In case you don’t keep up with such things, this week has been extraordinarily active on the Ring of Fire. There are 11 active volcanos, four of them newly active this week, on the Ring of Fire, an almost frighteningly alive and busy volcanic zone that circles the Pacific from just north of Oceania (Australia) all the way up the eastern edge of the Pacific Rim, around over the top of the Bering Sea and back down the western edge of the Pacific Rim to the southern tip of South America in Juan de Fuca. These are active SUBDUCTION zones where the pacific plate is sliding beneath the bordering continental plates (excluding the Antarctic side). It is the most active volcanic area in the world, and as a matter of fact, none of the volcanoes anywhere else in the world seem to be doing anything remotely noteworthy at the moment.
New this last week, Mayon in the Philippines, one of the 9 active stratovolcanoes on the Rim, is threatening to blow it’s little top in a big way and people are evacuating those islands. In two small northern Japanese Islands an ongoing volcano called Aira, which is the only caldera on this week’s active list, caused a deadly avalanche. Kilauea, which is NOT really a Ring of Fire volcano and is what is called a Shield volcano, has been mildly entertaining. This morning, off the coast of Alaska, there was a 7.9 earthquake and the tsunami warning for Alaska is still in effect. The rest of the active volcanoes are all stratovolcanos, 2 in central America, (San Miguel in El Salvador new this week), 2 in South America (Nevados de Chillan in Chile new this week), 1 in Kamchatka, and the rest around Japan and the Philippines, and one called Kadovar, northeast of New Guinea that is also newly active this week. So, there has been a whole lot of shaken going on, in case you wondered. http://volcano.si.edu/reports_weekly.cfm
A stratovolcano is what you picture when you think volcano. Also known as a composite volcano, it is a conical volcano built up by many layers (strata) of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic explosive eruptions and effusive eruptions, although some have collapsed craters called calderas. The US volcanoes along the western Pacific subduction zone are mostly stratovolcanoes which are found along the line made by the Cascades and Sierras because that is the visible, surface evidence of the subduction going on deep in the earth. Mount St Helens made herself famous in the 80s when she blew up. Mount Mazama is where Crater Lake is. Mount Mazama blew up thousands of years ago.
So, obviously, a caldera is a blown-out stratovolcano. A caldera is a large cauldron-like depression that forms following the evacuation of a magma chamber/reservoir. You might call it a crater, and the best example we have of that is Crater Lake, the deepest lake in North America. The native Shasti legend is that Shasta, the fire god warred with Mazama, threw rocks at him and Crater Lake was the result.
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their low profile, resembling a warrior’s shield lying on the ground. This is caused by the highly fluid (low viscosity) lava they erupt which travels farther than lava erupted from stratovolcanoes. This results in the steady accumulation of broad sheets of lava, building up the shield volcano’s distinctive form. The shape of shield volcanoes is due to the low viscosity of their mafic lava. The Hawaiian volcanoes are shield volcanoes. Moana Loa is the tallest mountain in the world, actually, if you measure her from her base deep in the ocean, and not from sea level.
In case you wondered.
I share with you a poem from my favorite contemporary poet, Nan Hannon, an archaeologist from the Pacific Northwest. She wrote this poem about Mount Mazama.
Count tanged arrowheads.
Run your thumb along an edge.
Watch your blood bead up.
Ask: Can these points still kill?
Dark layers of habitation interlaid with sterile soil.
Blow the dust from your hands.
Ask: How thick is life?
Sift Mazama ash
washed downriver to this site seven thousand years back.
Ask: What makes a mountain die?
This house pit scar.
Circular. As if a flying saucer set down here.
Sleep in it.
Ask: Who dreams?
Nan Hannon, from Sky River