Hasenaka Carmichael 1985

7/27/2019 Hasenaka Carmichael 1985

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T h e M e x i c a n V o l c a n i c B e l t ( M V B ) ( M o o s e r , 1 9 6 9 , 1 9 7 2 ) is d e f i n e d b y a nE - - W - t re n d i n g b e l t o f c o m p o s i t e v o l c a n o e s , r h y o l i t i c c o m p l e x e s , a n d s m a ll e rv e n t s ( F i g . 1 ) . L i k e o t h e r a c t i v e c o n t i n e n t a l v o l c a n i c a r cs , t h e M V B s h o w s as u b -p a r a ll e l d i s tr i b u t i o n o f Q u a t e r n a r y v o l c a n i sm , e a r t h q u a k e f o c i , a n d a no c e a n ic t r e n c h ( D r u m m o n d , 1 9 8 1 ; N i x o n , 1 9 8 2 ) . H o w e v e r , t h e M V B d o e sn o t p a ra l le l t h e M i d d l e A m e r i c a T r e n c h ( M A T ) , b u t r a t h e r m a k e s a n a n gl e o fa p p r o x i m a t e l y 1 5 t o i t ( F ig . 1 ) a nd d e e p e a r t h q u a k e s ( > 1 5 0 k m ) h a v e n o tb e e n o b s e r v e d u n d e r t h e a c ti v e v o l c a n o e s (M o l n a r a n d S y k e s , 1 9 6 9 ; H a n u sa n d V e n e k , 1 9 7 8 ; N i x o n , 1 9 8 2 ) . W i th i n t h e M V B in c e n tr a l M e x i c o is a l ar g ec o n c e n t r a t i o n o f c i n d e r c o n e s , l av a c o n e s , a n d c e n t ra l v o l c a n o e s . A b o u t

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F i g . 1 . I n d e x m a p o f t h e M i e h o a c ~ i n - - G u a n a j u a t o V o l c a n i c F i e l d . T h e l o c a t i o n o f t h eM G V F i s s h o w n a s a r e c t a n g l e . T h e i n s e t e n l a r g e s t h e r e c t a n g l e a r e a . C o n t o u r s o f 1 0 0 0 ma n d 2 0 0 0 m s h o w g e n e r a l t o p o g r a p h y o f t h e a r e a . M e x i c a n V o l c a n i c B e l t ( M V B ) i s s h o w na s t h e c h a i n o f v o l c a n o e s . P l a t e b o u n d a r i e s a r e d r a w n a f t e r D r u m m o n d ( 1 9 8 1 ) .V o l c a n o e s : 1 = S a n g a n g u e y ; 2 = C e b o r u c o ; 3 = S i e r r a L a P r i m a v e r a ; 4 = N e v a d o d e C o l i -m a ; 5 = N e v a d o d e T o l f l c a ; 6 = P o p o c a t 6 p e t l ; 7 = P i c o d e O r i z a b a ; 8 = S a n A n d r O s T u x t l a ;9 = E1 C h i c h 6 n .a = P a r i c u t i n ; b = E 1 J o r u l l o ; c = E 1 J a b a l i ; d = E 1 M e t a t e ; e = L a T a z a ; f = E 1 H u a n i l l o ; g =L a M i n a ; h = E 1 P u e b l i t o ; i = L a s C a b r a s ; j = L a P i l i t a ; k = S a n t a T e r e s a ; I = T a n c i t a r o .P l a c e s : Z = Z a m o r a ; V = V a l l e d e S a n t i a g o ; S = S a l v a t i e r r a ; C = C e l a y a ; U = U r u a p a n ; H =L a H u a c a n a ; M = M e x i c o C i t y ; LC = L a k e C h a p a l a .


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1071 0 0 0 e r u p t i v e v e n t s , m o s t l y c i n d e r c o n e s , a r e f o u n d i n t h e n o r t h e r n h a l f o ft h e s t a te o f M i c h o a c ~ n a n d i n t h e s o u t h e r n p a r t o f t h e s ta t e o f G u a n a j u a t o .T h e y o u n g e s t o f t h e s e , V o lc ~in P a r i c u t in , e r u p t e d i n 1 9 4 3 - - 1 9 5 2 ; t h e s e c o n dy o u n g e s t , V o l c ~ n E 1 J o r u l l o , i n 1 7 5 9 - - 1 7 7 4 . T h i s r e g io n , th e M i c h oa c ~ in - -G u a n a j u a t o V o l c a n i c F i e ld ( M G V F ) h a s an a r e a o f 4 0 , 0 0 0 k m ~ a n d f o r m s au n i q u e p a r t o f th e M V B , s in c e it la c k s t h e y o u n g l ar g e c o m p o s i t e v o l c a n o e sd o m i n a n t in o t h e r p a rt s o f t h e M V B .T h i s v o l c a n i c f i e ld w a s f o r m e r l y r e f e r r e d t o a s: ( 1 ) t h e M i c h oa c ~i n v o l c a n i cp r o v i n c e ( F o s h a g a n d G o n z a l e z , 1 9 5 6 ) ; ( 2 ) t h e P a r i c u t i n r e g i o n ( W i l li a m s ,1 9 5 0 ) ; a n d ( 3 ) t h e Z a m o r a v o l c an i c f ie l d ( S i m k i n e t al ., 1 9 8 1 ) d e p e n d i n g o nt h e p e r s p e c t i v e s o f r e s e a r c h e r s . A s t h e v o l c a n o e s d e s c r i b e d i n t h i s p a p e r a r ed i s t r i b u t e d i n b o t h M i ch oa c~ in a n d G u a n a j u a t o , w e s u gg e s t t h e b r o a d e r la b e lo f t h e M i c h o a c ~ i n - - G u a n a j u a t o V o l c a n i c F i e l d ( F i g . 1 ) .I n g e n e r a l , c i n d e r a n d l a v a c o n e s a r e a c t i v e f o r o n l y a s h o r t p e r i o d o f t i m e ,p e r h a p s a f e w m o n t h s t o t w e n t y y e a r s , a n d r a re l y b e c o m e a c ti v e a g ai n. I nc o n t r a s t , c e n t ra l v o l c a n o e s a r e f e d b y a s in g le c o n d u i t o r a r o w o f cl o s e l ys p a c e d c o n d u i t s w h i c h r e p e a t e d l y d e l iv e r m a g m a t o t h e s u rf a c e ; t h e y m a y b ee i t h e r c o m p o s i t e o r s h i e ld . C i n d e r a n d l a va c o n e s e i t h e r o c c u r n e a r a l ar g ec e n t r al v o l c a n o ( e. g. , P o r t e r , 1 9 7 2 ; D o w n i e a n d W i lk i n s o n , 1 9 7 2 ; M c G e t c h i ne t al ., 1 9 7 4 ; L u h r a n d C a r m i c h a e l , 1 9 8 1 ; N e l s o n a n d C a r m i c h a e l, 1 9 8 4 ) , o rc l u s t e r t o f o r m a v o lc a n i c f i el d l a ck i n g c o m p o s i t e v o l c a n o e s (e .g ., C o l t o n ,1 9 3 7 ; S i n g l e t o n a n d J o y c e , 1 9 6 9 ; S c o t t a n d T r as k , 1 9 7 1 ; B l o o m f i e l d , 1 9 7 5 ;M o o r e e t a l. , 1 9 7 6 ; W a l k er , 1 9 8 1 ; M a r t in d e l P o z z o , 1 9 8 3 ) , b u t ar e s o m e -t i m e s a c c o m p a n i e d b y t u f f r in g s, m a a r s , a n d s m a l l sh i e ld v o l c a n o e s .

