Certain mountain ranges (most notably the Himalayas, the tallest mountain range on the planet) are formed due to the collision of two tectonic plates. One way that mountains and thicker crust are formed in a tectonic collision occurs when the crust material from each is compressed together, "oozing" both upwards (to form mountain ranges) and downwards (thus thickening the crust in that area). You can picture this if you imagine two soft clay bricks lying on a table. If you take those two bricks and slam them into each other, the clay will be compressed where they collide, and will be thicker.

The other process which creates mountains and thickens the crust is called "subduction", and it's the phenomenon that's responsible for the Himalayas. In this case, as the two plates collide, one slips underneath the other, forcing the top one upwards and the bottom one downwards. Furthermore, since the plates are "sticky" and don't slide smoothly past each other, the leading edges tend to get folded and crumpled, thus exaggerating the thickening effect even more.

By the way, here's a picture showing a decent diagram of tectonic subduction. Note how the mountain range has been created, the crust is thickened in the process, and there is a lot of resulting volcanic activity (a common situation around subduction zones).

Because the crust has magma under it

See the magma in the volcano will dry and form rock that is why the crust is thicker under mountains hope u like my answer I'm in middle school top of my class if u need answers comment