And when the mountains are blown away.
meaning, they will be removed and no sight or trace of them will remain.
This is as Allah says,
وَيَسْـَلُونَكَ عَنِ الْجِبَالِ فَقُلْ يَنسِفُهَا رَبِّى نَسْفاً
And they ask you concerning the mountains;
Say,
"My Lord will blast them and scatter them as particles of dust." (20;105)
Allah also says,
وَيَوْمَ نُسَيِّرُ الْجِبَالَ وَتَرَى الاٌّرْضَ بَارِزَةً وَحَشَرْنَـهُمْ فَلَمْ نُغَادِرْ مِنْهُمْ أَحَداً
And (remember) the Day We shall cause the mountains to pass away (like clouds of dust), and you will see the earth as a leveled plain, and we shall gather them all together so as to leave not one of them behind. (18;47)
Then He says,
وَإِذَا الرُّسُلُ أُقِّتَتْ