Siwa Oasis (Egypt): Desert Oases
1. Zoom from the Namib to the Sahara.
2. The sahara is unimaginably large. At 9.5 million km2, it is almost as big as the
US (and that’s including Alaska – 9.8 million km2!)
a. From Arabic word for desert – Sahra.
b. Only the desert of Antarctica is bigger. (By far the largest hot desert.)
c. Describe its extent, and the reason for its existence (top of the
northern hemisphere Hadley cell; cool dry air falls and absorbs
available moisture from the ground.
d. But it is also extremely varied. Actually, only 20% is sand (despite
popular views). There ARE places where camels cross seas of sands,
but it is a small percentage of desert.
i. Giant sand ergs [show map]
ii. Mostly desert pavement (“hamada”), saw last lecture
iii. Also mountain ranges.
iv. Some strange features like the Richat Structure: 50 km wide
“bull’s eye” that is an eroded uplifted dome in Muaritania (far
west)
e. There have been a few major cities in the Sahara over time, like
Timbuktu in Mali, but few and far between
i. Berbers – native inhabitants of north Africa, from western
Egypt all the way to the west. Cave paintings show their
inhabitancy going back 12,000 yrs. They were assimilated into
the Islamic empire in the 11th century, but maintain their own
identity.
ii. And there has been travel and trade, long caravans, that have
crossed the desert between settlements.
iii. The Tuaregs have crossed the desert with salt caravans for
centuries (still do); Tuareg caravan with over 10,000 camels,
stretching 25 km, crossing the sahara in 3 weeks, such as from
the Timbuktu (Mali) to the Mediterranean.
f. The Sahara is divided into:
i. western Sahara,
ii. the central Ahaggar (Hoggar) Mountains (southern Algeria)
iii. the Air Mountains (a region of desert mountains and high
plateaus, a mile high, in Niger; with granite outcrops, some old
volcanoes)
iv. Ténéré desert (dry desert between Niger and Chad)
v. the Tibesti Mountains (Chad) (a set of continental intraplate
volcanoes, like Yellowstone, over a mantle hotspot plume)
1. The volcano Emi Koussi (80 km wide!) is the tallest
mountain in the Sahara – elevation of 3.5 km! (2.3 km
above the local plains)
vi. the Libyan desert (the most arid region – Libya and western
Egypt).
1. About 1000 km x 1000 km.
2. This is where the site of the Siwa Oasis is. In the middle
of a dry, arid, barren area, with almost no rainfall, there
is water: springs, lakes, etc.
3. It is a remarkable thing – in the midst of a broad desert, to find so much
water. There are more than 1000 springs in the oasis; most are too salty to
drink, but a few hundred are fresh water.
a. Siwa sits in a broad depression, 80 km long and 30 km wide. On the
north there is an escarpment that is cut by many wadis (stream
valleys). On the south it is bordered by the dunes of the Great Sand
Sea, which stretches 500 km to the south.
b. The depression is up to 60 m below sea level in places
c. There are several salt lakes in the oasis, fed by the springs. The largest
is >30 km2. Arab legend is that one of these lakes was magic:
i. No bird could fly over it.
ii. On an island of the lake are buried the saber and seal of the
Prophet Mohammed (in another version, it is the ring, sword
and crown of King Solomon that are buried here).
d. Because the water is generally so salty, very little will grow there.
However, date palms have adapted to salty water, so they have
traditionally been the staple of the Siwa Oasis. There are over 200,000
date palms there, and all parts are used – roof beams, furniture, mats,
basket.
i. The heart of the palm is a delicacy, but removing it usually kills
the tree, so it is only rarely harvested.
ii. Date wine is made, but it also kills the tree. It has traditionally
been used in all festivals, but is illegal under Islamic law, so is
made in secret.
iii. There are also >25,000 olive trees in the oasis, and because
olives fetch a higher price, it actually earns more than the date
crop.
iv. Siwa traditionally traded these with other communities, and
there are probably more caravan tracks leading out of Siwa
than any other oasis – some so narrow that a 4x4 can’t even
make it – only camels.
e. Because the water is salty, the mud that is used to make the bricks of
their buildings is filled with salt. Unfortunately, a few times a century
Siwa actually gets a rain storm, and in 1928, the ancient town of Shali,
founded in 1203, was destroyed so badly, that the people abandoned
it. The remains are still there.
