The volcanoes are natural features that are a result of the geologic processes. They form alongside all types of plate boundaries, though they are most commonly found along the subduction zones. Some of the volcanoes actually form in the middle of the tectonic plates, and they are a result of the hot spots. Whichever the way it is, all volcanoes have one common characteristic, the rising magma. The raising magma is actually what makes up the shape of the volcanoes. The magma can come in several different ways, depending on its composition, which in turn results in a particular type of eruption. The magma that cools off, be it on the surface in the form of lava flows, or deeper into the ground, creates igneous rocks, or practically new crust.
Base on the scenario been described in the question, the correct is 2.9 mb, subtracted from
The substraction is because the base station is below or under the sea level and the atmospheric pressure decreases or reduces with the height. I hope this explanation is helpful
