Inside an Eruption


Magma rises from below Earth’s surface.


Rising gases put pressure on the sealed vent. 


Gases explode, breaking the seal.


A hot column of vulcanic ash and gas shoots into the air.


During large eruptions, pyroclastic flows made of gas and volcanic matter move at high speeds.


Lava flows slowly across the land.


Lahars, mixtures of water and ash, move rapidly down streams and rivers.


Once the gases have escaped, the explosion ends. The seal closes again.


magma and gas

How It Works

The plan for the expedition was to hike to the top of Santa María. There they could observe the Caliente lava dome from above. From their research camp, they could see the surface of the erupting lava dome.

Magma and gas were steadily rising from deep inside Earth. Before an explosion, the vent at the top of the dome is sealed. This traps the magma and gases inside the volcano. As the gas rises, pressure builds up. Eventually, the pressure is too great. The seal breaks.

When the seal blows, it releases the gas
and allows magma to ooze out as lava.
lava flows and forces ash skyward hundreds of meters above the vent.
When all the pressure has been released, some of the material resettles over the vent. The vent seals again. Then the pressure begins to slowly build until the next event.

The size of the eruptions is never the same. They tend to be small, producing ash plumes that can reach a height of nearly 500 meters (1,640 feet) or lava flows that travel about 3.2 kilometers (2 miles) from the vent.