Spain’s Instituto Geografico Nacional (IGN) reported the nearly 9,000 seismic movements on the island since July 17 have caused measurable changes in the terrain.
El Hierro’s only eruption during recorded history was in 1793, when the Volcano de Lomo Negro vent rumbled for a month and produced lava flows.
Seismologists say they don’t know if the rising ball of lava causing the latest rumblings will break through the surface, but they have issued a “yellow” alert as a precaution.
IGN Director Maria Jose Blanco told reporters that any eruption would most likely have a “low explosive value.”
Tremors about 50,000 years ago produced an enormous landslide that cascaded into the ocean, creating a large tsunami that probably reached the North American coast.