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.).