The Monolith Monsters

In the desert region of San Angelo, California, geologist Ben Gilbert brings a strange black rock back to his office, where he and bored local reporter Martin Cochrane examine it but fail to determine its origin. That night, a strong wind blows a bottle of water over onto the rock, which begins to bubble and smolder.The next day, the head of the geological office, Dave Miller, returns to town from a business trip and finds the office destroyed by a huge growth of black rock and Ben dead, in a rock-hard, apparently petrified state. Meanwhile, Dave's girl friend, schoolteacher Cathy Barrett, brings her students to the desert, where little Ginny Simpson pockets a piece of the black rock, later washing it in a basin outside her parents' farm. In town, Doctor Reynolds performs an autopsy on Ben, but when he cannot explain Ben's rigidity, he informs Dave and Police Chief Dan Corey that he is shipping the body to a specialist. Martin returns to the demolished office with Dave, and there recognizes the rock formations as resembling the piece of black rock Ben had been examining.Cathy joins them and, also recognizing the rock, races with the men to the Simpson farm. They find it in ruins under a pile of black rock, and Ginny's parents dead. The girl, however, is in a catatonic state, and they rush her to the care of specialist Dr. Steve Hendricks, at the California Medical Research Center. He soon reports that she is slowly turning to stone, and posits that her only hope of survival lies with identifying the black rock within the next eight hours. Dave brings the rock to his old professor, Arthur Flanders, who assumes that it is from a meteor. Together they visit the Simpson farm, where Arthur notices a discoloration in the ground and deduces that the rock is draining silicon from whatever it touches, including humans.They then go to the desert, where they trace fragments of the rock to a huge meteor. Fretting that the meteor contains billions of years worth of space secrets, which they have only hours to discover, Dave sends the new information to Steve, who prepares a synthetic silicon solution and injects it into Ginny. While a storm brews outside, Dave and Arthur continue to investigate what causes the rock to grow, and after a piece of rock falls into the sink and begins to bubble, they realize that water is the culprit. Noticing the rain, they drive to the desert, where the small pieces of rock are mixing with water to form huge monoliths that rise from the earth and then crash into hundreds of pieces, each becoming another monolith.They report their findings to Dan, who plans to evacuate the town, even though the weather bureau reports that the rain will soon stop. At the hospital, Ginny finally stirs, and Dave deduces that Steve's silicon solution can be used to control the rocks. Soon, however, more locals are rushed to hospital in the throes of the petrification process, and the rock continues to grow. With little time to announce their findings to the town, Dave and Dan turn to Martin to round up the paperboys and spread the word. The governor soon declares a state of emergency, and Dave and Arthur struggle to convert the formula to one that will retard the rock growth, failing until they realize that the key lies in the simple saline solution Steve used on Ginny.While casualties mount, Dave figures out a way to dynamite the local dam and flood the nearby salt flats, thus creating a large supply of salty water near the canyon edge. Knowing that they must halt the rock's growth at the canyon edge or lose all hope of survival, Dave ignores the governor's refusal to give permission to the risky project, and sets up dynamite stations around the dam. Arthur doubts that the water will be able to absorb enough salt for the plan to work, but Martin cites hopeful statistics he has learned from years of reporting on the salt flats, and the team is cheered by his certainty. With only minutes left until the monoliths reach the canyon edge, Dan orders the dynamite to be detonated, and the group watches in fear as the water flows over the monoliths. The plan at first seems to fail, but finally the rock growth slows, and Dan holds Cathy as the last huge formation crashes to a standstill.

Question: What causes the rock / monolith to grow?
Answer: Water