I’m old enough to remember when the Mount St. Helens volcano erupted. I was a teenager living in Minnesota at the time. The news had reported that “ash” could be raining down and I remember I was surprised as I ran my fingers across the plastic lid of a coffee can where I kept worms for fish bait. It had been in the backyard overnight as the ash fell and formed a thin layer over everything. The ash wasn’t the soft, fluffy white stuff left after a campfire; it was hard, abrasive and sharp. Now that I am a geologist I know that the material is called tephra, or small pieces of volcanic glass carried from Washington by the wind after being blasted up to the stratosphere.



Today, we’re thinking about volcanoes again with the eruption in Hawaii all over the news. Above all, it reminds us we live on an active planet ruled by Mother Nature. The volcano can make land, and the volcano can take it away. As a geologist, I have a keen interest in the rock cycle and the formation of new ground, like what is happening in Hawaii today.

The type of volcanism that is happening in Hawaii is tame as volcanism goes. That doesn’t mean that we are not empathetic to the people in Hawaii who have suffered loss. We know that things could be a lot worse. You see, volcanoes come in three main types; shield volcanoes, stratovolcanoes, and calderas. The primary difference between each is the composition of the lava that erupts from it.


Shield Volcanoes

Shield volcanoes, like those that form the Hawaiian Islands erupt relatively fluid lava that flows out over the ground creating a low dome. In fact, if you look at the Hawaiian Islands from the side, at a distance, they have a profile of a Roman shield, hence the name. Consequently, shield volcanoes are not the picturesque mountain cones like Mount Fuji. When the lava from shield volcanoes cools it forms a rock called basalt. The minerals that makeup basalt generally contain metals, like iron, so you can imagine why the lava flows like molten metal, and the soils that form from weathering basalt are usually rusty red.



The stratovolcanoes are the ones we see in art and the characteristic volcano landform. They are frequently steep sided isolated peaks reaching high into the sky with snow-capped peaks. Mt Fuji, Mt. Kenya, and Mt. Shasta are stratovolcanoes, as is the type of volcano that extends up the west coast from California to Alaska, forming most of the North American part of the “Ring of Fire.” The lava that erupts from these volcanoes is more viscous and doesn’t flow smoothly. Thus, volcanoes have steeper slopes.

The minerals making up the lava from stratovolcanoes can vary more than shield volcanoes. But, generally, this lava contains mostly silica, the same material that forms glass. Oddly, this type of magma can have water mixed in, and when it does the super-heated steam can cause the eruptions to be explosive, like the one at Mount St. Helens in 1980. These eruptions can be powerful enough to blow the mountain apart and cause destruction many miles away. Fortunately, most of these volcanoes are in undeveloped areas, but not all; Mount Etna in Italy is an example of a volcano that has the potential to create a catastrophe when it erupts.



Calderas are something else, sometimes referred to as super-volcanoes. We haven’t had a caldera type eruption in historical time. They are huge, and the caldera crater can be many miles across. Yellowstone is a famous caldera crater, and it is over 30 miles across. When it erupts, it will change the landscape and have global implications for our climate. The chances are that a caldera eruption won’t happen in our lifetime, and that’s a good thing.



The hazards presented by volcanoes are in some ways similar to earthquakes; we know where they will occur but don’t know when. They both have the potential to impact a large area if they “go big” but generally only create interest amongst the scientists that study them. To get a perspective on how “in-tune” you are to the volcanic activity that is happening around the world right now, you can look at an interactive map of Active Volcanoes and Recent Earthquakes worldwide. I think you will be surprised how many volcanoes are currently erupting around the world.


The Volcano Near You

If you are in a region potentially affected by a volcanic eruption, you should be aware of the volcano’s location, potential hazards, and emergency procedures. The United States Geologic Survey (USGS) has produced maps depicting the positions of the known active volcanoes in California ( * USGS 1989). The USGS identifies hazard zones around the volcanoes, which include areas that are susceptible to volcanic debris flows and tephra fall that can travel many miles from the volcanic vent, in addition to the more intuitive lava flows.

Should you live in a county that has volcanic hazards, the safety element of the county’s general plan most likely includes guidelines for addressing the dangers. The most effective mitigation for volcanic hazards is avoidance. Development should be restricted in areas that may potentially experience the effects of volcanism, and the same goes for other geologic hazards such as landslides and fault rupture, too. Knowledge is your best defense against natural hazards. We encourage you to do a little research and be aware of the geologic hazards that exist in your region.

* USGS, 1989, Potential Hazards from Future Volcanic Eruptions in California, USGS Bulletin 1847, by C. Dan Miller.


Darrin Hasham, PG, CEG

Darrin Hasham is an Engineering Geologist with Earth Systems specializing in geologic hazards investigations, slope stability studies, and landslide analyses. He is the chairperson of the Association of Engineering Geologists (AEG), Southern California Chapter.