وَإِذَا البِحارُ فُجِّرَت

When the seas are merged.

EXEGESIS

Fujjirat (merged) is from fajjara, which means to pierce through and tear, or to split wide open.^{‎[1]Raghib, f-j-r.} The time of dawn is called fajr because its light breaks through the night; and sinful rebellion is called fujūr because it shatters the barriers of religion set by God. In verse 14 of this surah, the vicious are called fujjār for the same reason.

But when referring to seas and springs, this verb is used in the Quran to mean gushing forth (see also 2:60, 17:90, 17:91, 18:33, and 54:12). The meaning of merged, therefore, is in the sense of all the large water bodies of the planet breaking their barriers and the seas and oceans swelling over and becoming one.

EXPOSITION

Many past exegetes typically explained this verse with a report from Qatādah, that it means the sweet waters (springs, lakes, rivers) will mix with the salty (seas, oceans) until they all become one body.^{‎[2]Tabari, 30/54; Zamakhshari, 4/714; Alusi, 15/267; Ibn Kathir, 8/339; Nawawi, 2/609; Tabrisi, 10/681.}

For the most part though, the oceans of the world already appear to be joined even today. And in 81:6, instead of merged, When the seas are set afire (sujjirat) is given. So the phenomenon described here is meant to be far more encompassing. Explaining how both apocalyptic events – merging and being set on fire – could occur simultaneously, Makārim Shīrāzī suggests the merging of the seas could result from violent earthquakes and the crashing of the mountains into the seas, with some perhaps even spewing lava and erupting as they crash. Water itself is made up of two combustible elements – hydrogen and oxygen; if by some means a separation of their chemical composition were to occur, it would take a tiny spark to make the whole planet a burning ball of fire.^{‎[3]Nemuneh, 26/211.}

This theory agrees with a report from Zamakhsharī that after the seas fill up and become one, the water will somehow evaporate and the earth will be dry of water and become smooth and level.^{‎[4]Zamakhshari, 4/714.} Ālūsī mentions the same report and offers 20:106-7 as proof of the earth becoming an even and level surface in preparation for the Day of Resurrection.^{‎[5]Alusi, 15/267.}

INSIGHTS FROM HADITH

1. From Imam al-Ṣādiq (a), when asked concerning the dead body: ‘Does it decay?’ He replied: ‘Indeed, not even its flesh or bones remain except the [original] substance from which it was created, for this never disintegrates. It remains in the earth, mixed, until it [the dead body] is recreated from it, as it was created the first time.’^{‎[6]Kafi, 3/251, h. 7.}

[1] Raghib, f-j-r.
[2] Tabari, 30/54; Zamakhshari, 4/714; Alusi, 15/267; Ibn Kathir, 8/339; Nawawi, 2/609; Tabrisi, 10/681.
[3] Nemuneh, 26/211.
[4] Zamakhshari, 4/714.
[5] Alusi, 15/267.
[6] Kafi, 3/251, h. 7.