T h e p u r p o s e o f th i s s t u d y o f th e M G V F a re : ( 1 ) t o d e s c r i b e t h e s p a ci n g ,s i ze , a n d m o r p h o l o g y o f c i n d e r c o n e s a n d t h e i r a s s o c i a t e d l a v as ; ( 2 ) t o d e v e l-o p m o r p h o l o g i c a l a g e in d i c e s f o r c i n d e r c o n e s ; ( 3 ) t o c a l i b r a t e t h e s e u s in gr a d i o m e t r i c a g es ; a n d ( 4 ) t o e s t i m a t e t h e e r u p t i o n r a te o f m a g m a i n t h er e g i o n .THE MICHOAC~,N--GUANAJUATO VOLCANIC FIELD

M o s t o f t h e v o l c a n i c c e n te r s o f t h e M G V F o c c u r o n t h e E - - W - t re n d i n gp l a t e a u w h i c h c o r r e s p o n d s t o t h e a xi s o f t h e M V B ( C o r d i ll e r a N e o v o l c a n i c a )( F ig . 1 ) ; s o m e e x t e n d t o t h e n o r t h i n t h e B a j io d e G u a n a j u a t o , a n d a f e w t ot h e s o u t h i n t h e l o w l a n d s ( F i g. 1 ) .T h e l i m i t e d e x p o s u r e o f th e b a s e m e n t r o c k s u n d e r l y i n g t h e v o l c a n ic f ie l dc o n s i s t s m a i n l y o f f e l si c i n t r u s iv e r o c k s ( W i l li a m s , 1 9 5 0 ) . G r a n i t e s , q u a r t zm o n z o n i t e s , a n d q u a r t z d i o r i t e s c r o p o u t in t h e s o u t h e r n p a r t o f t h e v o l ca n i cf i el d , s o u t h o f U r u a p a n a n d n o r t h o f L a H u a c a n a ( F ig . 1 ). C l ar k e t a l. ( 1 9 8 2 )r e p o r t e d a n E a r ly O l i g o c e n e ag e o f t h e L a H u a c a n a b a t h o l i t h . T h e s e b a s e-m e n t r o c k s o c c u r a s p a r t i a l l y f u s e d i n c l u s i o n s i n s c o r i a e o f P a r i c u t i n ( W i l c o x ,1 9 5 4 ) a n d o t h e r c i n d e r c o n e s in t h e s o u t h e r n p a r t o f t h e v o l c an i c f ie l d.

B a s a lt o r a n d e s i t e la v as o f t h e E o c e n e t o M i o c e n e ( ?) Z u m p i n i t o F o r m a -t i o n ( W i ll ia m s , 1 9 5 0 ) u n d e r l y t h e M V B a n d o u t c r o p a s e r o d e d h i ll s. L a t e


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Miocene (?) ignimbrites, probably of the Sierra Madre Occidental province,form elongated ridges and mesas to the nort h and east of the volcanic field.They are also observed to the west of Lake Cuitzeo (Fig. 8), and near Salva-tierra, Guana juato (Demant , 1978). Shield volcanoes generally are older thanthe cinder cone eruptions. They range from 4 to 13 km in diameter, with aslope angle o f 5 to 15 . App rox ima tel y 120 shields are distr ibut ed thr ough-out the volcanic field, the largest being Cerro Tancitaro (3845 m) in the areaSW of Volc~in Paricutin (Fig. 1).D I S T R I B U T I O N O F V E N T S

A total of 1040 volcanic vents in the MGVF were identified from topo-graphic maps and air photographs (published by D E T E N A L , Mexico City,scale 1:50,000) in conjunction with field observations. This total includes901 cones, 43 domes, 13 yo ung shield volcanoes with surm ount ing cones, 22maars or tuff rings, and 61 lava flows with hidden vents. (A table of the loca-tions of these volcanoes with their dimensions and morphological parametersis available from the aut hor s u pon request.) Old, highly dissected shield vol-canoes and eroded hills, the nature of which is not easy to discern fromtopograp hy, are excluded from this compilation.

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F ig . 2 . D i s t r i b u t i o n o f v o l c a n o e s i n t h e M i c h o a c ~ i n - - G u a n a j u a t o V o l c a n i c F i e ld . T h e a r e ais t h e s a m e a s t h e i n s e t o f F i g . 1 . C i r c l es i n d i c a t e a ll v o l c a n i c c e n t e r s : c i n d e r c o n e s , l a vad o m e s , m a a r s , t u f f r i n g s, s h i e l d v o l c a n o e s w i t h a s u m m i t c o n e , o r l a v a f l o w s w h i c h a r en o t a s s o c i a t e d w i t h c o n e s . T h e p o s i t i o n o f e s t i m a t e d v e n t s a r e u s e d f o r l o c a t i n g t h e l av af l o w s .


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Few volcanoes occur closer than 200 km to the Middle America Trench(Fig. 2). The co ncen trati on of volcanoes is greatest about 250 k m from thetrench, and approximately 75% of the volcanoes are found between 200 kmand 300 km. Farther than 300 km, the number of volcanoes decreases; themost distant cinder cone is 440 km from the trench. A local concentrationof volcanoes at 380 km corresponds to the maar cluster of Valle de Santiago(Figs. I and 2).

In general, the cinder cones are randomly spaced and indicate no preferredorientation (Figs. 2 and 3). A small number of closely associated cindercones, however, show local alignments; these trend E--W in the no rther n partof the volcanic field, ENE in the middle part, and NE in the southern partnear the volcanic front (Fig. 2). In any given area, cinder cones are restrictedto relatively low elevations as most cones formed either on alluvial plains orlow on the flanks of eroded shield volcanoes (Fig. 3).

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Fig. 3. Map of the volcanoes and associated lava flows in the Paricutin region const ruct edfrom air photographs. UR = Uruapan; P R = Par ach o; P = Volc~in Par icu tin ; J = Cerro E1Jabali; M = Cerro E1 Metate; J n = Cerro Janamo; C = Cerro Capatacuiro.

