Central Valley
Extending from about 220 meters down to 640 meters below the surface is the crater's central valley. At three to five kilometers wide, the valley wraps around the central plateau and separates it from the surrounding island. The sea floor in this region of made of hard, sharp volcanic rock that hasn't been eroded as well as sediment from regions closer to the surface. Many cave entrances can be found in the depths of this area which leads to remnants of a past magma chamber while also providing means for seawater to penetrate heated rock and boil. The caves are thus turbulent with cold and hot water mixing and are remarkably acidic.
Central Plateau
The Plateau in the center of the caldera exists entirely below the ocean's surface, but only to about 2 meters during low tide. Most sea life in the crater lives in this warm, sunny region of soft ground. The topography of this structure is relatively smooth up to the boundary of the central valley where it suddenly drops a short distance.
Points of Interest
Plateau Crater
At the very center of the plateau is a second crater with a large, hemispherical structure of smooth igneous rock. The feature is almost perfectly spherical with a diameter of 200 meters, and is up to 30 meters deep. Large amounts of water boiling from subsurface heat seeps up from around the rock dome and heats the water to an average of 70 degrees centigrade. Because of the high heat and acidity, there is no life bar extremophiles and a pale, semi-transparent tree like organism with no biological explanation mounted to the very top of the igneous dome. The walls of the crater, which rise to just below 2 meters below surface at low tide, insulate the surrounding environment from the extreme heat and acidity.
The Buttress Staircase
From the northeast edge of the plateau, extending into the valley until the edge of the rift, is a series of large stone archways resembling a flying buttress. There are five such structures, varying slightly in size with the largest one being over 70 meters tall and extending outwards for about 230 meters. It is unclear how these structure formed but is theorized to have been old magma tubes being eroded away revealing an intrusion of erosion resistant rock.
Rift Bridge
The Rift Bridge is a single geographic formation found at 248 meters in depth that crosses the rift at its thinnest point.
Tuba Cave
Near the deepest part of the valley in the northern part of the caldera is a large cave entrance that connects to a portion of the old upper magma chamber. The cave got its name from explorers due to a constant low-pitched sound that reverberates in the opening. This sound comes from boiling water streaming up through the rock walls in the back of the cave as it makes its way to the Plateau Crater.
Ecosystem
The islands and the caldera form a unique microclimate. The tough, rocky, but fertile soil of the rim supports a lot of brush and small trees. To no extent is there any native fauna that dwells in the land. However, the trees and cliff faces provide shelter and nesting spots for the alien avian creatures of the area.
Inside the caldera is a colorful but somewhat homogeneous set of microbiomes. They are organized into four zones: the shore, valley, rift, and plateau. The shore life consists of limited underwater flora and a plethora of corals. Around 30% of all known species in the caldera reside in this region. The valley, with its dark, cool waters and harsh bed of volcanic crystal, is quite packed with life. Super tall kelp resembling trees in a rain forest rise upwards of a hundred meters to obtain light also provide food and shelter to creatures from top to bottom while even deeper bioluminescing cast color over the region with the light of a full moon. The rift, however, is sparse. Most of it can be found on the upper edges and some at the very bottom where nutrients settle from the surface. The plateau is the most environmentally rich region of the caldera sporting a large variety of corals, plants, fish, and various other fauna. Half of all life in Vassal Crater lives in this region.
Geological Activity
Because Vassal Crater sits immediately above a magma plume that created a tectonic rift, volcanic activity is almost constant. The crater was originally formed by a large basaltic shield volcano that collapsed when magma in underground chambers receded. Eruptions happen in predictable cycles roughly every one million to 700,000 years. The last one happened around 25,000 years ago. Micro-eruptions are relatively common when magma breaches the surface on the outside of the crater usually, but sometimes in the valley or rift. Plate divergence is growing the rift at a rate of 7.6 centimeters per year, so eventually the crater and its ecosystem will be destroyed and dragged beneath the waves.
Heat from the underground magma constantly heats up ocean water that seeps through the ground or through the valley caves. Very high volumes of water boil off under pressure and escape through vents on the periphery of the caldera but through a few places in the crater, such as the igneous rock formation at the top of the plateau.
Quakes are fairly common here, and due to the nature of young plate tectonics on Cerulean, very large quakes are possible elsewhere. This causes frequent landslides and thus creates tidal waves in the regions. Tsunamis are also common and have impacted Vassal Crater before as seen by limited tree growth below a certain altitude. The island does well to protect the interior ecosystem, but flooding does occur regardless.
Climate
Vassal Crater sits at a latitude of about 4 degrees north of the Cerulean equator. Here, the island is warm all year round but remains at a nearly constant temperature and humidity thanks to winds coming off the ocean. The regions experiences sunshine for around half of the year, but frequent tropical storms and hurricanes bring heavy rain and strong winds. Due to how close the crater is to the planet's equator, hurricanes are unlikely to directly impact the island. Storm surge can sometimes be a threat from the most powerful hurricanes if they pass close enough.
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