#subduction

When an earthquake happens, the first thing I do is look at the “moment tensor”, which uses the direction and intensity of seismic waves to tell us how the fault that caused the earthquake moved and how much energy was released. The fault plane solution, derived from this data, shows us two options for how the fault was oriented.

Southern Alaska is dominated by a subduction zone, where the Pacific Plate crashes into and goes under the North American Plate. When we look at the earthquake history over the last 5 years (look at all those earthquakes!) we can see that this is a seismically active area. Based on the depth of the earthquakes, we can also tell if they occur in the North American Plate, the Pacific Plate, or in the area of the mantle between the two.

Not gonna lie, every time I think about the Pacific Ring of Fire, this song gets stuck in my head (and I love this song so play away).

The Pacific Ring of Fire is a ring of volcanoes around the Pacific Plate Subduction zones.

It is about 25,000 miles long (40,000km) and it contains somewhere between 750-915 active and dormant volcanoes. This is about 2/3 or the world's total volcanoes!

It is also where about 90% of the world's earthquakes occur as well. Speaking of which, if you didn't hear there was an earthquake that happened on December 5th just off the coast of northern California. The recording will be posted on my Patreon so go join and check it out!

Now, this ring of fire is a complex system. I remember being taught as a kid that the Pacific Plate is subducting below like four different plates but that just isn't right. It's actually several plates interacting with each other. Let's zoom in.

We usually only hear about the large plates but there are tons of smaller ones around the globe. For instance, all the ones in this map. The red is the Philippine Plate which is subducting beneath the Eurasian Plate. As you can sea, the Pacific Plate is further out. Not subducting.

The Pacific Plate is subducting under parts of North America, but so is the smaller, older Juan de Fuca and Cocos Plates.

The Nazca Plate is being subducted along the west coast of South America as well as part of the Antarctic Plate. Just for funsies, the map below shows how much has been subducted and what the depth is.

The Ring of Fire has existed for a little over 35 million years. That means subduction of these plates began in the Late Eocene Epoch.

The largest volcanic eruptions in human history occurred in the Ring of Fire. We will start with the 1902 Santa Maria eruption in Guatamala. The volcano is estimated to have started erupting about 103,000 years ago during the Pleistocene Epoch. However, by 102 it was considered dormant. The 1902 eruption had a VEI (Volcanic Explosive Index) of 6 (the highest being a 7) which makes it "colossal".

Mt. Pinatubo activity began about 1.1 million years ago (known as Ancestral Pinatubo). The modern Pinatubo began erupting about 79,000 BC. It's eruption in 1991 is the most famous though. It was the second largest eruption in the 20th century but a lot more devastating. It was also rated a 6 on the VEI and it ejected about 10 cubic km worth of material (10 times larger than the Mt St Helen eruption a decade earlier).

Before and after shot of the river valley near Pinatubo.

Lake Pinatubo now fills the crater left behind by the eruption.

One of the most famous is the 1883 eruption of Krakatoa. It's eruption was so violent that it was heard in Perth Australia nearly 2,000 miles away.

It caused a massive pressure wave that was recorded on barographs around the world (in some cases, the wave was recorded 7 times!). It created devastating tsunamis and killed at least 36,000 people. The eruption lasted from May until October peaking in August when nearly the entire island collapsed into a caldera. Ash fell over approximately 1500 squares miles and the eruption caused a volcanic winter. This one is also categorized as a 6 VEI.

The largest eruption in the Holocene was that of Mt. Tambora in 1815. The volcano had been dormant for centuries caused by gradual cooling of hydrous magma in its closed magma chamber. At depths of abot 5,000-15,000 ft, the exsolution began to form causing an over-pressurization in the chamber (about 58,000-73,000 psi) with temps ranging between 700-850 degrees Celsius (1,290-1,560 degrees F). It is thought that the explosions from the initial eruption could be heard clear over in Thailand over 2,000 miles away.

Pyroclastic flows wiped out the village of Tambora and 13 ft tsunamis hit many Indonesian islands. The explosion had an estimated VEI of 7 with an estimated 10 cubic miles (41 cubic km) of ejecta weighing 10 billion tonnes. The caldera left behind is about3-4.5 miles across and nearly 2300ft (700m) deep. The eruption released the amount of energy equivalent to 33 gigatons of TNT. This was such a large eruption that it caused the Year Without A Summer in the northern hemisphere. The dramatic cooling directly and indirectly killed 90,000 people. Way to go Tambora.

The largest earthquakes on Earth as well including the Sumatra earthquake in 2004 whose 100ft high tsunami killed over 200,000 people in 14 countries the day after Christmas.

There is also the Greta Chilean Earthquake in 1960 estimated at a magnitude 9.4 on the moment magnitude scale. An 82ft tsunami battered the Chilean shores and effected Hawaii, Japan, the Philippines, New Zealand, Australia and the Aleutian Islands.

All in all, the Ring of Fire is a deadly mix in geologic terms and insanely fascinating as it gives a spectacular look into the inner workings of our planet.

IT'S MID-OCEAN RIDGE BASALTIN' TIME

After getting too many episodic-tremor-and-slip stomach rumbles, Cascadia goes absolutely feral and tries to eat/subduct the Juan de Fuca Ridge. The words in the last panel are courtesy of @iamthepulta. The line gets even sillier when you take into consideration that I imagine Cascadia to have an Austrian accent.

The Cascadia subduction zone is nowhere near doing this IRL, but it (or its precursor) did subduct part of the Farallon-Pacific Ridge in the past. I have drawn Cascadia grabbing Farallon-Pacific by the throat and dragging her into his trench, and in this comic he's doing it to the Juan de Fuca Ridge, so I guess he just uses chokeholds on ridges he wants to subduct. The Farallon-Pacific Ridge did not get completely subducted, with large portions of her ending up being strewn over the continent. And of course Cascadia’s southern portions got replaced by the San Andreas Fault in the process.

I imagine Cascadia’s true mouth is not the one he has on his face but the trench on his underside, and the big flaming maw on the right represents that in cross section. I have drawn the Juan de Fuca Plate as subducting at a much steeper angle than it actually is (it actually is almost horizontal) for dramatic effect.

“How about now? I’m free right now.” Portrait in acrylic inks of Wes Chatham as the superbly deadpan Amos Burton from The Expanse. This is the memorable scene in Season 4’s “Subduction” where he’s facing off against Murtry.