They are so concerned that the Volcanoes National Park has been closed.
It is the newest threat from the Kilauea volcano, which started to erupt eight days ago on the US state’s Big Island, the National Park Service said.
Lava levels in the crater are going down, say scientists, which means it may be clogging up before a massive blast.
US Geological Survey geophysicist Ingrid Johanson said movement of the molten rock opened space for lava to drain underground which reduced the height of a lava lake at the summit.
As the lava lake levels fall below the groundwater table, water can start interacting with the magma, heating it up and creating steam, said USGS scientist Donald Swanson.
If rocks fall from the walls surrounding the magma in the volcano, the rocks can form a dam and if the steam builds pressure, “it can eventually burst out in an explosion,” Mr Swanson said.
NASA produced a satellite image of how high the plumes could be in the area. in some parts they are expected to reach 1.5km (0.9 miles).
The lava could interact with the groundwater by the middle of this month, scientists have estimated.
Kilauea is one of the most active volcanoes in the world and one of five on the island.
A magnitude five earthquake under its south flank preceded an initial eruption last week and several severe aftershocks followed.
A quake last Friday was measured at magnitude 6.9, the most powerful to hit the islands since 1975.
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The flowing lava has destroyed dozens of structures in an area called Leilani Estates, although nobody has been killed.
Hundreds of people have been forced to evacuate their homes because of the lava and the threat of toxic fumes.