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Many people already make their world maps from continental and oceanic crusts shifting around, which is a great system because it’s actually how it works. But most tectonic maps of constructed worlds that I’ve seen go very large in scale- they have big plates for ocean and for land. But if you look at East Asia, you can see that the boundary of the oceanic and continental crusts are broken up into smaller minor plates. MANY of THE MOST INTERESTING PLACES in the world, East Asia, The Mediterranean, The Caribbean, are interesting because they made of a bunch of MINOR PLATES.
Around the edge of the Pacific is the ring of fire, where the pacific plate subducts underneath the surrounding plates. This means that the pacific plate, being a dense oceanic plate, slides underneath the less dense continental and oceanic crusts to its west. The crust on top gets pushed upwards, and volcanoes are created that build islands. These plate interactions create what we see of East Asia today, the peninsulas, the giant islands, and the small island arcs.
The peninsulas are Korea and Kamchatka, Korea being land that was pulled away from Eurasia as the Amurian plate rotated . As this happened, the land rifted open in the plate’s west, and Lake Baikal was formed. The Kamchatka Peninsula was made by continental crust that was pushed up and built with volcanos from the Pacific’s subduction underneath the continental Okhotsk plate.
The major islands are Japan + Sakhalin, Taiwan, and the Philippines. I… would talk about Indonesia but I don’t know anything about what’s going on here. Japan was basically formed by the Pacific plate subducting under the continental Okhotsk plate, the oceanic Philippine plate subducting under the Okinawa plate, and the Amurian plate subducting under the Okhotsk plate. This complex meeting of plates is why Japan has so many earthquakes and volcanoes. The line along which Japan is formed is easily visible from its shape, going linearly along the Okhotsk plate, and curving to the edge of the Amur plate under which the Philippine plate subducts.
Sakhalin is part of Japan, I think. At least in the island chain. This is the real Japan. During the last glacial period, it looked like this- Honshu being a peninsula connected to Korea, and Hokkaido and Sakhalin being peninsulas off of Russia. This points out that if you’re unsatisfied with what your map looks like, you can always play around with its sea level, or switch your big islands to peninsulas and vice versa. These can make for a more natural and less artificial looking landmasses.
Taiwan was made by the oceanic philippine plate subducting under the continental Yangtze plate. During glacial periods, it was attached to mainland Asia.
The Philippines are created on a belt that sits between the Sunda and Philippine plates. Both of these plates subduct underneath this belt to create the islands. It’s a super complex convergent boundary… I’m taking a geology class this semester, I’ll tell you how it really works then.
The islands in between the Philippines and Taiwan are part of the Luzon Volcanic arc, made by the Philippine plate subducting underneath the belt that the Philippines sit on.
The Ryukyu Islands between Taiwan and Japan were created by the Philippine plate subducting underneath the tiny Okinawa plate. On the other side of the Philippines plate, the Pacific plate subducts underneath it to create the Mariana Islands, ending at Guam.
Coming off of Hokkaido on the eastern edge of the Okhotsk plate are the Kuril Islands, where the Pacific subducts and forms volcanic mountains.
And all the way up north, the pacific plate subducts underneath the continental North American plate to create the Aleutian islands and Alaska Peninsula.
Given all of this, I think that we can reasonably make a rule that may be a bit geologically dubious, but in effect works for world building. On the minor and micro tectonic plates where major continental and oceanic crusts meet, landforms such as peninsulas, and big and small island chains will be created. These will form either along the side of a plate, or at the intersection of the plates. The more plates there are meeting in one place, the more certain that there’ll be a landform there.
But remember! Your world building does not serve the scientific models. Here, plate tectonics only exist as a tool to make realistic and good looking landmasses. You can always creatively override the science!
All of this is to say that around the coast of East Asia, there is a multilayered string of islands between the Pacific plate and the Eurasian mainland.

Some maps By "Eric Gaba for Wikimedia Commons", CC BY-SA 3.0, [ Ссылка ]
Some changes made for intervals in video

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