References to a burning mountain are in Deuteronomy where Moses talks to the people about "...the decrees and laws I am about to teach you..." (4:1). "Remember the day you stood before the Lord your God...at the foot of the mountain while it blazed with fire..." (4:11-12). "When you heard the voice out of the darkness, while the mountain was ablaze with fire..." (5:23). "Go near and listen to all that the Lord our God says... We will listen and obey." (5:27). Also see 9:15. All of Deuteronomy chapters 4 through 12+ seem relevant here.
There maybe a connection between the first four trumpets (Revelation 8:7-12). Where all are different aspects of the same phenomenon. If so, the hailstones of the first trumpet become a large mountain during the second trumpet which falls on the sea, whereas the hailstones fell on the earth.
The burning mountain may be a volcano. Perhaps a series of volcanoes is involved. The hailstones in the first trumpet may result from a volcanic eruption. The "huge mountain, all ablaze, thrown into the sea" maybe the top of a mountain, that is on top of a volcano, that is thrown into the sea. The waves may sink ships and the debris maybe poisonous to creatures living in the sea. The debris may also give the sea the color and consistency of blood.
"...A third of the sea turned into blood, a third of the living creatures in the sea died, and a third of the ships were destroyed." (8:8-9).