The overall density o f volcanoes in the MGVF is 2.5 vo lcanoes/100 km 2(1040 vents/4 0,000 km2). The highest densit y of 11/100 km 2 is calculatedfor th e Paricutin region (141 v ents /125 0 km 2 in Fig. 3). Cone separa tiondistance, the distance from one cinder cone to its nearest neighbor, has aPoisson distribution (thus, mode < median < mean). A representativemedian value is 2 km for 100 selected cinder cones from the entire field, and1.15 km for the Paricutin region (Fig. 3). If cone separation distance is cal-


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cula ted assuming equa l cone dens i ty th ro ugh out the Pa r icu t in r egion , themean dis t ance increases to 3.2 km, a ref le c t io n of the ten den cy for cones tocluster or form pairs .

Samples r epresen ta t ive of the d ive r s i ty of the 1040 c inder and lava conesinc lude o l iv ine basal t and basa l t ic -andes i te (usua l ly conta in ing phe nocrys tsof o l iv ine ) and pyro xene or hornbl ende andes i te s , a ll ca lc -alka l ine , andalkal i ol ivine-basal t and basanite . There is a progressive change in compo si-t ion wi th d is tance f rom the t r ench . Fo r the same s i lica conten t , in compa t ib l ee leme nts such as K, P , and Zr genera l ly increase with dis tance f rom th etrench. MgO, Cr , and Ni are r ichest in samples near the t rench and po ore stin the more a lkal i ne volcanics far thes t f rom i t (Table 1) (Hasenak a, 1981) .However , e xcept ion s to th i s genera l t r end a re found; one example i s o f anolder (ca . 0 .2 Ma) cone nex t to Volcf in E1 Joru l lo , one of the c losest conesto the t rench and ye t has 2.8% K20 and 49.2% SiO2 (Luhr and Carmichael ,in press) . The main rock typ es of the region, ca lc-a lkal ine basal t and basal t icandesi te , a re genera l ly less s i l ic ic than those o f the c om pos it e volca noes inthe MVB (Will iams, 1950; Gunn and Mooser , 1971; Pal e t a l ., 1978; Nelson,1980; Luhr and Carmichae l , 1980; R obin and Cantagre l , 1982) .TABLE 1Representative chemical analyses of scoria and lava

Sample: 663 416A 432B 571T 555ASiO: 53.06 53.06 52.82 54.12 52.48TiO 2 0.89 0.98 1.79 1.79 t.69AI:O3 17.00 17.84 16.97 17.12 16.98FeOt 6.85 7.04 8.70 8.33 8.10MnO 0.13 0.12 0.14 0.14 0.13MgO 7.96 6.61 4.86 3.75 4.47CaO 7.75 8.61 7.38 6.54 7.86Na:O 3.74 3.83 3.86 4.21 3.94K~O 0.91 0.96 t.46 1.96 2.17P:O~ 0,18 0.21 0.39 0.59 0.74Cr 364 142 100 47 n,d.V 169 139 204 182 223Ni 212 142 ,13 45 44Sr 530 723 549 569 1419Zr 117 99 207 243 240Ba 267 297 399 5%1 850DFT 201 249 305 345 401Major element oxides in wt.%.Trace e|menets in ppm.FeOt = total iron as ferrous.n.d. = not detected.Analyses by X-ray fluorescence.DFT = distance of the cone from the Middle America Trench in km.


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C I N D E R C O N E S I Z EM o r p h o m e t r i c p a r a m e t e r s o f c in d e r c o n e s a n d t h e i r a s s o c i a te d l av a f l o w s

w e r e o b t a i n e d f r o m 1 : 5 0 , 0 0 0 t o p o g r a p h i c m a p s . T h e s e p a r a m e t e r s i n c l u d ec o n e h e i g h t ( H ) , c o n e b a s a l d i a m e t e r o r w i d t h ( W co ), a n d c r a t e r d i a m e t e r o rw i d t h (W c r) ( F ig . 4 ) . M o s t o f t h e c i n d e r c o n e s s h o w a s y m m e t r i c a l t r u n c a t e dc o n e s h a p e , b u t c o n e s t h a t e r u p t e d o n a n i n c li n e d b a s e m e n t s u r f a c e a re o f t e nb r e a c h e d o r e l o n g a t e d . T o a l l o w f o r t h is , W c o a n d W cr a r e d e f i n e d a s t h ea r i t h m e t i c m e a n s o f t h e m a x i m u m a n d m i n i m u m v a lu e s o f c o n e a n d c r a t e rw i d t h s . H i s t h e e l e v a t i o n d i f f e r e n c e b e t w e e n t h e s u m m i t o f t h e c i n d e r c o n ea n d i ts b a s e . T h e b a s a l e l e v a t i o n i s t h e m e a n o f t h e h i g h e s t a n d t h e l o w e s tb a s a l v a l u es . T h e c i r c u m f e r e n c e s o f c r a t e r r im s a n d c o n e b a s e s w e r e d e t e r -m i n e d w h e r e t h e g e n e r a l l y e q u i d i s t a n t t o p o g r a p h i c c o n t o u r s a b r u p t l y w i d e n .W h e r e t h e c r a t e r i s n o t v i s i b l e o n t h e t o p o g r a p h i c m a p s , t h e d i a m e t e r o f as o m e w h a t f l a t te r s u m m i t w a s t a k e n a s t h e c ra t e r d i a m e t e r va l u e . F r o m t h e s ev a lu e s , t h e v o l u m e o f a s y m m e t r i c a l t r u n c a t e d c o n e w a s c a l c u l a t e d .

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F ig . 5 . F r e q u e n c y d i s t r i b u t i o n o f c i n d e r c o n e s i ze in t h e M G V F . ( A ) c o n e h e i g h t , ( B )c o n e b a sa l d i a m e t e r , ( C ) c o n e c r a t e r d i a m e t e r , ( D ) c o n e v o l u m e c a l c u l a te d a s a s y m m e t r i -c a l t r u n c a t e d c o n e s h a p e . M i n d i c a t e s t h e m e d i a n v a l u e.


