CHAPTER XIII

JORULLO, A YOUNG VOLCANIC MOUNTAIN

You must not suppose that when we speak of Jorullo as a young volcanic mountain that we mean young in the sense that you or I might be called young, but young as regards mountains; for Jorullo, now a great mountain range, had no existence before the year 1759, and that would make the mountain a little less than 150 years old, which so far as mountains are concerned may properly be regarded as quite young.

The story of Jorullo is very interesting, and affords an excellent example of the great scale on which modern volcanic eruptions take place during historical times.

If you examine the map of Mexico on page 86 you will see that Jorullo lies 170 miles southwest of the city of Mexico, and 108 miles from the Pacific Ocean, which is the nearest large body of water. This mountain is of especial interest because, if old traditions are to be believed, it was thrown up during practically a single night. This wonderful event took place on an elevated plain or plateau, called the Plain of Malpais, that lies between 2,000 and 3,000 feet above the level of the ocean. The plain was situated in a part of Mexico that was celebrated for the growth of the finest cotton and indigo in the world. It formed the large estate of a wealthy planter, Señor Pedro de Jorullo, who lived at his ease as a wealthy planter is apt to do in tropical countries like Mexico.

Jorullo's plantation was covered by an especially fertile soil, since it was formed by the deposits of volcanic ashes, dust, tufa, etc., produced, most probably, by neighboring volcanoes long before man appeared on the earth, for the plain of Malpais was bounded by hills that were composed of volcanic materials. There had, however, been no signs of volcanic activity in the neighborhood. It had indeed been quiet, so far as volcanic eruptions were concerned, since the time of the discovery of America by Columbus, until the middle of the last century. The fertile fields of the Jorullo plantation were watered by two rivers, or as we would probably call them, brooks, the Cuitamba and the San Pedro.

Signs were not wanting of the coming calamity. During June, 1759, subterranean sounds were heard of a low rumbling character, which every now and then increased until they resembled in intensity the sounds produced by the firing of large guns. These sounds were accompanied by earthquake shocks that greatly terrified the people and caused them to flee from their homes. Nothing, however, occurred, so, becoming accustomed to the noises, the people returned to their houses. The noises and tremblings ceased for over two months, until, on the 29th of September, 1759, they were again heard, and a terrible eruption began. A long fissure opened in the earth, extending generally from northeast to southwest. From this fissure flames burst out, fragments of burning rock and stone, together with large quantities of ashes were thrown to great heights in the air, and were followed by streams of molten rock. Six volcanic cones were formed along the fissure. The highest of these cones is what now constitutes the volcanic mountain of Jorullo, which then reached a height of at least 1,600 feet above the level of the plain. From its cone were thrown out great quantities of lava of the same type as that which escaped from the craters of many volcanic islands such as Hawaii and Iceland, namely, basaltic lavas. This eruption, which began on the 29th of September, 1759, continued until the month of February, 1760.

The account as above given was obtained by Humboldt, who visited the country some fifty-six years after the eruption. This story was told him by the Indians, but was also recorded in verse by a Jesuit priest, Raphael Landiva, a native of Guatemala. According to the account given Humboldt by the Indians, it appears that when a long time after the eruptions had quieted down, they had returned to their old homes with the hope of cultivating part of the grounds, they found the plains still too hot to permit their living on them.

According to Lyell, there was around the base of the cone, spreading from them as a centre over an area of some four square miles, a convex mass, about 550 feet in height, most of the surface of which was covered with thousands of small flattish conical mounds from six to nine feet in height. These, together with numerous large fissures that crossed the plain in different directions, served as points for the escape of sulphur vapors, as well as for the vapors of hot water.

During the escape of lava from the craters in 1759, the molten rock, spreading over the plain, ran into the channels of the river or brooks before named, driving out the water. This water reappeared at the base of the mountain in numerous hot springs.

Humboldt thought that the conical mountains had been lifted or raised by the formation of huge bubbles formed under the lava, thus causing it to assume a shape not unlike that of a huge bladder. This opinion, however, has not been accepted by geologists at the present time. Scrope points out that this was probably the origin of the little conical mounds that covered the surface of the principal conical mounds but was not, in all probability, the cause of the mound itself. He says:

"With regard to the disputed question as to the origin of the raised plain of the Malpais, M. de Saussure, the last and most trustworthy visitor, entirely confirms the opinion which I ventured to proclaim in 1825, that Humboldt was mistaken in supposing it to have been 'blown up from beneath like a bladder,' and that it is merely an ordinary current of lava, which, owing to its very imperfect liquidity at the time of its issue from the volcanic vent, as well as to the overflow of one sheet or stream upon another, had acquired great thickness about its source, gradually thinning off towards the outer limit of the elliptical area it covered."

