Lab 4: Igneous Rocks - Magmas and Volcanoes


Now that you've learned the basics of the main igneous rocks:

  • Felsic: granite/rhyolite,
  • Intermediate: diorite/andesite,
  • Mafic: gabbro/basalt, and
  • Ultramafic: peridotite/komatiite.


We can discuss the significance of those trends mentioned before, such as the relationship between silica content, viscosity of magma, and "explosivity" of volcanic eruptions. Let's start with a couple of real volcanoes you may have heard of:




In Hawaiian, this word means "spewing" or "spraying." It is basaltic lava (the less viscous, more fluid type) that gets spewed. The volcano formed from a deep source, mafic magma, as magma came out onto the Pacific sea floor and piled up into a gigantic layer-cake of lava flows. The lava is so fluid that it can run across the countryside and out into the ocean, over a period of miles. The Hawaiian volcanoes are called shield volcanoes, in reference to the broad shape, akin to a warrior's shield. The broad shape is a reflection of the fluidity (low viscosity) of the magma, as it has spread out widely over the ocean floor, culminating at the top of the pile of lava flows with the part we know above the water. There is a tremendous volume under the water. In fact, if counted from the sea floor, the Hawaiian volcanoes are the largest mountains on Earth. Finally, and with emphasis, take note that, although Kilauea might spew lava into lava sprays and fountains, it does not "blow its top," and the spewings are just local to the volcano peak.


Mount St. Helens


This volcano, in Washington state of the Pacific Northwest, you have have heard, "blew its top" in 1980, after swelling up and rumbling for months before the eruption. It was a gigantic blast that blew out the top and one whole side of the volcano, obliterating the forest for miles around, and spreading volcanic debris across the area, and blasting tremendous quantities of ash into the stratosphere, finally settling out several states away. Mount St. Helens is an example of a stratovolcano. The strato- prefix refers to the way that lava flows, and the occasional blasted-out pyroclastic debris is interlayered (stratified) within the body of the volcano. Stratovolcanoes usually lie along subduction zones where intermediate to felsic magmas rise up from the partially melted subducted slab of lithosphere. Why are stratovolcanoes so explosive? It is the silica content, which leads to greater viscosity. The greater viscosity holds gas bubbles in better, and traps pressure in the volcano, even plugging up the vent with highly siliceous, viscous magma. Mount St. Helens was plugged up before it blew, as shown by the swelling of the sides, which raised alarm bells for the geologists watching the volcano.


From that comparison, we can make a simple observation:


Felsic and intermediate magmas are associated with more explosive volcanoes and eruptions. Mafic magmas are associated with the quiet style of volcanoes and eruptions.


In graphical form, here's the diagram you learned, with these two volcanoes plotted in the position of the associated magma composition: