Much of the data for Central America comes from the CENTAM file put together by Rose and Carr. Arenal in Costa Rica erupted in 1968 gradually building up a huge lava field. Pacaya, a little further south is intermittently active and at the present moment (Jan.2000) is firefountaining 2000ft. The old Guatemalan capital of Antigua was destroyed when the two opposing volcanoes Fuego and Agua erupted together. Probably ignimbriteР”В«eginning near the Guatemalan border a chain of cones lies parallel to Pacific coast through Guatemala, el Salvador, Nicaragua and Cost Rica. We found a handful of rhyolitic scoria on Tlalapon east of Mexico City, but this was probably crustal material.Р”В¬olima - an active cone towards the western Trans-Mexican Volcanic group.Р”В«asement, ie of the region of Paricutin. Even though rhyolitic ignimbrites make up much of the trans-Mexican basement rhyolites are extremely rare in the central andesitic volcanoes. Even Minettes have been found, (Luhr & Carmichael, 1989, JGR 94). Cameron et al, 1989, JGR 94) found similar high-Al basalts in Chihuahua near the US border.

However, data from the similar Colima basalts (Verna & Luhr 1993) include Nb and show the expected negative Nb anomaly. McBirney et al (1987, C.Min.Pet.95) give the composition of Paricutin, but unfortunately without Nb. Basalts are also found in the youngest flow of Volcan Xitle in the centre of Mexico City, beside the Universidad de Mexico. These basalts proved to be similar to those found 50 miles south in the region of Paricutin, an area of numerous short lived basalt cones including Jorullo. In the canyon of the Rio Santiago near Guadalajara were half a dozen superposed sheets of massive ignimbrite, sometimes studded with sanidine crystals. Our early studies in Mexico showed a surprising range in type. only miles from one of the largest cities in the world - Mexico City. Vent of Popcatatapetl, at 18,000ft, one of the highest active cones in See chapter on ' Homogeneity of the Earth's Mantle'. These include lherzolite xenoliths which are compatible with being a source for the arc andesites.

The high K2O at 8 - 10 % MgO are from the alkali-basalts - basanites found in the region of Durango and Sonora, well behind the arc. Variation diagram for all Mexican Arc data. The photo below is rather atypical, usually the upper 5000ft is buried in snow. The whole 500 miles is studded with small short lived cones like the well known Paricutin, while at the other end of the scale is Popcatatapetl, one of the highest active cones in the world and towering above Mexico, a city of 18 million people. It includes 35 recently active centres including Ceborruco, Colima, Jorullo, Paricutin, Popcatatapetl, Orizaba and El Chichon, the latter exploding most disastrously in 1982. Mooser has called this the Trans Mexican Volcanic Belt. An associate Mexican volcanologist, Dr F. Orogenic Andesites Links within this page:Р”Р„ chain of andesitic volcanoes extends from near Puerto Vallarta in the NW for 600 miles toward Yucatan Peninsula in the south-east, Mexico City being in the approx.

Andesite ternary diagram