The magma is a product of the mantle layer. The temperatures in this layer are very high, and the pressure is also much higher than on the surface. This leads to creating a melt out of the rocks/metals in it, thus creating a thick liquid out of them. Apart from the liquid of molten rocks/metals, a major component of any magma is the silica, which actually determines its behavior ones it reaches the surface of the Earth. The third important component of the magma are the gasses. The gasses are forming bubbles in the magma, and depending on the amount of silica, they can remain trapped and than cause explosions on the surface, or they can escape the magma easily if the magma is not highly viscous.
