The text is careful to differentiate between a great mountain and something like a great mountain (ὡς ὄρος μέγα [hōs oros mega] ). The object itself is evidently not a mountain, nor is it said to erupt or spew into the oceanas we might expect from a great volcanic disturbance. Instead, it is thrown into the sea by some agent, possibly an angel, although the text does not say. This could describe the impact of a large meteor burning upon entry to the earths atmosphere from outer space,1 or it could be something entirely supernatural. Jeremiah referred to Babylon as a destroying mountain which God would make as a burnt mountain (Jer. Jer. 51:25). Here it seems that God uses a destroying mountain in judgments which will ultimately overthrow Babylon (Rev. Rev. 17:1+, Rev. 18:1+).
a third of the sea became blood
In the first plague which God brought upon Egypt where Aaron stretched forth his rod, all the waters of Egypt became blood (Ex. Ex. 7:19; Ps. Ps. 78:44). Here, as in Egypt, a literal judgment is in view because the result affects living creatures and ships ((Ps. Ps. 105:29 cf. Rev. Rev. 8:9+). Later, in the pouring of the second bowl, all of the sea will become blood as of a dead man (Rev. Rev. 16:3+). Like Moses, Gods two witnesses will also have the power to turn water into blood (Rev. Rev. 11:6+). In the third bowl judgment, the remainder of the rivers and springs of water which were not poisoned in the judgments of the third trumpet also become blood (Rev. Rev. 16:4+). Later, it will be said of the Harlot that she is drunk with the blood of the saints and with the blood of the martyrs of Jesus (Rev. Rev. 17:6+). During the Tribulation, God gives her literal blood to drink. The destruction of the sea will cause tremendous upheaval to the food chain which supports all life on the planet. The divine hand is systematically removing every means by which man can continue to imagine himself as independent of the one true God.
The price of sin will include the pangs of hunger, for not only will God ensure that the worlds food supply is depleted, but He will also ensure that its meager reserves will not be bolstered, and then ensure that mans alternate food source, the sea, will be drastically depleted.2
God created the sea to be a blessing to mankind, to provide food, oxygen (much of Earths oxygen comes from the phytoplankton and algae in the worlds oceans), and water from the rainstorms on the land that is originally gathered up by evaporation from the oceans. But people have repaid Gods gracious provision with ingratitude and idolatry, revering the sea as the supposed source of their remotest evolutionary ancestors. As He had devastated the land environment, the true God judges the sea.3In the plague of Egypt, the water turned into literal blood so it would seem a similar miraculous judgment occurs here, although it is also possible that blood may simply denote death which results from the polluted waters:
The word translated blood can mean death and this provides a ready explanation for this passage. The prophecy may mean that the chemical composition of the waters will be so altered by the meteorite, or whatever it is that God will plunge into the ocean, that the marine life in the effected area will be killed and even metal vessels will be destroyed or ruined. As the text does not say the sea became like blood but that it became blood, and, as the same word is used in Rev. Rev. 16:6+, this should not be taken as only describing the color of the oceans, but must be understood either literally, or with its secondary meaning of death.4See The Plagues of Egypt and the Tribulation.
1 This is evidently a giant meteorite or asteroid, surrounded by flaming gases set ablaze by the friction of the earths atmosphere, on a collision course with the earth. The current doomsday scenarios about an asteroid hitting the earth will come true with a vengeance.John MacArthur, Revelation 1-11 : The MacArthur New Testament Commentary (Chicago, IL: Moody Press, 1999), Rev. 8:8.
2 Monty S. Mills, Revelations: An Exegetical Study of the Revelation to John (Dallas, TX: 3E Ministries, 1987), Rev. 8:8.
3 MacArthur, Revelation 1-11 : The MacArthur New Testament Commentary, Rev. 8:8.
4 Mills, Revelations: An Exegetical Study of the Revelation to John, Rev. 8:8.