Fast lava vs Exploding lava
Rocks are made of different stuff, therefore when it’s in a more fluid form, it also is made of different stuff. The different stuff still bonds together in special ways depending on what stuff is in it, and creates a different viscosity, which is the thickness of a fluid. Example, honey has a higher viscosity than milk. Felsic lava is high in silica, potassium, aluminum, calcium (stuff that makes feldspars and quartz) and forms pretty strong bonds which makes it more viscous than other lava. The strong bonds however, make it possible for it to explode! Imagine snapping a twig in your hand: you don’t put that much force into it and it just kinda snaps and is done. Now imagine the Hulk snapping a tree in half: LOTS of force is required, so when it actually breaks it releases a lot of energy in the form of chunks of wood flying everywhere. Thick, felsic lava requires a lot of energy to break the chemical bonds to allow it to move, so it explodes. Intermediate lava is in between felsic and mafic, which is next. Mafic lava is high in iron and magnesium, and don’t have as much silica and aluminum. The bonds in these are pretty chill and totes okay with breaking up and forming new bonds, allowing it to move a lot faster than felsic. And since they’re so chill, there’s no explosions. There’s also a sub-catagory of ultramafic which is like mafic but it can move really, really fast, even as fast as water. I would also like to note here the random fact that enraged my engineering roommate: everything is a fluid. Gas is a very low viscosity fluid, liquid is in the middle, and EVEN SOLIDS ARE FLUID they just move really slow.