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T A B L E 2D i m e n s i o n s a n d a g e o f c i n d e r c o n e s b~

C i n d e r c o n e L a t i t u d e ( N ) L o n g i t u d e ( W ) W c o W c r H V o l u m e A g e S a m p l e m e t h o da V o l c ~ n P a r f c u t i n 1 9 2 9 ' 3 3 " 1 0 2 1 5 ' 0 4 " 0 . 9 5 0 . 2 5 0 . 2 2 0 0 . 0 6 9 1 9 4 3 - - 1 9 5 2 A D H i s t o ri c a l r e c o r db V o l c ~ n E 1 J o r u l l o 1 8 5 8 ' 1 9 " 1 0 1 4 3 ' 0 3 ' ' 1 . 4 5 0.42 0 . 2 9 0 0 . 2 1 9 1 7 5 9 - - 1 7 7 4 A D H i s t o ri c a l r e c o r dc C e r r o E 1 J a b a l i 1 9 o 2 6 ' 5 6 ' ' 1 0 2 o 0 6 ' 4 6 ' ' 0 . 9 3 0 . 3 8 0 . 1 6 0 0 . 0 5 7 3 , 8 3 0 1 5 0 y . B . P . 7 3 7 C c h a r c o a l ~ 4Cd C e r r o E 1 M e t a t e 1 9 3 2 ' 2 0 '' 1 0 1 0 5 9 ' 3 3 '' 0 . 8 8 0 . 2 0 0 . 1 5 0 0 . 0 3 9 4 , 7 0 0 2 0 0 y . B . P . 7 6 1 C c h a r c o a l ~ 4 Ce C e r r o L a T a z a 1 9 3 1 ' 3 3 " 1 0 1 4 3 ' 2 8 ' ' 0 . 7 0 0 . 1 8 0 . 1 7 0 0 . 0 2 9 8 , 4 3 0 - + 3 3 0 y . B . P . 7 5 9 C c h a r c o a l ~ 4 Cf H o y a E I H u a n i l l o 1 9 4 1 ' 0 1 '' 1 0 1 5 9 ' 0 4 ' ' 0 . 9 5 0 . 3 5 0 . 1 9 0 0 . 0 6 8 9 , 1 8 0 2 5 0 y . B . P ~ 4 1 1 C 3 c h a r c o a l ~ 4 C

9 , 4 1 0 2 3 0 y . B . P . 4 1 1 C 2 c h a r c o a l ~ 4Cg V o l c ~ i n L a M i n a 1 9 4 2 ' 4 5 ' ' 1 0 1 2 6 ' 0 2 '' 1 . 1 5 0 . 3 5 0 . 1 9 0 0 . 0 9 2 1 7 , 1 7 0 4 3 0 y . B . P . 622Ccharcoal~4Ch E l P u e b l i t o 1 9 4 9 ' 2 9 '' 1 0 1 5 5 ' 2 4 ' ' 1 . 0 0 0 . 3 8 0 . 1 8 5 0 . 0 7 5 2 9 , 0 0 0 3 , 3 0 0 y . B . P . 4 3 5 C c h a r c o a l ~ 4Ci C e r r o L a s C a b r a s 1 9 4 9 ' 3 4 " 1 0 1 5 3 ' 3 7 " 1 . 1 8 0 . 5 5 0 . 1 9 5 0 . 1 2 0 > 4 0 , 0 0 0 y . B . P . 6 7 4 C c h a r c o a l ~4Cj C e r r o P e l o n 1 9 1 7 ' 5 2 " 1 0 1 5 4 ' 4 7 ' ' 0 . 6 8 0 . 1 8 0 . 0 8 5 0 . 0 1 4 0 . 3 7 0 5 M a 4 2 6 L l a v a K - A rk S a n t a T e r e s a 2 0 2 9 ' 5 0 " 1 0 0 5 9 ' 5 3 '` 0 . 6 3 0 . 1 5 0 . 0 3 0 0 . 0 0 4 2 . 7 8 + _ 0 7 M a 5 5 5 A s c o r i a K - A rS y m b o l s ( a - - k ) a r e t h e s a m e a s in F i g. 1 .W c o = b a s al d i a m e t e r , W c r = c r a t e r d i a m e t e r , H = c o n e h e i g h t ( i n k in ) .V o l u m e o f c i n d e r c o n e ( i n k m 3) i s c a l c u l a t e d a s a s y m m e t r i c a l t r u n c a t e d c o n e , i .e . :

~ HV o l u m e = - - . ( W ~ r + W c r W c o + W ~ o )1 2C o n e si z e i s m e a s u r e d u s i n g l : 5 0 , 0 0 0 t o p o g r a p h i c m a p s ( D E T E N A L , M e x i c o C i t y ).~ 4C a g e s a r e o b t a i n e d f r o m t h e c h a r c o a l i n t h e s o il u n d e r t h e a s h a n d l a p p i li l a y e r s ( a n a l y s t : T e l e d y n e I s o t o p e s ) .C h a r c o a l s a m p l e s w e r e c o l l e c t e d a t t w o d i f f e r e n t s i t e s f o r H o y a E t H u a n i l l o .K - A t a g es a r e o b t a i n e d f r o m t h e w h o l e r o c k s a m p l e s o f l a v a o r s c o r i a ( a n a l y s t : G . M a h o o d ) .


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1 1 3

For cones with discernable crater rims, frequency histograms for H, Wco,Wcr, and cinder cone volume are shown in Fig. 5. Median values are 90 m forthe height, 800 m for the basal diameter, 230 m for the crater diameter, and0.021 km 3 for the volume. Mean values are 100 m, 830 m, 240 m, and 0.038km 3, respectively.

Younger cinder cones dated by 14C are larger than the median (Table 2).Older cinder cones are often partly buried by later lava flows, and aeolianand alluvial deposits as well as erosional processes can diminish the dimen-sions o f relatively old cinder cones. Large cinder cones are more frequen t inthe southwestern part of the MGVF where cones erupted on the lowlands(Fig. 1), indicating that there may be some elevation (crustal thickness) in-fluence on cinder cone size (Vogt, 1974; Ben-Avraham and Nur, 1980).L A V A F L O W V O L U M E

Volumes of lava flows whose margins were clearly observable on the airphotographs (scale 1:50,000) were estimated by tracing flow margins ontotopographic maps and then calculating their thickness from the countourintervals (either 20 m or 10 m). The thickness estimates were made at equalintervals along the margins o f a flow, and th en averaged. When the thicknesswas smaller than the contour interval, an arbitrary value of one-half the con-tour interval was assumed. Lava flow area was measured with a planimeter.The volume of each flow unit was then calculated from its average thicknessand area. Note that the volume of lava flows hidden under later flows wasnot estimated and that the thickness of a lava flow may be underestimatedbecause of the accumulation of colluvium at the foot of the flow marginsand later alluvial deposit ion surroun ding the lava flows. Therefore, the calcu-lated volumes represent minimum values. The thickness, length, and volumeof the lava flows are compiled in Table 3.T A B L E 3D i m e n s i o n s a n d v o l u m e s o f la v a f l o w s

M e a n M e d ia n M i n i m u m M a x i m u mT h i c k n e s s a ( m ) 4 0 3 0 2 - - 3 1 2 0L e n g t h a ( k m ) 3 . 5 3 . 0 0 . 7 1 5V o l u m e b ( k m 3) 0 . 2 3 0 . 2 0 0 . 0 1 4 . 8a C a l c u l a t e d f o r 2 7 9 l a v a f l o w s .b V o l u m e c a l c u l a t e d a s t o t a l l av a e r u p t e d f o r a s i n gl e c o n e .

