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I have a magical setting I am working on in my world that is based on a large volcanic island that has been created by a sunken, fantastically large caldera. Up to this point, my story has centered around a fairly large city with a grand royal palace built into the side of a mountain, but I am just now realizing that the building materials required to achieve such a setting might not be available.

I am still working on the details, but so far the island is based around an ancient super volcano which has sunk into the ocean creating a huge caldera sea (like a giant Santorini). The island itself post-volcanic collapse is around size of Great Britain and is a mix between Hawaii and New Zealand in terms of climate and topography. The volcano itself is central to the magic system in my story.

Up until this point I have been basing the fictional volcano on the real-life Tamu Massif shield volcano in the Pacific Ocean, where it was caused by a huge up-flow of magma at the convergence of 3 separate tectonic plates. However, I am now realizing that this would mean the primary rock of the island is basalt.

I had previously pictured mountain ranges made of granite, including granite quarries and the subsequent worldbuilding associated with that industry, including plentiful ancient ruins and large and sturdy cities with mansions and palaces made of stone. Now I am thinking I need to retool it because according to my research granite just isn't found on volcanic islands.

My primary questions are as follows:

  1. Is there a geological combination of tectonics that could theoretically create a volcanic caldera AND a granite mountain range on the same island? Like a volcano at the edge of two converging plates? If the island is situated at the intersection of three tectonic plates, is it geologically possible for two plate to be converging and thrusting upwards to form the granite mountains and the third plate to be diverging, creating the super volcano and subsequent caldera?

  2. If granite just isn't a possibility, could a large ornate palace be constructed of basalt (or another volcanic stone) that would be able to last a minimum of 500 years (assuming general upkeep is maintained)? From what I can tell in my research basalt is a common building material, but I am concerned since it isn't as hard and dense as granite that it won't hold up for the generations my story requires. I am hoping for stone ruins from previous civilizations on the island (pre-caldera collapse) as well as a "modern" industrial-revolution inspired city that has areas that have been around for at least 500 years.

Thank you in advance to all you geologists out there!

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  • $\begingroup$ An island "around size of Great Britain" is sufficiently large to have each and every kind of rock known to man, basalt, granite, porphyry, schist, marble, limestone, shale, whatever. Even Iceland, which is basically a piece of uplifted oceanic floor, as volcanic as volcanic can be, has some granite; not a lot of it as geologists would count, but enough to quarry and export. (And I really do not see why you so much want granite instead of basalt. Both are excellent construction rock and come in many colours.) (As for durability, we have 2,000 years old Roman brick buildings.) $\endgroup$
    – AlexP
    Dec 28, 2022 at 23:36
  • $\begingroup$ @CemHill, for future reference, we permit one and only one question per post. It's actually a reason to close questions (click "close" and read "Needs More Focus"). $\endgroup$
    – JBH
    Dec 29, 2022 at 1:19
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    $\begingroup$ @jbh Thank you! I am still fairly new here and didn't realize (I did check the guidelines before posting, but I must have missed this one). I will remember that for next time! $\endgroup$
    – CemHill
    Dec 29, 2022 at 12:45
  • $\begingroup$ Have you considered coquina? It's a rock-like substance composed of coral, seashells, and other such material, and would be commonly found around islands, volcanic or otherwise, and it has long been used as a building material in such locations. As for longevity, this was built in 1695 and is still standing. $\endgroup$
    – Darrel Hoffman
    Dec 29, 2022 at 14:35

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Basalt good!

I know the ancient Hawaiians built out of basalt but those ruins are not too impressive. Edinburgh castle sits on a basalt plug but is made out of sandstone which I am sure is easier to work with.

This place is made of basalt.

qasr al-azraq

https://en.wikipedia.org/wiki/Qasr_Azraq#/media/File:Qasr_Al-Azraq.jpg

It did not assume its present form until an extensive renovation and expansion by the Ayyubids in the 13th century, using locally quarried basalt which makes the castle darker than most other buildings in the area.

It looks like something you would find in Minecraft. A place where basalt buildings are common! In any case, basalt is a fine building stone. The trouble is getting it out and into good blocks, because it is hard to cut.

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  • $\begingroup$ Fortunately, columnar basalt comes almost cut already. $\endgroup$
    – AlexP
    Dec 28, 2022 at 23:39
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    $\begingroup$ @ALexP super bonus points if you can post images of a building made of columnar basalt where they built using intact columns. I am hoping for a log cabin type thing. $\endgroup$
    – Willk
    Dec 28, 2022 at 23:48
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    $\begingroup$ Your wish is my command. (Not sure if they are intact columns, but they are basalt logs.) $\endgroup$
    – AlexP
    Dec 29, 2022 at 0:04
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    $\begingroup$ How cool! And even the log cabin callout! Thank you very much @AlexP $\endgroup$
    – Willk
    Dec 29, 2022 at 1:53
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Not a geologist but: basalt has been used as a building and construction material since the dawn of civilization. That said granite is slightly stronger/longer lasting/resistant to weathering than basalt when used in construction. Albeit in your specific scenario (durability of 500 plus years) as long as large blocks are used for for key load bearing structures either would suffice.

The answer to a similar question to your own in QUORA might help. The person answering purports to be a geologist. From reading his answer I think apparently what you would require a preexisting long term volcanic province with uplifting of older granite magma beds as a result of more recent plate movement. Your volcano then lays down basalt on the surface but over time erosion also exposes underlying granite formations. (You really do need a geologist to give a better explanation though.)

Quora Question: Can basalt and granite coexist

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  • $\begingroup$ Generally speaking, basalt comes from melting mantle rock while granite comes from melting continental rock. So, the Hawaiian mountains are not likely to contain any granite core as they are formed from mantle rock. $\endgroup$
    – David R
    Dec 29, 2022 at 15:40
  • $\begingroup$ As I said, not a geologist I wasn't particularity thinking of the Hawaiian Islands BTW Just some other part of the crust that got uplifted by geologically recent vulcanism whihc also contained granite. An example would be good to have if you know of one. $\endgroup$
    – Mon
    Dec 30, 2022 at 5:57
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    $\begingroup$ If we look at Washington state or Oregon, they have both granite and basalt. However, basalt tends to flow freely, and the huge basalts flows went around the granite mountains and filled in the river valleys. usgs.gov/observatories/cvo/… The OP was building on a large basalt flow and wanting granite to show up later which is different. The granite of the Sierra Nevada is said to form in the core of stratovolcanoes, not basalt flows. $\endgroup$
    – David R
    Dec 30, 2022 at 15:33

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