the yellow margin) and then covered in a black manganese rind.

Manganese crust is a type of deposit that crystallizes out of seawater. In this picture, a rock has first been altered by contact with seawater (the yellow margin) and then covered in a black manganese rind. Click image for credit and larger view.

A contact is a sharp boundary between two geologic units. Here we see a clear boundary where sand has slid down to partially cover lava flows on a steep slope.

A contact is a sharp boundary between two geologic units. Here we see a clear boundary where sand has slid down to partially cover lava flows on a steep slope. Click image for credit and larger view.

 

None of these properties alone is enough to establish local geologic history; the precise age of a rock still needs to be measured in the laboratory. However, by mapping out changes in these properties on the seafloor, we can identify geologic “units”—that is, rock groups that formed at the same time.

A very important idea in geology is that new rocks form over old ones. For example, lavas on a volcano flow out one after another, forming a pile with old lavas at the bottom and young lavas at the top. Since we do not get this cross-section from remotely operated vehicle dives, we have to look for “contacts” where two rock units meet each other. For example, debris could partially cover a lava flow, indicating that a landslide happened after the volcano built up. By identifying contacts between units, we can begin to put events in order. This will help us understand the kinds of processes that are changing seamounts even long after they have stopped erupting.

 

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