Plots of lava flow volumes against the volumes of associated cinder conesfor the MGVF show scatter of up to 2 orders of magnitude. The data, how-ever, fall on the least-squares line defined by cinder cones with greater sizeranges (Fig. 6), fr om which Wood (1980a) con clud ed tha t in general cone


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11 4v o l u m e b e c o m e s a g r e a t e r p o r t i o n o f th e t o t a l e r u p t i o n a s t h e e r u p t i o n v o l-u m e i n c r e as e s . T h e r a t i o o f c o n e t o l a v a v o l u m e i s a f u n c t i o n o f e x p l o s i v i t yp o s s i b l y r e f l e c t i n g t h e v o l a t i le c o n t e n t o r v i s c o s i ty o f m a g m a . O n t h e a v e r-a g e i n t h e M G V F , t h e v o l u m e o f a c i n d e r c o n e is r o u g h l y 1 / 1 0 t h a t o f it s a s-s o c i a t e d l av a f l o w , w h i c h i s t h e s a m e r a t i o o b s e r v e d a t P a r i c u t i n , w h e r e m o r ep r e c i s e m e a s u r e m e n t s e x i s t ( F r ie s , 1 9 5 3 ) .

10 1 1 0 2E> 1 0 - 3

1 0 - 4

T ~ i f#/

1 0 - s

1 0 6 A I t L1 0 4 1 0 3 1 0 2 1 0 - 1 1 1 0

Associated lava f low volume I k m 3 1Fig . 6 . T he r e la t ionsh ip be tw een vo lum e of c ind er cone s and as soc ia ted l ava f lows in theM G V F { d o ts ), co m b i n ed w i t h W o o d ' s ( 1 9 8 0 a ) d a t a f r o m S an F r an c i sco v o l can ic f ie l d,E tna , Reunion , Tolbach ic , and E ldfe l t {cros ses ) . The l ine ind ica tes the l eas t - square f i td e r iv ed b y W o o d ( 1 9 8 0 a ) .G E O M O R P H O L O G I C A L P A R A M E T E R S O F C I N D E R CO N E A G E

O n e o f t h e e a r l i e s t s t u d i e s o f c in d e r c o n e s w a s C o l t o n ' s { 1 9 3 7 ) w o r k i nt h e S a n F r a n c i s c o M o u n t a i n v o l c a n i c f ie l d o f A r i z o n a , i n w h i c h h e c l as s if ie dt h e s ta g e s o f e r o s i o n o f c o n e s a n d t h e i r a s s o c i a t e d la v a f l o w s . O t h e r s e m i-q u a n t i t a t iv e i n d i c a to r s o f c i n d e r c o n e a ge in c l u d e : th e m a x i m u m c o n e s l o p ea n g l e ( S c o t t a n d T r a s k , 1 9 7 1 ) , t h e r a t i o o f c o n e h e i g h t t o c o n e b a sa l d ia -m e t e r ( S c o t t a n d T r a sk , 1 9 7 1 ; W o o d , 1 9 8 0 b ) , t h e t a n g e n t o f t h e c o n e s lo p e( B l o o m f i e l d , 1 9 7 5 ) , a n d t h e c h a n g e o f s u r f a c e f e a t u r e s o f l av a f l o w s a s so c i-a t e d w i t h c i n d e r c o n e s ( B l o o m f i e l d , 1 9 7 5 ) .

C i n d e r c o n e s i n t h e M G V F a ls o s h o w v a r i o u s s ta g e s o f d e g r a d a t i o n , f r o mw h i c h t h e r e la t i v e a g e s o f th e c i n d e r c o n e s c a n b e e s t i m a t e d . T h e y o u n g e s tc o n e s , l ik e P a r i c u t i n , h a v e a p e r f e c t c o n e s h a p e w i t h s l o p e a n gl e s o f 3 4 , t h ea n g l e o f r e p o s e o f c i n d e r . T h e i r c r a te r s , w h o s e r i m s a re s h a r p a n d l i t t le m o d i -f i e d b y e r o s i o n , h a v e l i t t l e i n f il l in g o f a s h o r s c o r i a e . A l a rg e n u m b e r o f ri ll sa n d s h a l lo w g u ll ie s h a v e d e v e l o p e d o n t h e s l o p e s o f t h e y o u n g e s t c o n e s . T h el a rg e s t n u m b e r o f r a d i al l i n e a m e n t s o n t h e s l o p e o f P a r i c u t i n o b s e r v e d i n a i rp h o t o g r a p h s , h o w e v e r , a re n o t g u ll ie s b u t a l t e r n a t i n g b a n d s o f s c o r i ae a n dl a p il li . V e g e t a t i o n is s p a r s e o n P a r i c u t i n w h e r e s o i l is a b s e n t , b u t m o s t o f t h es u r f a c e o f V o l c ~ n E 1 J o r u l l o is a l r e a d y c o v e r e d w i t h t r e e s o n l y 2 0 0 y e a r s


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1 1 5

after its eruptio n. Under the tropical to te mpera te climate of the MGVF,vegetatative recovery after eruption is rapid compared with the age span ofcinder cones. The total annual precipitation in the MGVF is between 500mm and 1800 mm/year, and the mean annual temperature is between 16Cand 29C (total annual precipitation and mean annual temperature maps,published by DETENAL, Mexico City). The area on the plateau has a relative-ly cool and wet climate; in contrast, the lowland south has a relatively hotand dry climate, resulting in different erosional conditions.Older, degraded cinder cones have lower slope angles, a smaller number ofgullies (which are larger and deeper), greater soil development, and moreweathe red a nd oxidized ejecta than the young est cones. Craters of the oldercones are filled by ash from other volcanoes and by debris eroded from thehigher slopes of the craters themselves. Some of the oldest cones may be al-most completely buried and have very shallow slope angles. The oldest groupincludes cones with flattened and rounded shapes and deeply dissected coneswith breached craters.

Lava flow morphology also changes with age. Holocene lava flows displaywell preserved original surface features, such as flow margins, boundaries ofindividual flow units, pressure ridges, and levees. Lava flows lose these char-acteristics with time and become covered with soils which may be cultivated.In a densely populated area where almost all the useful land is cultivated,lava surfaces covered with trees and shrubs indica te the absence of arablesoils and henc e relative y ou th . The lava flows of Paricutin and E1 Jorullo areexceptional; th ey only have sparse vegetation.

" J 0 2 0 [ ; - - . - _ 10 . 2 4 ~ 0"16L ~ ' e - - ~ - e - - ~8 m a x

3 3 ~ - 0 . . . .32 L

343 0 1 - _ J8 L e

Gul ly 40~Q.~dens i ty 3 0 ~ .

2 0 / I - ~ ~ ~ ~ ~ 0 - *9 0 10 L 0 ~

' Age

F i g . 7 . G e o m o r p h o l o g i c a l p a r a m e t e r s o f c i n d e r c o n e a g e p l o t t e d a g a i n s t ~ 4C a g e. T h e d a t aa r e f r o m P a r i c u t i n , E l J o r u l l o , E l J a b a l i , E1 H u a n i l l o , L a M i n a , E l P u e b l i t o , L a s C a b r a s,a n d E1 P e l o n ( T a b l e 2 ) . T h e d a t a f o r C e r r o E 1 M e t a t e a r e e x c l u d e d b e c a u s e t h e c o n e s h a p ei s g r e a t l y m o d i f i e d b y l a v a f l o w s o n t h e s l o p e . D a s h e d l i n es a r e f i t t e d b y e y e .


