Effusive volcanic eruptions are outpourings of low-viscosity lava from a volcano. The low viscosity of the erupted material allows for the slow dispersal of the lava in large sheets across Earth’s surface. An example of effusive eruptions is the large lava flows that creep across the surface of Hawaii. This eruptive style is comparatively calm, easy to avoid, and described as low energy. Rocks formed from effusive eruptions have very small crystal sizes due to rapid cooling. As such, the cooled lava has what is called a massive texture that is relatively featureless with no or little structure. However, gasses dissolved in this type of low-viscosity lava are able to escape the liquid, so small holes or vesicles produced by the escaping gas are often present in the rock.
Explosive volcanic eruptions occur when high-viscosity lava is suddenly released from a volcano. This eruptive style imposes significant violence upon Earth’s surface in a high-energy event. An example of an explosive eruption is the catastrophic eruption of Mount St. Helens. Rocks produced by explosive eruptions also have small crystal sizes due to rapid cooling, but they are chaotic and preserve features of the high energy of their deposition.
We can now use those basic features we identified in the outcrop to classify the two volcanic units. Our goal is to use our observations to recognize the processes that formed the volcanic units, from which we can then classify the two units.

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