THE CINDER CONES OF MICHOAC/i~N---GUANAJUATO, CENTRAL MEXICO: THEIR AGE, VOLUME AND DISTRIBUTION, AND MAGMA DISCHARGE RATE

Hasenaka , T. and Carmichael , I.S.E., 1985. The c inder cones of Michoac~n -Guana jua to , cen t ra l Mexico: the i r age, vo lume and d i s t r i bu t ion , and magma discharge rate . J. Volcanol. Geotherm. Res., 25: 104-124 . The Michoac~in-Guana jua to Volcanic Field ( M G V F ) in cen t ra l Mexico con t a in s over 1000 late Q u a t e r n a r y volcanic centers , of which a p p r o x i m a t e l y 90% are c inder cones. This area is d i s t inc t f r om o t h e r par t s o f the Mexican Volcanic Bel t (MVB), where c o m p o site vo lcanoes p r e d o m i n a t e . O t h e r volcanic fo rms in th is f ield inc lude lava cones, lava domes , maars , t u f f rings, small shield volcanoes , and coneless lava flows. Most of the shield vo lcanoes are e r o d e d and p reda t e the cu r r en t l y observable c inder cones. Wi th in the M G V F , c inder cones are s i t ua t ed b e t w e e n 200 k m and 440 k m f rom the Middle Amer i ca Trench . Nearly 75% of the vo lcanoes are d i s t r ibu ted b e t w e e n 200 k m and 300 k m f rom the t r ench , and cone dens i ty is h ighes t a t 250 km. Overall cone dens i ty is 2.5 c o n e s / 1 0 0 k m 2, and med ian sepa ra t ion d is tance is 2 km. The m e d i a n c inder cone has a he igh t of 90 m, a basal d i ame te r of 800 m, a c ra te r d i ame te r o f 230 m, and a volu m e of 0 .021 k m 3. The c inder cones typica l ly e r u p t e d ol ivine-basal t or basal t ic andes i te ; these rock types are less silicic t h a n those of c o m p o s i t e vo lcanoes in the MVB. In general, samples f rom the M G V F show higher MgO, Cr, and Ni and lower K20, P~O5, and Zr t han those f a r t he r f rom the t rench . Cinder cones show various stages of degrada t ion , f rom which relat ive ages can be estim a t e d ; r a d i o c a r b o n da tes of seven c inder cones were o b t a i n e d for ca l ibra t ion . Of several morpho log ica l indices of age, gully dens i ty and surface m o r p h o l o g y of associa ted lava f lows are the m o s t sensitive. The morpho log ica l c lassif icat ion, based on gully dens i ty and lava f low surface fea tures , revealed t h a t 78 vo lcanoes are younge r t h a n 40 ,000 years. All of t h e m are s i tua ted in the sou th , and some have a rough NE a l ignment , parallel to the relat ive m o t i o n vec to r b e t w e e n the Cocos and N o r t h Amer ica plates. Such NE a l ignments are also f o u n d local ly for o lder cones, a l t hough in general cones are r a n d o m l y spaced. Local c inde r cone a l ignmen t s are E--W in the n o r t h e r n pa r t of the volcanic field, where E--W norma l faul ts also occur . Despi te the large n u m b e r of sca t t e red c inder cones and o t h e r small vo lcanoes in the M G V F , to ta l e r u p t e d vo lume suggests a low magma supply rate. The e s t ima ted to ta l volume of lava flows, ash, and cones e r u p t e d dur ing the last 4 0 , 0 0 0 years for an area of 15 ,000 k m : is 31 k m 3. The ca lcu la ted magma e r u p t i o n ra te of 0.8 k m 3 / 1 0 0 0 years is small in c o m p a r i s o n to a single c o m p o s i t e vo lcano in the MVB. 0 3 7 7 0 2 7 3 / 8 5 / $ 0 3 . 3 0 © 1985 Elsevier Science Publ i shers B.V.