If you have been able to follow the above you will see that Mr. Scrope means that in his opinion the cone of Jorullo is a lava cone like that we have already studied on Mt. Loa or Mt. Kilauea, or, in other words, that the lava as it came out from the opening on the top of Jorullo, flowed in all directions around the opening, thus building up a mountain in the form of a flat lava cone.

Perhaps one of the reasons Humboldt had for believing the entire elevation of Jorullo to be due to the formation of a huge bladder was the fact that the plain on which the cone is situated, when struck, gave out a sound as though there was a vast hollow space below it. This was especially the case when the hoofs of the horses driven over its surface produced sounds as though they were moving over the summit of a hollow dome-like space below. But, as Lyell points out, this was probably only due to the fact that the materials forming the cone were very light and porous.

According to Burkhardt, a German mining engineer who visited Jorullo in 1827, there appears to have been no other eruptions of the volcano since the time of Humboldt's visit. Mr. Burkhardt descended to the bottom of the crater and observed that small quantities of sulphurous vapors were still escaping. The small cones or _hornitos_, however, on the slopes had entirely ceased emitting steam. It appeared, too, that the twenty-four years that had passed since the time of Humboldt's visit, the rich soil of the surrounding country had permitted the successful cultivation of some crops of sugar cane and indigo.

Russell appears to doubt the reliability of the information obtained by Humboldt concerning Jorullo. He suggests that a poetical account by the Jesuit missionary from whom Humboldt obtained much of his information was not apt to possess marked scientific accuracy. While, however, this may be true, yet to a certain extent it seems entirely probable that the principal facts were as above given. The following account as given by Humboldt, is taken from a translation made in the early part of 1800:

"The affrighted inhabitants fled to the mountains of Aguasarco. A tract of ground from three to four square miles in extent, which goes by the name of Malpays, rose up in the shape of a bladder. The bounds of this convulsion are still distinguishable in the fractural strata. The Malpays, near its edge, is only twelve metres above the old level of the plain called the Playas de Jorullo; but the convexity of the ground thus thrown up increases progressively towards the centre, to an elevation of 160 metres (524.8 ft.).

"Those who witnessed this catastrophe from the top of Aguasarco assert that flames were seen to issue forth for an extent of more than half a square league, that fragments of burning rocks were thrown up to prodigious heights, and that through a thick cloud of ashes, illuminated by the volcanic fire, the softened surface of the earth was seen to swell up like an agitated sea. The rivers of Cuitamba and San Pedro precipitated themselves into the burning chasms. The decomposition of the water contributed to invigorate the flames, which were distinguishable at the city of Pascuaro, though situated on very extensive table-land 1,400 metres (4,592 ft.) elevated above the plains of Las Playas de Jorullo. Eruptions of mud, and especially of strata of clay enveloping balls of decomposed basalt in concentrical layers, appeared to indicate that subterranean water had no small share in producing this extraordinary revolution. Thousands of small cones, from two to three metres in height, called by the indigenes ovens, issued forth from the Malpays....

"In the midst of the ovens, six large masses, elevated from 400 to 500 metres each above the old level of the plain, sprung up from a chasm, of which the direction is from N. N. E. to the S. S. E. This is the phenomenon of the Montenovo of Naples, several times repeated in a range of volcanic hills. The most elevated of these enormous masses, which bears some resemblance to the puys de l'Auvergne, is the great Volcan de Jorullo. It is continually burning, and has thrown up from the north side an immense quantity of scorified and basaltic lavas containing fragments of primitive rocks. These great eruptions of the central volcano continued till the month of February, 1760. In the following years they became gradually less frequent.... The roofs of the houses of Queretaro were then covered with ashes at a distance of more than forty-eight leagues in a straight line from the scene of the explosion. Although the subterranean fire now appears far from violent, and the Malpays and the great volcano begin to be covered with vegetation, we nevertheless found the ambient air heated to such a degree by the action of the small ovens, that the thermometer at a great distance from the surface and in the shade rose as high as 43° C." (109° 4' F.).