The best-known variety of volcanic vent is the central vent, which underlies the summit crater, or mouth, of a volcano. According to San Diego State University's Department of Geological Sciences, the mountainous structures that volcanoes can develop result from layers of lava and other materials accumulating and solidifying around these types of vents. The vents connect summit craters to chambers of molten rock, known as magma, that exist deep below the Earth's crust.
As the volcanic sciences website Volcano Pictures & Information notes, fumaroles are vents that form near volcanoes either as clusters or as long cracks. Unlike larger, central vents, which release a plethora of materials, fumaroles only release gases, specifically those containing sulfur. Although some fumaroles remain active for only a few weeks or months, others can persevere for multiple decades or even centuries.
Parasitic cones are essentially mini-volcanoes that form around the main structure of a much larger parent volcano. The vents of parasitic cones branch off from central vents, providing conduits for magma and pressurized gases. According to the Rochester Institute of Technology, there are two main types of parasitic cones: spatter cones, which shoot out droplets of lava from their vents because of the release of dissolved gases, and tuff cones, which have seawater flowing in to their vents, causing tiny fragments of magma to shoot up and outwards on plumes of steam.
Although vents, fumaroles and cones represent the most commonly occurring varieties, as San Diego State University notes, fissure vents are the most significant when it comes to releasing high quantities of materials and creating new landmasses.
Unlike standard volcanoes, which erupt material out of central vents, fissures occur as long cracks and have no distinctive points of origin. So, while lava from volcanoes accumulates in single locations, forming mountainous structures, lava from fissures spills out in every direction, in what are known as basalt flows. As Virginia Tech notes, these flows can cause large-scale geological changes and are responsible for the formation of the Deccan Traps in India.