Mount St. Helens
Mount St. Helens

Where and Why are Igneous Rocks Formed?

2.1 Where? Volcanoes!

New igneous rocks are being formed on the earth all the time. We know this because igneous rocks can be observed forming at active volcanoes. The Smithsonian Institution's Global Volcanism Program (GVP ) maintains a database of volcanoes on earth. On their Current Eruptions page, 70 volcanoes were listed as active in 2019. The locations of some of these volcanoes are shown on Figure 2.01 below. Click on the figure to see an enlarged version with the locations of all of them. In their database of eruptions, GVP () lists 218 volcanoes that have erupted since 2000, and 1430 volcanoes that have erupted during the Holocene. Use the pulldown "DataSet" list on the large version of Figure 2.01 to see the locations of all these volcanoes.
Holocene volcano map

Figure 2.01. Locations of some of the Holocene volcanoes in the GVP database. Click on the image to see a larger, iteractive map with all Holocene volcano locations.

As you can see in Figure 2.01, some areas of the earth have active volcanoes, whereas many other areas have none. What determines where the volcanoes are located?

Most readers of this text will have learned in an introductory geology course that many volcanoes occur near the boundaries of tectonic plates. Return to the large version of Figure 2.01 and click on the "Show Plate Boundaries" button, you can see for yourself which volcanoes are near plate boundaries. If you need to refresh your memory on plate tectonics concepts, follow the links in the "Plate Tectonics Resources" web-resources page.

Use the pulldown "DataSet" list on the large version of Figure 2.01 to display the Holocene volcanoes by tectonic setting. The majority (1004) of the Holocene volcanoes on the Global Volcanism Program (GVP ) list are interpreted as being in subduction zone (convergent) plate boundary settings. A much smaller number (204) are interpreted as being in rift (divergent) plate boundary settings. The rest are either in intraplate settings (180) or not interpreted (42). These numbers, however, leave out a great many volcanic eruptions that are not observed directly. Rift volcanoes are underrepresented on the GVP volcanoes list because it does not include a large number of the volcanic eruptions that occur beneath the ocean, most of which are at rift (mid-ocean ridge) settings.