View Full Version : Geology question about islands

01-21-2007, 11:00 AM
Here's the setting:

Large inland, fresh-water lake, a kingdom of islands.
They are rich in gem deposits like rubies, emeralds, diamonds, etc. It's partly a supernatural event but I don't want to completely stretch all natural law, either.
One of the islands has geothermal springs, too.

In my mind, I was picturing the lake formed by a sunken crater of some kind, with the higher peaks of the former land-mass sticking up as islands.

Does this make sense geologically?

01-21-2007, 04:21 PM
Like a really huge volcanic crater? I suppose that would work.

01-21-2007, 05:43 PM
The Great Lakes in North America are the biggest fresh water lakes on earth.

No kingdom of islands there. Not sure how it would work. you can get salt water islands by having volcanoes rise up from the ocean floor and above sea level.

Freshwater is always from rainwater runoff and melting snow and whatnot.

The great lakes were formed by glaciers during the ice age. Some islands in the lakes were formed because of uneven erosion left hard rock in place. You could also have the glacier leave large deposits. I think the big arm of cape cod in massachusetts and long island in new york are both glacial deposits, but both of those are in the ocean, not in freshwater.

The other thing that would be required is a source of rainwater to feed the lake. The great lakes are around 1,000 feet above sea level and are continuously refilled by rain and melting snow from a large surrounding area.

It might be possible to get a string of islands in a fresh water lake. but they'd be on the order of size of Long Island or Cape Cod.

I don't think the geothermal springs would work, though. You'd need a fault line and mountains for that. Wikipedia shows the location of hot springs in the US here;


And it seems clear that they follow mountain ranges. If you have a mountain, then the glaciers couldn't form the lake, cause they'd go around the mountain or stop in front of it.

No clue about gemstone formation.

01-21-2007, 05:53 PM
diamonds are formed deep below the surface and brought up by magma flows from volcanos. To have a bunch of diamonds be in the left overs from a glacier that formed the islands would be possible, but they would have been picked up from some place north of the big lake, scraped off the surface and then dumped in the lake with everything else. Which would mean there'd also be more diamonds to the north.


01-22-2007, 11:11 PM
Thanks for the reference on diamonds, I think I can use this in descriptions.

Dawn R
01-24-2007, 04:36 AM
I live near Lake Taupo in the North Island of New Zealand. Taupo is actually a caldera which is a type of volcano but it has filled with freshwater to make a large lake. A useful web site to look at for this sort of thing is www.gsnz.org.nz (http://www.gsnz.org.nz).

01-24-2007, 04:52 AM
Hi Bylinebree - I'm not a geological expert but I can tell you that the three gems you mentioned (diamonds, emeralds, and rubies) don't appear naturally in the same locations because they are created by different causes and from different chemical compositions. If you're stretching for a fantasy type of situation, you could have them in fairly close proximity - just not in the same dig. Diamonds appear naturally in kimberlite which is pretty much deteriorated magma that was in a volcanic cone (Crater of Diamonds in Arkansas a good example). Emeralds appear in pegmatite formations (and I think rubies do too). Pegmatites are from magma/lava that was not extruded onto the ground - it formed in the subsurface and had a chance to cool more slowly which is what allowed the gems to develop.

The Great Lakes were formed by glaciers but not too many people realize that there is actually a deep fault under part of the Great Lakes that was once an open wound into the earth that disgorged magma. That's the reason for the iron and copper deposits up around Lake Superior (and the agates which also formed in a lava).

If you do some research on Wikepedia or even just through Googling, you should be able to piece together the origins of these gems and then set your world up so the different needed geologic formations are close to each other (so they would be on different islands). Good luck with your idea. Puma