Of Volcanoes and Boiling Water
Fifteen years ago, on a small island near Iceland, a lava flow was threatening to block a harbor. Plans were made to use explosives on the hot lava, with the idea that allowing the cold seawater to come into contact with the hot interior of the flow would speed up the cooling and help stop the flow. Then, the day before the attack on the lava was due, someone did a quick calculation of what might happen if the energy stored in the hot lava was released in further mixing of hot lava with sea water. The experiment was canceled -- fast.
A goodly fraction of the world's volcanoes are situated on islands or coastlines, or erupt underwater. Hot lava and water meet each other in these regions rather often, yet violent steam explosions are rare. What usually happens when lava meets water?
Suppose hot lava flows into the ocean. Lava fluid enough to flow has a temperature well above the boiling point of water, and the water in contact with the lava will be heated to its boiling point in a matter of seconds. Two things, however, work against any violent explosions. In the first place, the bubbles of steam that form adjacent to the hot lava are free to expand into the ocean. The result is boiling, not explosion. In fact, the burner on your stove is as hot as fresh, glowing lava, and has very much the same effect on water. Some small explosions may occur if water gets into cracks in the lava and turns to steam faster than the steam bubbles can escape.
The second reason that lava flowing into water produces boiling rather than explosions is the enormous amount of heat that it takes to boil water. In order to boil a thin skin of water, a skin of lava three times as thick must give up so much heat that it cools by 800 degrees C. The result is that the surface of a lava flow cools to the boiling point and hardens within seconds of contacting the water. The lava inside this hard shell is still hot and mobile, and the flow continues to advance as the hardened shell cracks to let more hot lava ooze out. This new flow also develops a hard, cool shell almost at once. The result is called pillow lava, and it does indeed look like a pile of pillows. Ancient pillow lavas occurring high in the mountain ranges of today are proof that those mountains were once under the sea.
The temperature at which water boils depends on pressure. On high mountains boiling water is too cool to make good tea, while in a pressure cooker set for 15 pounds pressure (double normal air pressure) the boiling point is 250 degrees F (121 degrees C). Pressure in the ocean is due to the weight of the overlying water, and increases by approximately one atmosphere for each 10 meters (33 feet) depth. Deep enough in the ocean, water won't boil at all. A liquid expands as it gets hotter and has very little volume change with pressure, while a gas tries at the same time to expand with heat and contract under pressure. The boiling temperature doesn't rise as fast as the pressure, so the density of steam at the boiling point becomes larger as the pressure increases. Eventually the compression of the steam under the increasing pressure is so great that there is no longer any difference between steam and water. This occurs at a temperature of about 374 degrees C or 705 degrees F (called the critical temperature for water) and a pressure of over 200 atmospheres, which is found at a depth of about two and a quarter kilometers in the ocean. So the extrusion of lava in the deep oceans is a very quiet affair.
Violent reactions between water and lava certainly occur, but they require special circumstances. In the first place, the ratio of water to lava is important. If there is a lot of water, much of the cooling of the lava comes from heating the water rather than turning it to steam. If there is very little water, there may be small explosions, but there isn't enough steam generated for really major explosions. If the volumes of hot lava and water are just about equal, almost all of the water can flash to superheated steam.
Furthermore, the more surface of the lava is exposed, the faster the heat can be transferred to the water. The eruptions with the most potential for steam explosions, then, are those where the amount of water available is somewhat limited, such as a wide submarine eruption near the surface, and the lava is already broken up into small drops when it contacts the water. The formation of Surtsey, near Iceland, fulfilled these conditions beautifully, and in fact the "rooster tails" produced by that eruption were a characteristic feature of what is now called a Surtseyan eruption.
