first, the basalt lava erupted to make the hallasan shield volcano about 2 million years ago. hallasan was still active into recorded history, but it is not active now. then this first layer of volcanic rock was covered up with sediment. then there was another set of eruptions, which made most of the rock on the surface of jeju island today. in the third round of volcanic activity, hallasan grew to its current, tall height, 1950 m.