Crystal, thank you for sharing this photo of the little-known Purple Cave in Dubrovnik, Croatia! From my readings online the purple seems to come from seaweed that grows on the rocks in the area, and it looks really beautiful! I’ll share some pictures if I ever go there myself.
Speaking of rocks, did I mention I absolutely love Physical Geography?
Today, thanks to Sherlynn’s request (she didn’t tell me what she didn’t know) I’m going to be talking about the Pacific Ring of Fire! It’s a commonly discussed area in Secondary School Geography here, but I’m going to try to recall as much as I can about it.
So what’s the Pacific Ring of Fire? It refers to a literal ring of volcanoes that surround the Pacific Ocean here on Earth. Here’s a diagram I found online on how it looks like:
See the lines highlighted red above? That’s what referred to as the “ring”. Don’t be underestimated by its simplistic look in the diagram above, though. This ring here is responsible for 90% of the world’s earthquakes, and 81% of the world’s deadliest ones. Also, 22 out of the 25 major volcanic eruptions over the past 12,000 years are caused along this ring. Amazing, isn’t it?
You already know that, don’t you? Let’s move on to something interesting – the formation of volcanoes themselves, and why they occur. Now, if you would gladly take a look the diagram below:
Volcanoes along the Ring are generally formed by the subduction (going under) of plates beneath each other. Plates are basically pieces of Earth that continuously move around and struggle for space while floating on the Asthenosphere (the magma underground). As two plates collide, the less dense counterpart will eventually give in and start going underneath the one that’s more dense. That’s why in the diagram above, the thin oceanic crust goes under the thicker continential crust.
As the poor oceanic crust goes under it gets hotter (it’s being submerged in magma, mind you) and it begins to melt. Some of this magma, due to high silica and acidic content, will start to rise from beneath the continential plate. If you’re not a science student, don’t worry, I forgot why it happens too! Either way, it rises up like a pimple and pops out (exactly like a pimple) on the continential surface. That’s how a volcano on land is formed!
Bearing in mind that this is just a cross-section, it’s now easy to visualise how entire mountain ranges form as a product of this process. The Andes Mountains in South America are a perfect example of this process! Caused by the subduction of plates under the South American plate, it’s the longest continential mountain range in the world today:
Not only can this range be seen a an extremely long row of magma “pimples”, it’s also an example of how orogenic (mountain-building) processes come into play as well. Remember those fold mountains we used to learn about? The western rim of the South American plate is simultaneously being compressed and pushed upwards. Many major eruptions and earthquakes still happen in the region due to this, and with initial and (now) continuous faulting and uplifting of the rocks in the area, the mountains grow slowly every year. In 4 million years, the Andes will be twice their current height! Amazing, isn’t it?
However, along the Ring of Fire, don’t some volcanoes appear by themselves on the ocean too? That’s because of a similar concept, this time with two oceanic plates colliding instead. The diagram below shows how this works:
The diagram above is pretty self-explanatory, isn’t it! In this case, however, magma that flows out of the “pimple” solidifies and piles up on itself, eventually poking through the ocean surface above.
That’s how Japan was born! Also, that’s why Japan looks absolutely linear. It’s parallel to the Ryukyu Trench that runs to the east. With it being right next to a subduction zone, it’s going to be hit with a ton of earthquakes, which we saw multiple times in recent history. Coupled with that comes some major volcanic activity as well. Mt. Sakurajima and Mt. Fuji are two of the more popular kids on the block, and a simple google search can yield you a ton of results.
However, that’s not the interesting part. A study of the distribution of earthquakes show that the earthquakes of the highest magnitudes occur at such zones as mentioned earlier. With rocks and magma constantly colliding and sliding against each other, friction and tension is bound to build up, and what happens when all that potential energy is released is a massive earthquake. Singapore is right smack in the middle of a plate, and nowhere near any plate boundary. See why we have earthquakes hitting our neighbours but not us? (Look at the first diagram above and see why)
There’s a lot more I can say about the Ring of Fire, but I guess this will have to suffice for now!
Though not part of the Ring of Fire, I’m more interested in the islands right in the middle of the Pacific Ocean – the Hawaiian Islands, and more specifically, the Midway Atoll.
The Hawaiian Islands aren’t near any plate boundary, so why is the Kilauea volcano situated there? The answer lies in plate tectonics, as always. Plates float on the Asthenosphere, leaving them free-floating. However, there are several spots on the Asthenosphere that just want to rise no matter what due to some properties exhibited near the Earth’s core, and these “hotspots” display staticity – the ability to remain at virtually the same spot while the land above moves. Here, take a diagram to the face!
Hotspots remain stationary as shown in the diagram above, but the land above it is moving. What happens? Multiple pimples, as usual! This time, the hotpot leaves a trail of volcanoes, abandoning each one as the land passes the hotspot, and creating new ones as new land passes through. As you can see, the main island of Hawaii is currently over the magma plume head, and that’s why Mauna Loa and Kilauea are still active as shield volcanoes. They look pretty awesome, and flow pretty slowly too. A simple google will sate your curiosity!
So what has this got to do with the Midway Atoll?
The Hawaiian Islands (along with Midway) are all part of this long island chain called the Hawaiian-Emperor chain, and named so for good reason too. This chain consists of a staggering 5,800km worth of underwater mountains whose peaks peek (heh) above sea level. The best part? All the islands are formed by the same hotspot that sits beneath Hawaii this very instant! This means that the Midway Atoll was once at Hawaii’s current location. How cool is that! Through millions of years, northwestward drifting of the plate caused a string of underwater mountains to develop, and this chain will extend even further from the same spot in future.
This means that Hawaii and its volcanoes will eventually undergo isostatic adjustment (where after piling so much weight on itself over the hotspot, it begins to sink after moving away from it) and move northwest. A new Hawaii will take its place, and this cycle will repeat itself till the world explodes. Nothing is static, not even the biggest of islands.
Isn’t Physical Geography wonderful?
As usual, I couldn’t have written this without a few internet readings to help me go along (though I recalled my textbook information for the most bit). Give the following a read!
http://www.geology.sdsu.edu/how_volcanoes_work/subducvolc_page.html
http://www.greenpacks.org/2008/06/06/the-andes-mountains-to-double-their-height-in-4-million-years/
http://geology.com/usgs/hawaiian-hot-spot/
http://goleaddog.com/gis-data-products/satellite-maps/satellite-maps-index/japan-satellite-maps/
Disclaimer: All the above images do not belong to me. All rights belong to their respective owners.
THE RABBIT HATCH. 