A Cross-Section Of The Earth’s Crust
The outer shell of the Earth is called the ‘Lithosphere’ and is encased by the crust, Continental or Oceanic; the area outside of the ‘Astenosphere’.
The Earth’s Three Primary Layers-
The Crust (10 to 37 miles) & Uppermost Solid Mantle & the Asthenosphere (about 490 miles thick)
The mantle (about 1,800 miles thick)
The core (about 7,500 miles; consisting of the Outer Core & the Inner Core) or centre.
Volcanoes Sit on the Earth’s Crust
The Earth’s crust is made-up of a mixture of igneous, metamorphic, and sedimentary rocks; they sit on-top of the Upper Mantle
Volcanoes are fueled by the Mantle
The Upper Mantle, composed of peridotite rock, is the cooler, outer layer of the Mantle; it is topped by the Asthenosphere and the lithosphere. The outermost layer of the lithosphere is home to the tectonic plates upon which the Earth’s crust sits.
The tectonic plates cap the Earth’s surface and overlap to form a continuous ‘shell’ around it. That shell is covered by material (dirt, etc.) deposited there by erosion over millions of years.
The Earth’s Core
Contained between the crust and the mantle is an area containing magma. Magma is super-heated rock (liquefied). The heat to melt those rocks comes from geothermal energy generated at the Earth’s core.
Magma is molten rock and also contains a combination of various, gasses, and chemicals, at temperatures ranging between 700 °C – 1300 °C (or 1300 °F – 2400 °F). The area exerts a constant pressure on the crust and when it comes in contact with groundwater the pressure increases, the gasses expand and it propels the load to the surface.
Millions of years of the crust experiencing this pressure, produce a swelling on the surface, as the magma rises to the crust through fissures and gaps in the tectonic plates. The magma further rises through a shaft through the middle of the volcano till it reaches the top forming vent at the top. When the pressure blows through the vent it forms a crater at the top. The escaping lava causes a volcanic eruption, flames and a smoldering peak.
On other occasions the release is not as powerful, no larva but lots of ash and toxic gasses and is called a phreatic eruption.
In either case, most of the material discharged by the volcano will add to the height of the cone after each eruption.
Volcano Eruptions Below Craters
After an initial eruption on a cone a large crater forms at the top of the vent. Residual material will seep back into the vent, harden and form a plug, and there it will remain until the pressure builds sufficiently to blow the vent again. Not all of the pressure passes through the vent at the top, some escapes via ‘weaker vents’ on the slopes along the way; these places offer less resistance than the central vent at the top of the shaft and will allow the volcano to discharge toxic gasses and steam generated by superheated ground water here; this is a ‘Fumaroles’.