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116

The morph ologi c indicators of age include: (1) the ra tio of c inder coneheight to basal diameter ( H / D ) ; (2) maximum slope angle (0max), taken asthe average of several field measurements from different directions; (3) aver-age slope angle (0ave) , wh ich is calc ula ted as tan -~ (H/r ) , where r is given inFig. 4; (4) the difference between the maximum and the average slopeangles; (5) gully density, defined as the number of gullies determined fromair photo graph s normalized to 90 o f arc ; and (6) the geomor pholog icalclassification of lava flows, as summarized in Table 4. Important measuredvariables are displaye d in Fig. 7. These indices are relative ly easily ob tai nedfrom topographic maps, a i r photographs, or f ie ld observat ions.TABLE4Geomorphological classification of lava flows

Flow Indiv. Pressuremargins flow ridgesunitsSoil Tree Culti- Cindershrub ration conecover

Age

Hv

Ply4Plv~

Plv~Ply,

X

X X

x Paricutin 1943--1952 ADEl Jorullo 1759--1774 AD E1 Jabali 3,830 y.B.P.E1 Metate 4,700 y.B.P.La Taza 8,430 y.B.P.

: La Mina 17,170 y.B.P.. . . . '. El Pueblito 29,000 y.B.P.

Las Cabras ~40,000 y.B.P.'~ Pelon 0.37 MaX ?

-, = feature is distinctly observed or abundant.= feature is somewhat recognizable or moderately abundant.= feature is obscure or scarce.RADIOMETRIC DATES AND CINDER CONE AGESR a d i o c a r b o n d a t e s

Carbon 14 dates were obtaine d for 8 charcoal samples from cinder conesin the earlier stages of degradation (Table 2). The charcoal was collectedfrom soil just beneath the ash and lapill i at distances of 500 m to 2000 mfrom a c inder cone. At this dis tance, the cont in ui ty of the tephra from thecone is obvious. Charcoal was found at the contact of the soil and ash and upto 20 cm below it . This indicates a close temporal relationship between char-coal form ati on and airfalls; thus the 14C dates are used to repres ent the ageof eruption. Most of the cones dated are relatively large and from the plateau(with a relatively cold and wet climate).


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117Cinder cone geomorphological parameters are plotted against the '4C agesin Fig. 8. Clearly, no significant change with age occurs in the H/D ratio or

in the cone slope angles; both maximum and minimum values are found toremain constant within the '4C span of 40,000 years. The maximum slopeangle, 33 to 34 , is the initial cone slope angle and remains con stant f orabout 40,000 years; the average slope angle shows a greater but inconsistentvariation. Accordingly, the difference between maximum and average slopeangles will not be meaningful. These three parameters indicate tha t th e initialform of these cinder cones has been slow to change over a period of at least40,000 years. During this period, majo r erosional processes of landslidingand gullying (Segerstrom, 1950; Wood, 1980b) have yet to mod ify the shapeof cinder cones which are largely comprised of permeable scoriae and lapilli.

L .

2 0 N

lgh

Park utin e w e

i " / / m

~ C 1 02 WO C O S - N O A M M O T I O N

% _

A

o%

~ L ~ ~ '~ : ~ ~"

!

1101W

5 0 k m

Nocmal fault~ Lake

C a n y o n= ~ A l i g n e dc i n d e r c o n e s~- Or i en ta t i on o f

parasi t ic cones,d i k e s

C i n d e r c o n e s &lava flowsy o u n g e r t h a n40,000 y .B .P .

F ig . 8 . A l i g n m e n t s o f c i n d e r c o n e s , t h e i r v e n t s a n d d i ke s , a n d n o r m a l f a u l t s in t h e M G V F .L a r g e d o t s i n d i c a t e c i n d e r c o n e s a n d l av a f l ow s y o u n g e r t h a n 4 0 , 0 0 0 y e a r s . A r r o w s h o w st h e r e l a t iv e m o t i o n v e c t o r o f t h e C o c o s a n d N o r t h A m e r i c a p l a te s . M a p a r e a s a m e a s i nF i g . 2 e x c e p t a t t h e e a s t e r n m a r g i n .

Conversely, gully den sity shows a consistent change from 30/90 to 10/90 over 40, 000 years, althoug h there is some scatter of th e data due to in-fluence of vegetation, visibility of gullies, prevailing wind and weather, andcinde r cone size. Smaller cones are expected to develop fewer gullies due totheir smaller area and the shorter runoff distance of rainfall water. Also,gullying may play a minor role in the degradation of cones under dry cli-mate. In general, cinder cones in the lowla nd (relatively ho t and dry climate)are r ound ed, show little gullying, and are similar to th ose of th e San Francis-co volcanic field in Arizona (mean annual precipitation, 490 mm by Ruffner,1980) (Colton, 1937; Wood, 1980b).


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118Geomorphological classification of lava flows is another useful age indexfor their associated cinder cones, and the surface features of lavas are corre-

lated with radiometric dates in Table 4. The lava flows and associated cindercones are classified into thre e groups: Holo cen e volcan oes (Hv), which areyounger than 10,000 years B.P., and the youngest two groups of Pleistocenevolcanoes (Plv4, Plv3). Cerro Las Cabras and other similar lava flows are arbi-trarily subgrouped into Plv~.3, since they have surface features betweenPlv2 and Plv3.K-Ar dates

K-Ar dates were obtained for lavas and scoriae of more degraded cindercones (Table 2). Cerro Pelon (0.37-+ 0.05 Ma) sho ws a significant diff eren cein all parameters compared to cones younger than 40,000 years B.P; H/Dratio decreases to 0.12, ma xim um and average slope angles decrease to 28 and 19 , respectively. An apparen t "y ou ng " value of gully density (11/90 )may be coincidentally due to a dry climate, as discussed above. The lavaflows associated with this cone are classified as Plv2 (Table 4). Note fromTable 4 Ply2 (and p ossibly Ply2.3) lava flow mo rp ho lo gy rep rese nts a longe rtime span t han t hat of the y oungest three groups. For this longer period,erosional processes became significant in changing the form of cinder cones.

