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In my world, there is a chain of offshore volcanoes which are the result of a smaller plate hitting a larger one. There are two to three active volcanoes.
My question is: two of these volcanoes are close enough they are only separated by a narrow strait of sea, so would the volcanic activity from them, lava flows, hydrothermal vents, etc., be enough to turn the strait into a nearly boiling, seething sea of hot water? If not, what could? Or is this impossible?
Highly unlikely.
For a practical demonstration, take a look at videos or pictures of pillow lavas being formed in Hawai'i. The divers are within meters of lava emerging into the water, and while the water in contact with the lava is clearly heated, the divers only a few meters away are in no immediate danger. An example:
In order to heat a sufficiently large mass of water between the volcanoes to boiling, there would need to be so much lava outflow that there very quickly wouldn't be any water at all, but built-up emerged land.
Possible for a short time frame during an eruption event, but highly unlikely during a long time period.
little blurb I found through google on the cooling of lava: http://ffden-2.phys.uaf.edu/212_spring2005.web.dir/Philip_Fitzgerald/physics.htm
The lava is assumed to be at 1000 degrees C and the water is assumed to be at 10 degrees C. The final temperature for the lava and the water is assumed to be 100 degrees C. These numbers are only approximate. The actual specific heat and latent heat of fusion for lava varies depending on the mineral composition. In his piece “Cooling the Lava”, John Mcphee gives a figure of 1.7 kg of lava cooled per 1 kg of water. My calculations show 2.7 kg of lava cooled per 1 kg of water.
Water has a couple properties that leaves it quite unique in this space..it takes a lot of energy to warm up water compared to most other materials, including lava. You would need around twice as much lava as water to get the water up to a boiling point...and by that point the strait between your two volcanoes would be a filled land bridge.
The water has a lot of methods of cooling itself. The tide and ocean currents wouldn't leave the water in between two islands static (some of the strongest ocean currents exist between two islands)...it would be ever replaced as the current moves and the huge depth of the ocean has tons of room to absorb this warm water.
Wind also comes to play. As it blows over the surface, water evaporates. The act of evaporation for water takes an incredible amount of energy out of the surrounding environment (blowing on your hot coffee is the same principle). So the water at the surface would be consistently cooled.
It is possible to have these conditions right for a short term eruption event to increase water temperatures, but nothing that would be permanent over a month timeline. You could probably make a few alterations (isolate this water from the ocean) that could make this scenario a bit more likely.
Geothermal activity other than lava flows could possibly provide your world with enough heated water and bubbling from escaping gases to present a warm body of water that has the appearance of boiling.
One place on earth that we see heated water bubbling and even gushing is Yellowstone. If your sea is shallow with a low flow of fresh water into the area you could have a warm to hot sea that has under water geysers and hot springs with escaping gases to look like it's boiling. Perhaps a few geysers blast enough boiling hot water to reach the surface so some areas will boil a person alive.
I agree with the previous answers in that lava coming out of an ordinary volcano could not do this. Unless you took Kilauea and multiplied it by 1000. However, a possibility is to have a lava flood eruption something like Laki in Iceland whereby instead of lava coming out of the top (or vent) of an on-land volcano, you could have a massive, continuous fissure-type eruption in very shallow water. This could be combined with your two flanking volcanoes.
As an aside, I suggest you go to Wikipedia to look up "hypercane". A pretty scary cool concept.
Hope this helps.
While previous answers have explained that this isn't viable. Do remember it doesn't have to be boiling to be deadly. Much lower temperatures can still result in humans and fish being unable to enter the region. It may also be deadly for other reasons like the volcanoes increasing the acidity of the water
