Volcanism behind the main arc
Cinder cones, Guatemala
Cinder cones form from moderately explosive, basaltic eruptions. Magma approaches the surface in a dike, a vertical, lens-like magma body a km or more long, and a few meters wide. The eruption may commence in a curtain of fire along the length of the dike The curtain rapidly narrows to a few discrete vents that eventually become cinder cones. The ideal cinder cone has a broad summit ring surrounding a wide crater. The slopes, especially the external ones, are about 30 degrees. This is the angle of repose, for scoria (gravel size black tephra). A slope piled steeper than 30 degrees will generate a landslide that removes oversteepening.
To make scoria, the eruption must be strong enough to blast fist size and smaller particles high enough into the air so that when they land they are too cold to stick to their predecessors. Eruptions that are too weak allow hornitos of agglutinate to form. Agglutinate forms from hot, fluid spatter that welds together on impact. By welding together, the particles are not subject to angle of repose limitations and can pile up very steeply to make spatter cones and hornitos (little spatter cones with very steep slopes). Cinder cones have to be explosive enough to make unsticky scoria and ash but not so explosive as to make a Plinian eruption. Strombolian eruptions are often linked to cinder comes.
The characteristic cinder cones profile is seen in the 5 uppermost photos on the left panel. The shape is primarily the result of landslides that occur whenever gravitational potential energy is large enough to overcome the resisting frictional forces in the scoria pile. A slope stays at the angle of repose, 30-35º during an eruption. After the eruption, time, wind, rain, farming, etc degrade the slope through erosion and mass wasting processes.
Back arc lavas have much greater volume than do the more visible scoria/cinder comes. Shield volcanoes are common, like the one on the left near Tegucigalpa, Honduras. The gentle slope gets lower toward the summit. This shape is made by a pile of lava flows emanating from a central vent. A cinder cone sometimes marks the vent and may be the last gasp of eruption.
From the side Ixtepeque is nothing much to see. Up close it is a mountain of black glass, transparent in small shards. This volcano was a major source of cutting tools for pre Columbian cultures. Fragments of obsidian knives litter the surface in some areas.
Composite volcano, Chingo
Chingo is a small composite cone at the south end of the Ipala graben, the site of the majority of Central America's back-arc volcanoes.
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