Boulders serve the purpose of slowing or preventing access to biome or area containing a primary or highly recommended item that helps the player with game progression.
The boulders seen in the Volcano Obstacle are emitted from the Boulder Cave. And every time a player enters the Volcano Obstacle, these boulders will trigger and roll down the Volcano Obstacle one by one with a one-second interval. Boulders will totally destroy and knock the player into the Lava and Volcano Obstacles below.
After a period of time, boulders created from the Boulder Cave will begin to clump in corners and will tend to block access from the Volcano entrance, making it the players wait to exit the Volcano's summit.These boulders, as follows, will disappear with some time given, though they can last one to two minutes. These boulders are affected by logs, planks, and dynamite and will roll away if blown up, this is generally used as you could place a stick of dynamite between clumped boulders to push them outwards if the boulders are blocking the entrance to the Volcano. Boulders emit a faint red light from them, which can be visible at certain angles. If boulders fall into the lava, they will turn black after about a minute, and disappear shortly after.
Even if you are not on the Volcano Obstacle but in an area where you take damage and has the Volcano Obstacle's music playing, the boulders will come. Even if the area you are in would not become more difficult if the boulders were there (e.g. Lava Trail, Underground Grove).
Snow Cave Boulders
As of 10/1/17, The Snow Cave Boulders have no collision, possibly due to a glitch or bug in the game files. 50/50 chance it will be fixed and a 50/50 chance it will be stuck like this.
When blown up with Dynamite, these boulders/rocks will fade in the spot where the usual boulders drop.
However, after about a period of two minutes, these boulders will be recreated by the cave and will slide down in order to block the entrance to the Taiga again. It is recommended to bring a second stick of Dynamite, as this issue will happen to you when you are in the Taiga for too long.
A well-known issue with these objects is that they will generally deform after coming into contact with large trees, and the player can exploit this issue by rubbing trees on the undersides of these boulders, though no mentions of a later patch have occurred.The player may only need to remove the boulders when bringing a vehicle, as there are regular player-sized gaps that allow players to slip into the Taiga, which should not be uncommon given the ways the boulders must stack. These rocks can get frozen in mid-air, mainly due to a glitch. Also, the boulders look more like rectangular blocks too.