EL PASO, Canary Islands—As a new volcanic vent blew open and unstoppable rivers of molten rock flowed toward the sea, authorities on a Spanish island warned Tuesday that more dangers lie ahead for residents, including earthquakes, lava flows, toxic gases, volcanic ash, and acid rain. Several small earthquakes shook the island of La Palma in the Atlantic Ocean off northwest Africa on Tuesday, keeping nerves on edge after a volcanic eruption on Sunday. The island, with a population of 85,000, is part of the Canary Islands archipelago, a key tourist destination for Europeans. Authorities said the new fissure demonstrated that the area was unstable and unsafe, and kept people at least 2 kilometers (1.25 miles) away. The rivers of lava, up to six meters (nearly 20 feet) high, rolled down hillsides, burning, and crushing everything in their path, as they gradually closed in on the island’s more densely populated coast. …