The Santa Teresa cone west of Celaya, Guanajuato (2.78 Ma 0.07) ispro bab ly o ne of the oldest in the MGVF. It has an almost flat shape, beingpartly buried by sediments but is recognized as such by an openpit quarry.Another K-Ar age is known for the San Nicholas maar near Valle de Santi-ago; juvenile scoriae of the maar give an age of 1.2 Ma (Murphy and Car-michael, 1984).LATE PLEISTOCENE--RECENT ERUPTION HISTORY

The calibrated geomorphological classification of lava flows combinedwith the gully density allows estima tion o f the relative ages of cinder conesand lava flows. The num ber of Holo cene and Late Pleistocene volcanoes (Hv,Plv4, and Plv3) are 16, 27, and 35 respec tive ly; thu s an est ima ted 78 volca-noes erupte d within t he last 40,000 years. These volcanoes include mostlycinder cones, but also so me lava flows not associated with cones and twoshield volcanoes with sum mit cones. These you ng volcanoes are situated onlyin the southern part of the volcanic field, between 200 and 300 km from theMiddle America T renc h in an area of 15,0 00 km 2.Structural control of eruptive vents

Structural alignments of cinder cones and othe r volcanoes which haveerupt ed within the last 40,000 years are notable (Fig. 8). One such alignmentincludes Paricutin and extends about 100 km farther to the northeast. An-


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1 1 9

othe r 40 km long alig nment o f volcanoes which includes E1 Jorullo in thesouth also shows a NE direction. Altho ugh there is some scatter of cones,these alignments suggest crustal fractures along which magmas ascended.The direction of these younger cone alignments coincides with the relativemotion vector of the Cocos and North America plates (and the directionperpendicular to the minimum horizontal compressive stress) consistentwith the idea that tectonic stresses control the location of magmatic con-duits, (Nakamura, 1977; Nakamura et al., 1977). In the southern part of thefield, older parasitic cones and closely-associated cinder cones also show NEalignments. Additional tectonic information is difficult to obtain because theregion is almost completely covered by young volcanic products.

In the northeastern part of the field, the relationship between tectonicstress and cinder cone alignments is more obvious. The area surroundingLake Cuitzeo is characterized by E--W normal faults and parallel alignmentsof cinder cones. These faults may be related to the E--W-trending Chapalagraben structure s just west of the volcanic field.M A G M A O U T P U T R A T E

Multiple vents like those in the MGVF imply an absence of long-livedshallow magm a reservoirs characteristic of comp osite volcanoes, since on theevidence of Volc~n Paricutin and Volc~n E1 Joru llo, the vents had beenactive for less tha n 20 years. An expon entia l decrease of effusio n rate at Vol-cdn Paricutin indicates that no new magma batches were supplied duringerup tion (Scando ne, 1979), and it has been suggested that the magma camefrom a deep reservoir (Wadge, 1981). If the formation of a shallow magmareservoir is prevented by a small magma supply rate, a field of cinder cones isH v

V o l u m e o f l a v a + c o n e + a s h

I I I I I I I0.1 1

I n m i l l ilY 4

Sum

I 9 . 3 t03 0 . 5 k m 3I 11.3

9 . 9 - Jlv s I I I I l l l l l l l I I , I I0.01 0.1 1

k m 310

F i g . 9 . C a l c u l a t e d d e n s e r o c k ( 2 . 7 g / c m 3) e q u i v a l e n t v o l u m e s o f t h e L a t e P l e i s t o c e n e t oR e c e n t v o l c a n o e s i n th e M i c h o a c ~ n - - G u a n a j u a t o V o l c a n i c F i el d . T h e s y m b o l s , H v , P ly 4,a n d P l v 3 r e p r e s e n t t h e g e o m o r p h o l o g i c a l s t a ge s o f l av a f lo w s ( T a b l e 4 ) . C o n e v o l u m e w a sc a l c u l a t e d a s a s y m m e t r i c a l t r u n c a t e d c o n e . E a c h l a v a f lo w v o l u m e w a s c a lc u l a t e d as t h ep r o d u c t o f a r ea a n d a v e r a g e t h i c k n e s s . V o l u m e o f a ir f a li a s h w a s a s s u m e d t o b e 7 . 7 3 t i m e st h a t o f t h e a s s o c i a t e d c o n e a f t e r F r i e s ( 1 9 5 3 ) . A l l v o l u m e d a t a w e r e c o r r e c t e d f o r v e s ic u l -a r i t y b y a s s u m i n g t h a t l a v a f l o w s h a v e 3 0 % p o r o s i t y ( F r i e s , 1 9 5 3 ) , b o t h a i r fa l l a s h a n dc i n d e r c o n e s h a v e 5 0 % p o r o s i t y . V o l u m e s o f d e p o s i t s w e re n o t a d j u s t e d f o r e r o s io n a n db u r i a l b y a l lu v i al s e d i m e n t s , w h i c h m a y a c c o u n t f o r t h e s m a l l e r v o l u m e s o f P ly 3 v o l -c a n o e s .


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1 20m o r e l ik e ly t o f o r m i n s t e a d o f a c o m p o s i t e v o l c a n o ( F e d o t o v , 1 9 8 1 ;H i l d r e t h , 1 9 8 1 ; F i g . 15 ; W a l k e r , 1 9 8 3 ) .

O v e r a ll m a g m a o u t p u t r a t e w a s c a lc u l a t e d f o r t h e M G V F f o r t h e l a s t4 0 , 0 0 0 y e a r s . M a g m a e r u p t i o n v o l u m e s (d e n s e r o c k e q u i v a le n t ) f o r t h e H o l o -c e n e a n d l a t e P l e i s t o c e n e ( H v , Plv4 a n d P ly 3) a r e 9 . 3 , 1 1 . 3 a n d 9 . 9 k m 3, r e s p e c -t i v e l y ( F i g . 9 ) . T h u s , 3 0 . 5 k m 3 o f m a g m a w a s d i s c h a rg e d o v e r 4 0 , 0 0 0 y e a r si n a n a r e a o f 1 5 , 0 0 0 k m 2, y i e ld i n g a n o v e ra l l m a g m a o u t p u t r a t e o f 0 . 8 k m 3 /1 0 0 0 y e a r s ; t h i s c a n a ls o b e r e p r e s e n t e d b y a r a t e c a l c u l a t e d f o r u n i t l e n g t hp a r a l le l t o t h e M i d d l e A m e r i c a T r e n c h a n d i s a p p r o x i m a t e l y 0 . 0 0 5 k m 3 / k mp e r 1 0 0 0 y e a r s .

