**1. Introduction**

The chapter summarizes the main concept in Volcanology with an overview that will help to understand other chapters presented in this Volcanology Book. This chapter has been chosen, in particular, for under-graduate people who want to deepen their knowledge in Volcanology, producing several avenues that can help them to develop their own research interests—from volcanic geology to forecast volcanic eruption [1–13]. Some concepts about the constitution of the Earth must be described to better understand the volcanology base. We know that the Earth, broadly speaking, is stratified with a temperature around 6000°C at its center, as at the surface of the sun. The stratification gives some cloud on the production and genesis of the magma. The issue is very much complex but worth to investigate for the sake of clarity about magma generation. The Earth is stratified not only for temperature but also for density, minerals, and physics state as liquid versus solid. In fact, it is the fluid zone of the Earth (asthenosphere versus outer core), the areas much advocated for partial melting: The asthenosphere is the denser, weaker layer beneath the lithospheric mantle. It lies between about 100 and 410 km beneath the earth's surface. The temperature and pressure of the asthenosphere are so high that rocks soften and partly melt, becoming semi-molten. Earth's outer core is a liquid layer about 2400 km thick and composed of mostly iron and nickel that lies above the earth's solid inner core and below its mantle. Its outer boundary lies 2890 km beneath Earth's surface. Unlike the inner (or solid) core, the outer core is liquid (**Figure 1**). Hence, the Earth is made of several layers—(a) crust, mantle, inner and outer core. (b) Lithosphere, asthenosphere, mesosphere to core (**Figure 2**). The asthenosphere must be the most productive melt layer within the mantle structure.

**Figure 1.** *Earth with a window inside the planet (author's collection).*

#### **Figure 2.**

*Plot of the P and S wave velocity, and density versus depth from the surface to the core. Upper mantle with a profile of S wave velocity (Vs) and the seismic discontinuity (redrawn after Schmincke [14]).*
