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| 1 |
Figure 1, Street in Mzil adobe town.
From the Geology and Paleontology Area of the Alto Bierzo Museum and the Aragonito Azul Mineralogical Association, we thank Carlos García Blanco, geologist and member of INGEMI (a multidisciplinary geological and mining research group dependent on our organization), for this documentary contribution which serves to show the geology of other distant countries.
On March 19th, 2026, four daring explorers, during the Eid al Fitr holiday, went to visit the Al Wabah maar, in western Saudi Arabia. From Riyadh we went westwards along Route 40, towards Mecca; on the way, to break up the 700 km journey, we stopped to visit an adobe (mud brick) town in Mzil, where the outer wall has been rebuilt and the inside is slowly collapsing.
Once we did this visit, we continued our way, and
traveled through a long sandstorm, which sand blasted almost all the numbers
from our license plate.
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Figure 2, on a sandstorm. On the background,
trucks.
Near our destination and inside the volcanic field of
Harrat Kishb, we stopped again to visit very interesting neolithic ruins. A lot of structures are visible, built with
volcanic rocks. As sadly is usual, they had been visited by grave diggers.
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Figure 3, alignment of stones with a big
neolithic structure at the background.
Finally, we arrived at Al Wabah by sunset, we found a
cinder crater nearby to set the camp for the night with a nice bonfire.
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Figure 4, crater where we sleep, a small
accumulation of cinder and lava.
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Figure 5, some volcanic bombs from the small
crater, and a piece of basalt with abundant vacuoles, some of them filled with
carbonates.
In the morning, breakfast and to explore the Al Wabah
maar. But, before that, an introduction to the area and the maars.
The lava field where Al Wabah is situated is called
Harrat Kishb, it is the youngest of the lava fields in Saudi Arabia, having
developed in the last two million years. Lava are basanites to alkali olivine
basalts, with scarce phonolites. Frequently xenoliths of mantellic peridotite
and pyroxenite appear, indicative of young lavas coming from the mantle.
At the northern margin of the Al
Wabah maar there is a small volcanic cone that can be related with the maar, as
we will see. A maar is, modified from Wikipedia:
A maar is a broad, low-relief volcanic crater caused by
a phreatomagmatic
eruption (an explosion which occurs when groundwater comes into
contact with hot lava or magma). A maar characteristically fills
with water to form a relatively shallow crater lake,
which may also be called a maar. Maars generally range in size from 20 to 3,000 m across and from 5 to 200 m deep. Most
maars fill with water to form natural lakes. Most maars have low rims composed
of a mixture of loose fragments of volcanic rocks and rocks
torn from the walls of the diatreme.
Diatreme is the inverted cone product of the
explosion. In economic geology maars are important as a source of diamonds, the
big mines in Siberia, South Africa, India and Brazil are in diatremes. In Al
Wabah this is not the case, it has been investigated and there are no diamonds.
At the bottom of maars there are frequently lakes. In Al Wabah the lake only
appears in rainy season like during our visit.
A
short introduction to the geology of the area.
The basement in this area of Saudi Arabia are diorite
and granitic diorites, with some andesites, all Ediacaran age
(Precambrian).
Over the basement there are two volcanic flows,
quaternary, which belong to the Harrat Kishb. The lower is around 30 m thick
and of 1,24 million years ago, the upper of around 5 to 10m thick and 1,16
million years ago. Both are affected by columnar jointing, and at the base of
the upper Flow there are pillow lavas and hyaloclastites weathered, which are
of interest as we will see.
Over these lava flows appears directly the debris of
the maar explosion, which are poorly selected and slightly silicified
materials, where all the lithologies described above can be found, and some of
the residues of the lava which provoked the explosion included crystals of
olivine, pyroxene (augite), some of a notable size, centimetric.
The origin of the explosion seems to be related to
the small cone at the north side of the maar; first, the cone erupted and grew,
then it finished at this stage. Later, lava tried to go up again, but the
conduit was plugged, so it moved slightly to the south, but there was a small
lake there (hence the pillow lavas and hyaloclastites), with an associated
underground water system. When the lava ascending found the water, it created
vapor at high pressure, provoking a huge explosion that created the maar. On
the volcanic plug at the lateral cone a geo chronology has been done, giving a
maximum age of 170.000 years to the maar.
In the following figure I’ve done a sketch of the
maar.
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Figure 6, sketch of the Al Wabah maar. In red,
explosion deposits; in V, volcanic flows; In X, diorites; In blue, diatreme
filling; in green temporary lake, which dries out leaving magnesium sulfate salts.
After this introduction, back to the maar. At the end
there is a bibliography for more information, all free access on the net.
The crater is elliptical around 2,500 by 3,000 m
diameter, with a depth of around 250 m.
At the crater walls can be seen the materials described above: diorite, lava
flows and explosion debris.
Arriving at the maar, we went over a bit over the
explosion debris, to the crater side with an amazing view, then we descended to
the bottom of the crater. The path is a bit dangerous, with narrow passages and
cuts over 10 m below, there is no protection there.
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Figure 7, the crater bottom with the lake and salts.
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Figure 8, detail of the crater wall, both lava flows
can be seen, and atop the explosion debris.
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Figure 9, detail of the explosion debris with a bomb
inserted in the lower level.
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Figure 10, pillow lavas and hyaloclastites.
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Figure 11, the path, narrow and with cuts in
the rocks.
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Figure 12, lava flow with deformed vacuoles by
the flow.
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Figure 13, bottom of the crater, two people
for scale.
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Figure 14, at the right side, the path.
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Figure 15, bottom part of the path.
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Figure 16, once back at the top, we went to
the other side of the maar, towards the east, from here we can see the cone
next to the crater.
And from here, we went to see a nearby lava
field, not related directly to the maar, but worth a visit, with a lot of lava
structures: ropey lava, hornitos, collapsed tunnels…
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Figure 17, ropey lava.
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Figure 18, depression in lava, full of eolian
sand and silt.
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Figure 19, ropey lava.
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Figure 20, lava flow marks.
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Figure 21, ropey lava.
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Figure 22, micro plate tectonic, lava crust is
broken and the sides are submerging into the hot lava: a rift and two
subduction zones.
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Figure 23, small hornito.
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Figure 24, and finally, go back to Riyadh,
with the memories of this amazing visit.
Thanks to Mike C. for his help with the
translation.
Bibliography.
-The geology and geochronology of Al Wahbah
maar crater, Harrat Kishb, Saudi Arabia. (2014). Quaternary Geochronology ,21,
70-76
-The Al Wahbah Crater of Saudi Arabia and its
geotourism potential. (2022) Geotourism vol.
19, 3‒4 (70‒71)
-Geoheritage values of one of the largest maar
craters in the Arabian Peninsula: the Al Wahbah Crater and other volcanoes
(Harrat Kishb, Saudi Arabia) (2013) Cent. Eur. J. Geosci. 5(2), 254-271
-Factors controlling the internal facies
architecture of maar-diatreme volcanoes (2013) Bull. Volcanol. 75:761
That's how things are, and that's how we've told them to you.
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