A c o m p a r i s o n w i t h o t h e r c o m p o s i t e v o l c a n o e s a t c o n v e r g e n t p l a t e b o u n d -a r ie s i s m a d e i n F i g . 10 . M a g m a o u t p u t r a t e s o f 1 3 c o m p o s i t e v o l c a n o e s f r o mJ a p a n , U S S R , a n d A m e r i c a r a n g e f r o m 0 . 4 t o 2 7 0 k m 3 / 1 0 0 0 y e a r s ( C r i sp ,1 9 8 4 ) . I n M e x i c o , th e o u t p u t r a t e s o f C o l i m a a n d C e b o r u c o a r e 2 .7 k m 3 /1 0 0 0 y e a r s a n d 6 k m 3 / 1 0 0 0 y e a r s , r e s p e c t i v e l y ( L u h r a n d C a r m i c h a e l , 1 9 8 0 ,1 9 8 2 ; N e l s o n , 1 9 8 0 ) . I n c o n t r a s t S i e r ra L a P r i m a v e r a rh y o l i t ic c o m p l e x h a s ar e l a t iv e l y l o w m a g m a o u t p u t r a t e o f 0 . 5 k m ~ / 1 0 0 0 y e a r s ( C r is p , 1 9 8 4 ;M a h o o d , 1 9 8 0 , 1 9 8 1 ; W r i g h t, 1 9 8 1 ) . T h e e n t i r e M G V F is a m o n g t h e sm a l -l e s t ( F i g . 1 0 ) .

1 o 3

x 8

/ . , /10" i a I1 0 2 1 0 3 1 0 4 1 0 5 1 0 6

D U R A TI ON O F M A G M A T I S M [ y r ]F i g. 1 0 . M a g m a o u t p u t r a t e s o f v o l c a n o e s a t c o n v e r g e n t p l a t e b o u n d a r i e s s h o w n o n a l og a -r i t h m i c p l o t o f d i s c h ar g e v o l u m e a g a in s t d u r a t i o n o f m a g m a t i s m . D i a g o n al li n e s i n d i c a tee qua l ma gma ou t pu t r a t e s . F i l l e d c i r c l e s r e pre se n t t he Me xi c a n vo l c a noe s ' . C O = C o l i m aa f t e r t h e 1 8 1 8 m a j o r a s h f l o w e r u p t i o n ( L u h r a n d C a r m i c h a e l, 1 9 8 0 , 1 9 8 2 ) ; C E = C e bo-r u c o a f t e r th e P l i n ia n e r u p t i o n o f J a la p u m i c e ( 1 0 0 0 y e a r s a g o ) ( N e l s o n , 1 9 8 0 ) ; S P =S i e rr a l a P r i m a v e r a d u r in g a n d a f t e r t h e a s h f l o w e r u p t i o n o f th e T a l a T u f f ( 9 5 , 0 0 0 y e a r sa go) (Ma hood , 1980 , 1981; Wr i gh t , 1981; C r i sp , 1 9 8 4 ) ; M G F V = M i c h o a c A n - G u a n a j u a t oVo l c a n i c F i e l d ( t h i s s tudy ) . C rosse s r e pr e se n t t he vo l c a noe s f rom o t he r r e g i ons (Wa dge ,1 9 8 2 ; C r i sp , 1 9 8 4 ) : 1 = A r e n a l, C o s t a R i c a ; 2 = K a i m o n d a k e , J a p a n ; 3 = E d g e c u m b e ,U S A ; 4 = S a k u r a j i m a , J a p a n ; 5 = O s h i m a , J a p a n ; 6 = A s a m a , J a p a n ; 7 = F u e g o , G u a t e -m a l a ; 8 = A v a c h i n s k y , U S S R ; 9 = K l u y c h e v s k o y , U S S R ; 1 0 = F u j i, J a p a n ; / 1 = S h i v el u ch ,U S S R ; 1 2 = H a k o n e , J a p a n , 1 3 = C a l a b o z o s c a l d e r a, C h ile . M a g m a o u t p u t v o l u m e o f t h ePar icut in reg ion (Fig. 3) is a l so plot ted .

E 1 0 2x3


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Whether the magma discharge volume of th e MGVF is ~equivalent to acomposite volcano is difficult to estimate. Those volcanoes whose outputrates were calculated for comparison have surface areas less tha n 500 km 2(Crisp, 1 984), whereas the last 40,000 years of activity in the MGVF coversan area of 15,000 km 2. Although the surface area of a volcano does no tnecessarily reflect its source, the MGVF is likely to sample a larger area ofmagma source region than a compos ite volcano, since compos ite volcanoes inmost volcanic arcs have an average spacing closer than the dimensions of theMGVF (ca. 200 km). As an attempt to make a reasonable basis for compari-son, high cone density areas in Fig. 8 are assumed to represent a magmabatch equivalent to that of a single composite volcano. For example, theParicutin region (Fig. 3) yields a magma ou tp ut rate of 0.12 km3/1000 years,which is about 1/7 of the entire MGVF. This value is the lowest among thereported magma output rates at convergent plate boundaries. These calcula-tions indicate that the magma discharge rate for the MGVF is smaller thantha t of a single comp osite volcano despite the evidence of geographically ex-tensive volcanic activity. It seems that the myriad vents of the MGVF reflecta small magma discharge rate, and probably a small supply rate to the lowercrust.CONCLUSIONS

(1) The Michoac~in--Guanajuato Volcanic Field (MGVF) contains 1,040volcanoes within an area of 4 0,000 km 2. Most of these are cinder cones butalso include lava domes, maars, tuf f rings, shield volcanoes, and conelesslava flows.

(2) The median-sized cinder cone in the MGVF is 90 m high and has abasal diameter of 800 m, a crater diameter of 230 m and a volume of 0.021km 3. For lava flows, the med ian thickness is 40 m and t he med ian length is3 km.(3) Gully density norm alized to a 90 arc and t he geomorphol ogical classi-fications of lava flows are sensitive indicators of cinder cone age but haveonly been calibrated for the last 40,000 years.(4) Some of the late Pleistocene--Recent volcanoes form NE alignments,which parallel the relative motion vector for the Cocos--North Americaplates.(5) For the last 40,000 years, the average magma ou tp ut rate in theMGVF is 0.8 km3/1000 years, and in the Paricutin region, 0.12 km3/1000

years, a value small in compar ison to a compo site volcano.ACKNOWLEDGEMENTS

We tha nk Dr. J.F. Luhr for his contin ued guidan ce in volcanolog y andProf. G.A. Mahood for K-Ar dates. Radiocarbon dates were made by Tele-dyne Isotopes. Thanks are also offered to many U.C. Berkely students, espe-


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122c i a ll y R . L u d w i n , M . K o k i n o s , a n d S . K e a t i n g f o r t h e i r a s s i s ta n c e i n t h e f i e ld ,a n d t o M . W o p a t f o r h e l p i n g t o m e a s u r e c i n d er c o n e s . A . M iU ar a n d W .C h a v e z i m p r o v e d t h e E n g l is h . C o m m e n t s b y D r s. B .H . B a k e r, J .F . L u h r , K .N a k a m u r a , M . S a k u y a m a , H . F er r iz , S . L i n n e m a n , a n d a n a n o n y m o u s re -v i ew e r g r ea tl y i m p r o v e d t h e m a n u s c r i p t . T h e s u p p o r t o f N S F E A R 8 1 -0 3 3 4 4 , E A R 8 2 - 1 9 9 4 5 ( C a r m ic h a el ) is a c k n o w l e d g e d .

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Hasenaka Carmichael 1985