Why are there so many Islands?

I am currently studying geology on the island of Santorini. One of the concepts that is really important in doing that is how the islands in the Cyclades form. For most of the islands, plate tectonics hold the answer.

The idea of plate tectonics is that the Earth’s crust is made up of plates. These plates move around and crash into each other which is what causes most of Earth’s volcanism and earthquakes.

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Map of major plates [1]
At the edges of each of these plates are smaller plates that are called microplates. These are usually grouped with the larger plates because they behave in a similar fashion. However, in the Aegean, it is important to understand that these plates are separated from the main plate by fault lines and sometimes act separate from the main plate.

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In the Mediterranean, the African plate in moving under the Eurasian plate and the Aegean microplate. When one plate moves under another plate. It is called subduction. This creates a trench in the sea floor at the plate boundary which is marked in red in the above map. Parallel to subduction zones are volcanic arcs, including one within the Aegean Sea.

As the subducting plate sinks into the Earth, it pulls the overriding plate with it. This is known as subduction rollback and it makes the overriding plate move faster near the plate boundary.

The pulling from the subduction rollback creates uneven movement within the overriding plate. This leads to extension and thinning of the crust within the plate or microplate.

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Diagram of the subduction zone and corresponding normal faults in cross-section view

In order to thin the crust, normal faults form. Faults are breaks in the earth. Normal faults have a hanging wall and a footwall. The hanging wall is on top of the footwall. In a normal fault the hanging wall moves down relative to the footwall. The faults in the Aegean tend to point in a line from northeast to southwest.

As the hanging wall moves down relative to the footwall in a normal fault, the footwall also moves up relative to the hanging wall. After enough time has passed and the footwall has moved enough, the footwall will emerge above the water as an island. This is how most of the islands in the Cyclades are formed.

However, a few of the islands were constructed by volcanism and are either active or dormant volcanoes. These volcanoes are located on a different type of fault called a strike-slip fault. Strike-slip faults do not have a hanging wall and a footwall, but move horizontally whereas normal faults move vertically.

The strike-slip faults that are located with the volcanoes are special in that they are oblique strike-slip faults, so they move predominately horizontally but also move vertically. This type of motion creates a gap where the magma can move to the surface. Which is why there are volcanoes on these strike-slip faults, but no volcanoes along a strike-slip fault such as the San Andreas.

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Map of the Aegean with volcanoes circled

There are only four volcanic centers within the Aegean Sea. The rest of the islands all formed from normal faults. The four centers in order from West to East are Methana, Milos, Santorini, and Kos/Nysiros. These are circled in pink on the map of the Aegean above.

Plate tectonics created faulting, which then created islands and allowed magma to flow through the crust and make volcanic islands. I think this is a very important concept to understand in order to truly comprehend why there are so many islands within the Aegean Sea.

References:

1. Walter Friedrich, Santorini, Aarhus University Press, 2009

3 thoughts on “Why are there so many Islands?

  1. Hello!

    You have a good sense of organization, and you have a good impulse for which kinds of visuals to include. I hadn’t learned about strike-slip faults before, so I find this post very interesting. In fact, this text is intriguing and caught my attention.

    “The strike-slip faults that are located with the volcanoes are special in that they are oblique strike-slip faults, so they move predominately horizontally but also move vertically. This type of motion creates a gap where the magma can move to the surface. Which is why there are volcanoes on these strike-slip faults, but no volcanoes along a strike-slip fault such as the San Andreas.”

    I read it over 3x and wished you had included an image or two. In fact, when you’re teaching, as you are in this post, include visuals that keep us moving along with your discussion. You could, for instance draw the differences between the strike-slip faults. Take advantage of your field notebooks and use columns, cross-sections, overheads, and close-ups.

    It’s clear that you understand quite a bit and perhaps, your daily experience among other Geologically minded people makes it difficult to write for the rest of us. It’s a good idea to find a blog-buddy or three, and trade drafts before posting. Write for someone you know who is like me—very interested in your course, in Santorini, and I look forward to learning from your experiences and new knowledge.

    I find that it helps one to imagine a reader with a local snack:
    https://www.tripadvisor.com/Restaurant_Review-g482941-d6903571-Reviews-Hungry_Donkey-Oia_Santorini_Cyclades_South_Aegean.html

    A place called the Hungry Donkey, well, take pictures.
    I look forward to your next post.

  2. Sarah,

    You have a detailed post with lots of information, but for someone who has little to no background in Geology, they may have a difficult time picturing and understanding what you are describing. This would be a great opportunity to let your voice come through, use your experiences, diagrams, and even some everyday comparisons that may help breakdown the more complicated Geology lingo.

  3. Agree with both Sheridan and Professor Barron that you have a good understanding of the material, but this post is a little lingo heavy. It would be great, for example, to include a diagram of normal faulting explaining the hanging wall and footwall. Also, adding the stresses to your cross section (where extension is happening). The maps are well chosen, but they would benefit from some annotation. Where all all the islands you are talking about, etc.

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