A letter from Leo

I received a letter today from Leo, a middle school student in Berkeley, California. He and his classmates are studying plate tectonics, and Leo is doing a research project on Iceland. He writes:

You said on the College of Wooster website that the Icelandic flexure zones are dipping to a magmatic chamber.  Is that chamber what fuels Iceland’s volcanoes?  Do these flexure zones make fissures?  And it seems that these huge folds have to disturb the rocks, so why are there so few earthquakes in Iceland?  You also say that we can use this to learn how oceanic crust is formed?  Don’t we already know how?  Isn’t the sea-floor spreading how the crust is formed?  Or are you trying to figure out how the ORIGINAL crust formed?  Because figuring out the ORIGINAL crust sounds like it would be really fun.  Last question: Can you monitor the flexure zone’s movement?  And if you can, can you use that data to predict volcanic eruptions, if the flexure zones and volcanoes are somehow related?

I’ll do my best to answer all of your questions, Leo, but first, let’s chat for a bit about the geology of Iceland. Iceland is unique because it is a hotspot (an area of very high volcanic activity) that straddles the Mid-Atlantic Ridge (a divergent plate boundary).

The Reykjanes and Kolbeinsey Ridges extend onshore in the southern and northern parts of Iceland, respectively. Through Iceland, the divergent plate boundary is shifted to the east, where it sits on top of the Iceland Plume.

The rift wasn’t always in this location, though. The picture below shows the rift today (1n and 1v), the future rift (4), and the rifts that used to be located in the western part of the island (2 and 3).

The Western Rift Zone (1v) and Northern Rift Zone (1n) are active today. The Eastern Volcanic Zone (4) is starting to become more active. The Westfjords Rift Zone (3) was active until about 15 million years ago, when the rift migrated to the Snaefellsnes-Skagi Rift Zone (2). The Snaefellsnes-Skagi Rift Zone was active until about 7 million years ago, when the rift reorganized into the present-day configuration.

Flexure zones, which develop as a result of seafloor spreading, are one of the reasons that scientists know the position of the ancient rifts. As you already know, Leo, the plates diverge at mid-ocean ridges, and the underlying mantle ascends, melts, and transforms into magma. The magma either cools slowly  (creating a rock called gabbro, which makes up the thickest and deepest part of the ocean crust) or rises up to the surface (through conduits called dikes) and erupts on the seafloor as mid-ocean ridge basalt (MORB).

As the mantle rises beneath a mid-ocean ridge, it melts, creating magma. The magma that cools slowly in the crust forms a thick layer of gabbro. The magma that rises up through dikes and erupts on the seafloor creates the uppermost extrusive portion of the crust.

As you can imagine, a lot of MORB lava is erupted right at the mid-ocean ridge axis. So, early lava flows tend to be buried by later lava flows. As the lavas are buried, they tilt toward the ridge axis, creating a flexure zone. The picture below shows a side-view of a flexure zone. The mid-ocean ridge axis is in the middle of the pink lavas, which have buried the gray lavas, which have buried the green lavas, which have buried the blue lavas.

Side-view of a flexure zone showing lavas that dip toward the mid-ocean ridge axis (center). After Dowland, 2003.

So yes, flexure zones dip toward the magma chamber, and yes, magma chambers fuel volcanoes. However, the flexure zone that I’ve worked on is part of the Snaefellsnes-Skagi Rift Zone (#2), which went extinct about 7 million years ago, so its magma chamber is long gone.

Now, let’s move on to your other questions:

Do these flexure zones make fissures?

Not exactly. Fissures typically develop above magma that is moving around in subsurface dikes. Sometimes, the magma reaches the surface and creates a fissure eruption, like the one pictured below.

Fissure eruption at Krafla.

Can you monitor the flexure zone’s movement?  And if you can, can you use that data to predict volcanic eruptions, if the flexure zones and volcanoes are somehow related?

Good question. Geologists definitely monitor ground movements around volcanoes in an effort to understand when they might erupt, although most of the instruments measure changes to the surface of the volcano. You can find more information about volcano monitoring at the USGS website.

And it seems that these huge folds have to disturb the rocks, so why are there so few earthquakes in Iceland?

You might be surprised by the number of earthquakes in Iceland. You can get up-to-the-minute information about Icelandic earthquakes at the website for the Icelandic Meteorological Office. The map below shows all of the earthquakes that occurred in Iceland over the 48 hours before this blog post. You can see that most of the earthquakes are located in the active rift zone or in the South Iceland Seismic Zone.

Earthquakes in Iceland over the past 48 hours. Red indicates 0-4 hours old; orange is 4-12 hours; yellow is 12-24 hours; light blue is 24-36 hours; dark blue is 36-48 hours.

You also say that we can use this to learn how oceanic crust is formed?  Don’t we already know how?  Isn’t the sea-floor spreading how the crust is formed?

Yes, the ocean crust is formed through seafloor spreading, but we don’t know all the details about seafloor spreading yet. For example, there are a lot of questions about how the flexure zones form. How quickly are lavas buried? How deeply are lavas buried? How much of the flexure zone forms near the axis? How much forms far away from the axis? Are the lavas slipping on one another (like a deck of cards)? We have ideas about these questions, but we really need to do more research to understand the details.

I hope I answered your questions, Leo. Thanks for the email! Good luck on your research project!