It would be a divergent boundary, with the two tectonic plates moving away from eachother.
Some of the most populous ones are the Guarani, Tikuna, makuxi. The majority of the tribes were assimilated in 2019-2010. The country is going through a fight in the courts with president Bolsonaro on the legitimacy of some of the smaller tribes, and the land they own.
The best answer to the question presented above would be the first statement. Stalactites are chemical sedimentary rocks that form when the minerals in the cave are left behind by the water. They saturate and form into these stalactites.
