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For over a year I've been writing about a fictional race of people that has taken on rather a life of its own. They live on an earth like world with similar geological processes to our own but I don't feel like I can quite get their land portrayed accurately.

I know approximately what it looks like, it has a small inland sea with a dormant chain of volcanoes to the east. I've provided a mock up of the sort of thing I was thinking of. I know about different geographical processes that I feel may lead to something like this forming but not sure if I can stitch them all together;

  • Many millions of years ago a hotspot formed under the surface, this volcanic plume over many millennia formed the landmass of the Fortunate Coast as it is now known.

  • The land is a massive lump of igneous rock sitting across two continental plates, the land has, since it formed, been pushed to the north east by the movement of these plates meaning the north east is now geologically dormant as it is no longer directly above the plume while to the south there are many active volcanoes and new land formation along with numerous hot springs and earthquakes.

  • The island to the north and east is honeycombed with old lava tubes and caverns, now filled with water while the movement of the tectonic plates over which the island sits in opposite directions has led to the formation of large rifts in the center of the island that now form the main waterways.

  • Most volcanoes are shield volcanoes meaning that the land is hilly rather than mountainous with a vast shallow ocean surrounding the island.

Does all that sound geologically logical or can anyone point out any major errors I am making here?

enter image description here

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  • $\begingroup$ pretty sure over those times frames erosion would have smooth out the overall outline $\endgroup$
    – ErikHall
    Commented Sep 15, 2023 at 18:34
  • $\begingroup$ An image with outlines and symbols of where these features are would be very helpful. $\endgroup$
    – Martamo
    Commented Sep 15, 2023 at 18:37
  • $\begingroup$ I did wonder about the cragginess of the coastline, I wondered if there were other factors that might affect the curvature of the coast but I think glaciation would be the major one, which isn't really a factor here, or potentially soft rock being eroded away but as this land is mostly formed by volanoes I imagine it'd be fairly homogenous in terms of type of rock. I'll try and get a labelled picture sorted if I can :) $\endgroup$
    – Trotskisaurus
    Commented Sep 15, 2023 at 18:41
  • $\begingroup$ You are putting the cart before the horses. Geography has existed and was very happy for thousands of years before this fancy new science of geology was born. The task of the geologist is to explain, as far as they can, how come that the landscape looks like it looks; and the geological explanations are fickle and change when a young and eager geologist comes of age. In other words, just make up your landscape and describe it; if anybody wants to come up with a possible geological history, that is on them not on you. $\endgroup$
    – AlexP
    Commented Sep 15, 2023 at 19:18
  • $\begingroup$ Just one splendid example: the half-a-century long furious debate about the geological history of the spectacular Channeled Scablands in north-western United States. Eventually, J Harlen Bretz's hypothesis that they were formed by a short but unbelievably gigantic flood won... But he was 96 years old when the Geology Society of America awarded him the Penrose Medal. $\endgroup$
    – AlexP
    Commented Sep 15, 2023 at 19:25

1 Answer 1

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Island yes, continent no

Assuming your island is not that big just add some sea level change and coral reefs (atoll) along the coasts and you can have that shape with no problem.

When you combine volcanic islands and atolls to can get any shape you want. If sea level has recently changed jagged edges are common. Jagged coasts just mean rocky coasts, so you might go for a recent sea level drop exposing old reefs and areas previously below wave base, or a rise submerging beach and outlining high areas. Remember the shoreline is not the the shaped of the island, just where it meets the sea level.

Atolls can exaggerate shapes enter image description here

and volcanic islands have a huge number of shapes.

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The only question is how big the island is. The bigger it is the smoother the shoreline will be, just because at larger scales you are only seeing overarching geologic trends shaping the landscape. Your island cannot be that big to have crater features like that dominating the shape. Hawai'i or Ireland size would be fine but not much bigger.

If you want it to be a continent then no, it is completely impossible. Tectonic forces are the dominant shaper at that scale and can't give you those shapes. Continental margins are composed almost entirely of 130 degree angles or subduction smoothed arcs.

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  • $\begingroup$ Beautiful pictures thankyou :) I think the landmass i'm thinking about is probably TOO big then in that case (UK sized) so going to have to pull in a bit more comprehensive an idea of the geological factors in play. $\endgroup$
    – Trotskisaurus
    Commented Sep 17, 2023 at 11:31
  • $\begingroup$ @Trotskisaurus most of the problems are the massive inland sea and the huge arc around the left bay. the inland sea is too large and the bay is not round enough to be an impact feature and tectonics cant give you that southern wrap around peninsula. subduction arc are fairly flat arcs because they are created by movement in a single dirrection, that curve is far too extreme. otherwise most of the land shape is possible. on an england sized island. $\endgroup$
    – John
    Commented Sep 17, 2023 at 17:15
  • $\begingroup$ I love the idea of a bay and inland sea but I agree with that :) Might have to scrub that and create something more akin to the Bristol Channel. $\endgroup$
    – Trotskisaurus
    Commented Sep 18, 2023 at 11:24
  • $\begingroup$ If you make the bay circular you can keep it, it could be an impact structure. $\endgroup$
    – John
    Commented Sep 18, 2023 at 20:33
  • $\begingroup$ I like that idea :) i'm tempted though to increase the size of the land themically (maybe subcontinental size) and put it between two subduction zones so it'd have a similiar geology to Japan; it could therefore potentially have a large shallow ocean though I may have to read a little more about geology :) $\endgroup$
    – Trotskisaurus
    Commented Sep 22, 2023 at 17:14

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