أَثْقَالَهَا  (Its burdens) refer to what we see of volcanoes and treasures in the underground. (Burdens) may refer, as well, to what inside the graves, Allah (swt) says:

 ﴾وَأَلْقَتْ مَا فِيهَا وَتَخَلَّتْ ٰ﴿

(And has cast out that within it and relinquished [it].) (Al-Inshiqaq: 4)

 

That is, it threw out what is buried in it of the dead or anything else.