Volcanic Processes

erupting lava
Kīlauea summit eruption on October 12, 2021. Large fountaining bursts from the main vent threw lava about 100 ft (30 meters) high, depositing spatter on the outside of the cone. Hawaiʻi Volcanoes National Park, Hawai'i

USGS photo by B. Carr.


Volcanic processes consist of all the volcano-related processes that happen on volcanic landforms. Volcanic processes include phenomena that occur before, during, and after volcanic eruptions, and also volcano-related processes that impact them between and after eruptions.

Volcanic Earthquakes

Volcanic earthquakes are caused by movement of magma and gases within and underneath a volcano. Volcanic tremor consists of a long-lasting release of seismic energy that may be rhythmic or harmonic and is associated with magma in motion. Other types of shallow earthquakes may be caused by rock fracturing or cracks resonating in response to magma and fluid movements. These earthquakes are often precursors to an eruption.

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Volcanic Eruptions

steam cloud rises from an erupting mountaintop crater
Redoubt Volcano in Lake Clark National Park during its eruption on May 8, 2009 showing the summit crater, growing lava dome, and steam plume of Redoubt volcano. Lake Clark National Park & Preserve, Alaska.

AVO/USGS photo by Waythomas, C. F..

Volcanic eruptions consist of the expulsion of gases, rock fragments, and/or molten lava from within the Earth through a vent. Volcanic eruptions may produce lava flows, lava domes, blasts, eruption columns, pyroclastic flows, lahars, and landslides and debris flows.

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Lahars and Landslides

Lahars and landslides may occur during eruption, but may also occur without volcanic activity.

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Hydrothermal Alteration

barren valley surrounded by forested hills
Hydrothermally altered rocks at Bumpass Hell, a hydrothermal basin in Lassen Volcanic National Park, California. Acidic hydrothermal fluids alters volcanic rocks to white kaolinite (clay) and silica.

NPS photo.

Hydrothermal alteration takes place when hot volcanic gases mix with groundwater. Hydrothermal fluids are frequently acidic from sulfur-rich volcanic gases. Hydrothermal alteration alters volcanic rocks to form clay minerals, zeolites, and other minerals. Hydrothermal alteration weakens rocks and areas of volcanoes that have been hydrothermally altered are more susceptible to erosion and mass wasting processes.

Hydrothermal alteration occurs anywhere on volcanoes where there are hot fluids present, such as in or adjacent to craters, near fumaroles (link to article), and in other geothermal areas.

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Last updated: April 14, 2023