Sea Mount

* See also Mt_Ryan
* See also Lighthouse_Island
* See also Geography
* See also Maps
https://www.whoi.edu/know-your-ocean/ocean-topics/how-the-ocean-works/seafloor-below/seamounts/
https://oceanservice.noaa.gov/facts/seamounts.html
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An Undersea Volcano
Rapture is perched on the slopes of an ancient submarine volcano - a Seamount  Mt_Ryan http://en.wikipedia.org/wiki/Seamount - That's handy, as the Ocean around there is over 3000 feet deep.
How ancient ? If it was active/forming during an Ice Age ( Last ended approximately 11,700 years ago ) when Ocean levels were approx 400 feet lower, then it could have terrestrial (above water) volcanic formations at its top, rather than having hydrophonic (below water) structurings.
Rapture's location Sea Mount ( as seen in-game ) NOW barely reached the Surface of the Ocean ( a high spot where the Lighthouse got built ).
Crater edges protect Rapture ( Large Icebergs, Storm currents ). The crater edges are irregular with only one section still reaching the Ocean surface.
A Crater means it erupted sometime in the past ( possibly during an Ice Age when the Earth's sea levels were 400 feet lower ). And the width of the crater may indicate that it blew up and perhaps collapsed in on itself.
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Iceland itself was the volcanic product of the spreading Mid-Atlantic tectonic plates
Large ruptures in this 'spreading' built up towards the Ocean's surface
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Defined by Oceanographers as independent features that rise to at least 1,000 m (3,281 ft) above the seafloor, characteristically of conical form.
There are several different types of volcanoes, including shield volcanoes, composite volcanoes, dome volcanoes, and cinder cones. However, if you ask a child to draw a volcano, you'll almost always get a picture of a composite volcano. The reason ? Composite volcanoes form the steep-sided cones most often seen in photographs. They are also associated with the most violent, historically important eruptions.
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These underwater mountains have steep outer slopes with conical or flat summits and contain collapse features. Their flanks become steeper when sills ( horizontal intrusions ) occur between layers of preexisting rocks. Moreover, some seamounts have wider bases due to flank collapse, submarine landslides, or explosive volcanism.
More than 50% of Sea Mounts have craters or calderas, but some are already buried in thick layers of sediments. These craters and calderas are circular or elliptical and can have diameters from tens or hundreds of meters to greater than 5 km. The crater walls commonly occur as vertical and may cut through sub-horizontal lava flows. Most craters are characterized by single collapse features ponded with lava flows. Some have more complex features, while others have already been cut by collapse and landslides.
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""Composite volcano magma is felsic, which means it contains silicate-rich minerals rhyolite, andesite, and dacite. Low-viscosity lava from a shield volcano, such as might be found in Hawaii, flows from fissures and spreads. Lava, rocks, and ash from a stratovolcano either flow a short distance from the cone or explosively eject into the air before falling back down toward the source.""
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