4. The Berbers have inhabited the Siwa Oasis for at least 12,000 years. At that
time, Egypt may have been even drier than today.
a. But from about 9000-5000 yrs ago, Northeastern Africa benefitted
from a wet, most climate. Called “Wet Sahara” or “Green Sahara,” there
was a shift in air circulation patterns and Egypt benefitted from a
monsoon climate, with strong rains part of the year. Egypt was Green,
and people travelled easily from the central to the northern parts
(now the Sahara largely cuts them off!).
b. There are famous caves in a region south of Siwa called the Wadi Sura,
where there are Neolithic cave drawings that suggest plentiful water –
the Cave of Swimmers (which inspired a similar story in The English
Patient).
c. It was an independent Berber Oasis for thousands of years, but at
some point it was conquered by the dynastic Egyptians (probably
Ramses III, over 3000 years ago), and became part of the Egyptian
Empire.
d. When the Greeks settled in Libya, they heard of a wise oracle at the
Temple of Amun, and elevated it to a place of great honor. Soon, one
by one, all the great leaders of their times came to Siwa to visit the
Oracle, not always happy with what they were told. Cleopatra came,
and is known to have visited also to bathe in the hot springs.
e. The most famous visitor was Alexander the Great, who visited in 331
BC, and was told that he was a god and the rightful leader of the
empire.
f. It seems likely that Siwa became Christian at some point, and fended
off Islamic attackers from 700 onward, but finally was conquered and
became Islamic in 1150. At that time, there were only 40 inhabitants
left (now there are about 23,000!). it is currently Islamic, and people
speak Islamic, but they also still speak their original Berber language.
g. In 1819, the Egyptian leader Muhammad Ali began to try to make
Siwa, along with the other Oases, part of Egypt. He eventually
succeeded, but it took almost 20 years.
h. It also ended up playing important roles in battles during both WWI
and WWII, though much to the misery of the Siwans.
5. Why is there even water here? How is it that there are 1000 springs here.
a. The Sahara, and other deserts, is occasionally interrupted by a region
of water and green. It comes from a layer of water underground that is
saturated with water – an aquifer. The water gets to the surface by
one of several ways, such as the intersection of the aquifer with the
surface or an earthquake fault. [show diagram; explain]
b. In the case of the Siwa, there is a layer of sandstone (Nubian
Sandstone) that is both porous and permeable. Beneath Siwa it is 2.5
km thick!! Estimates are that the Nubian sandstone contains 50,000
km3 of water (equal to about 600 years of Nile river water).
6. Water is scarce – how did so much water get there? Have to look at Siwa’s
geological history. It wasn’t always so dry!
a. Layers of limestone and sandstone formed in Egypt, like other places
around the world, by times when the ocean flooded the continent and
deposited sediments. These layers were initially flat, but began to be
tilted upward in the south (associated with the Afar hotspot that is
opening the Red Sea and causing the African rift valley).
b. By 35 million years ago, there was a large river system, the Gilf River,
that drained much of Egypt. It emptied into the Tethys Sea at the exact
location where the Siwa Oasis is now (coast was farther south back
then). At that point, there was no Nile River.
c. By 25 million years ago, an early Nile was forming, but it ran south (!),
emptying into the desert somewhere! The Gilf River began to shrink.
And the coast moved north, as sediments were deposited and sea
levels dropped.
d. By 10 million years ago, the tilting had increased, and the nile was
now draining to the Mediterranean, and the Gilf river system was
drying up.
e. So where did all that water of today’s Siwa oasis come from?
Radiometric dating of it has found that it is older than 20 million
years. It sank into the ground when it was topped by a major river
system for millions of years. This is not renewable water – it formed
during a warmer, wetter period.
7. Though the lectures I am giving are on distinct geological wonders and
geographic locations, they still can be connected in often strange and
interesting ways.
a. Ex/ When the Med closed at the Rock of Gibraltar 5 million years ago.
b. The Nile river suddenly dropped kilometers at the delta. This became
a huge canyon that then cut back for many hundreds of miles into
Africa – would have dwarfed the Grand Canyon!
c. Eventually, the Sea rose again and the deep Nile canyon eventually
filled with sediments!
8. The Siwa has a limited future, as do all the oases. The heavy draw on the
water (>20,000 people) is making the soils so salty that even the date palms
are starting to die. Groundwater is often needed most in dry areas, but was
put in the ground during a time of much wetter climate, so is not a renewable
resource during our lifetimes.
9. Top 5:
a. Chebika Oasis (Tunisia) – was where they filmed the first Star Wars
movie. “Chewbacca” comes from this.
b. Huachina (Peru) – oasis in the middle of giant sand dunes
c. Ubari (Libya)
d. Timea (Niger) – in the Air Mountains
Questions:
1) How do you think it came to be that the Siwa depression and the larger
Qattara depression to the north are below sea level? (Hint: Why doesn’t this
happen more often in other parts of the world?)
2) The date palm tree has evolved to extract fresh water out of salty water.
What are the implications of this for agriculture and soil quality?