Magmas can be classified based on chemical composition. In between we have intermediate composition magmas with intermediate levels of silica content and which occur at intermediate temperatures. Mafic magmas such as basalt are very fluid. They will spread over large areas and can move very rapidly. Silicic magmas, on the other hand, are very viscous and do not readily flow.
Pressure increases with depth in the Earth caused by the weight of overlying material.
Increased pressure leads to an increase in melting temperatures, i.e., a rock which at surface conditions would melt at 1000C might melt at 1300C in the interior. That helps explain why most of the rocks in the Earth's interior don't melt. But as magma is generated and moves up in the Earth, it encounters lower pressure and therefore can generate more melting. Water has a very important effect on melting. As magma rises in the mantle into the crust it will encounter more water again allowing more melting.