地质之旅 东非裂谷.doc

Geologic Journey: African Rift 地質之旅東非裂谷The Great Rift Valley.Things have happened herethat happen nowhere else.Thats the underlying truth to this place.Within its walls, our species evolved,civilizations rose and collapsed.And theres an even bigger drama at play.The birth and death of oceans.Its all due to the impact of tectonics.Geologist Nick Eyles is on a questto explore its power.From the cradle of humanityto the hottest place on Earth,to the ruins of the oldest civilizationsand the bedrock of some of the worldsmodern religions- a journey along the Rift.Theres something primeval being up thereand looking down into the depths of the Earth.Holy cow!Its an absolutely fantastic experience.Not just as a scientist, but as a human being.This is the East African Rift,one part of the Great Rift Valleythat stretches from Mozambique to Turkey.Its a special place for humankind.Embedded in these cliffs,packed between layers of volcanic ash,is the story of our origin.The Rift system is our natural home,as it were. Thats where humans evolved.We developed in association with volcanoesand faults, earthquakes.So its a great view of where we originated.The whole landscape is covered withgreat thicknesses of ash,which is a good thing because those ash layersentomb the remains of early humansand their ancestors and they provideprecise marker horizons of time.Ever since Darwin suggested thathumans share common ancestorswith African apes,scientists have searched for further clues.The only place they found fossilsthat link directly to modern humansis in this long, narrow valley.The intriguing question is why.Canadian geologist Nick Eylesseeks to understand how human historyis linked to the geologyof the Great Rift Valley.He begins his journey in Kenyato explore the theory that the Riftwas the catalyst for human evolution.He meets German geologist Martin Trauth.Trauth has spent his careerdating sediments and cross-referencing themwith evolutionary changes.Hes concluded that each step in our evolutioncoincided with the appearanceor disappearance of large lakesin the Rift basin.If you have major events in human evolutionoccurring at the time where we havelarge lakes everywhere, thats probablyan evidence for that there should be a linkbetween climate, environmental changesand the evolution of early humans.So if large lakes were the keyto human origins, what happenedwhen these lakes disappeared?When theyve disappearedprobably they have to change dramaticallythe strategy to find food in the environment.But these lakes, they reappear in cyclesand so that makes them very good for humansto survive as they are generalists.The frequent environmental changesdrove our ancestors to evolve waysto better adapt to new landscapes and diets.Here at Kariandusi, Trauth has studieda particular lake deposit called diatomitethat dates back a million years.This is actually a lake sedimentthat consists of millions and trillionsof skeletons from little plants.As the lake dried up, early humansleft traces of their lives-hand axes embedded in the walls of the gorge.So what uniquely defines a tool and a handaxe?All the tools are just very, very simple,round shaped kind of hammers.So these ones are really for cutting meat.We see the development of stone toolsabout 2 million years agoand the development of human brainsand to me thats not coincidental.Youve got to find the rock,youve got to know what rock typemake good tools. Youve got tocommunicate that to others.You need a social organizationto manufacture those stone tools.To me that was the firstindustrial revolution because it requiredmanufacturing on a huge scale.To understand why this revolution happened,its necessary to delve deepinto the tectonic story of the Rift Valley.Geologists come to the East African Riftbecause its the largest tearaffecting any plate on the planet.Its a huge wound which is opening up.That wound arose because the African platelies above a huge plume of magmathats slowly melting the crust,causing it to stretch or rift.On the edges of warmed rock,high shoulders rise up.In the middle, the floor subsides,forming a rift.3 million years ago, repeated rapid swingsof climate from wet to dry,caused huge lakes to form-and suddenly disappear.Combined with frequent volcanic eruptionsand earthquakes, our ancestorsfaced unbelievable odds.For Nick, its part of a greater truththat hes exploring,that the Great Rift Valley is unique.Things happen here that happen nowhere else.One of the great findings of geologyover the past 40 years is thatoceans come and goas super-continents form and break up.But how do continents splitand how do new oceans form?To find the answer, Nick comes here.the Afar Depression in Ethiopia.The Afar Depression is one ofthe most dramatic landscapes on Earth.Few people live here and even fewer visit.But to geologists like Nick Eyles,its Shangri-la.This is a fantastic place to bebecause were driving along the middleof the East African Riftwhere the African plate is literallybeing torn in two.And Africa is over here, the Somalia plateis on my right and theyre moving apart.And the Rift between the twois slowly subsiding.This tectonic process has been underwayfor 30 million years.But the Afar is so remotethat its significance has eluded geologistsuntil recently.For Nick, its a once-in-a-lifetimeopportunity. His ultimate destination-a volcano called Erta Ale,or Smoking Mountain, in the local language.To get there he must first crossthe Danakil Desert.This is a serene landscape.These are enormous lava flowswhich have come down from margins of the Rift,from these big volcanoes.Very fluid lava extending many kilometresdown slope. But here theyve been broken upby faults. Its absolutely amazing.Its just geology happeningbefore ones eyes.The next morning the tarmac ends.Nicks transported back in time.To the dawn of life.This is the Dallol Crater,a volcano whose summitis still below sea level.The crust here is incredibly thin,the magma close to the surface.Whats unique about this place is thatthis volcanic activity is interactingwith the very thick salt deposits.Early life may have thrived in environmentslike these, 4 billion years ago.First to arrive are the hot fluidsmigrating up above the magma.And then puncturing the saltare those beautiful little industrial pocks.The hot water is coming upbringing sulphur bearing minerals,beautiful colours and theres bacteria-very simple blue-green bacteriain there that love these hot waters.These simple, colourful bacteriaare Earths oldest life form.And this maybe a glimpse of early earth.This whole area is underlain withwhat is referred to as a mantle plume,a huge mass of red-hot plastic rockwhich is rising up from the mantle,impinging on the base of the African plateand pushing it apart, splitting it into two.As we go further out in the Afar Depression,we go below sea level. And at the momentthe ocean is only preventedfrom flooding into this area by volcanoes.Eventually they will subsideunder the great weight of all the volcanic rockand the sea will burst in here.In 2005, volcanic eruptions and earthquakesrocked the Afar Depression.Fissures opened up.Large flows of black basalt lavaflooded the landscape,creating crust of a new ocean floor.Nick crosses paths with a group of scientistsfrom Britain trying to explainwhy that new ocean is forming here.They use seismometers to trackthe numerous earthquakes that hit the Afar.Each quake corresponds to the injectionof molten rock into the crust,slowly splitting the continent in two.So how many of these do you haveacross the Afar?In total I think theres somewherearound 45 thats combined between U.K.seismometers and U.S. seismometers.The moment of truth.In this particular area the main axisof the Rift is Erta Ale and Tatali volcanoes.Dramatically to the West.And then further to the South.So theres a well-defined rift axis.So youre right at the beginningof this whole new ocean.With each geologic eventthe Afar Depression evolves.Nick is now 20 kilometresfrom the volcano.His guide, Solomon Berhe,takes him to meet some remarkable peoplein a village called Ali Abdala.So whats it like living on a huge seaof lava?Its very difficult, tough life.Im amazed that people can survive here,living literally on lava.Theres no soil.Weve seen children running aroundin bare feet on hot rock.Its absolutely amazing.This is their homeand there have been people living herefor millions of years because this is wheresome of the earliest pre-hominid fossilshave been found. So theres a continuityof life here in an extremely arid,hostile environment.Finally, Erta Ale.Its not very high or impressivebecause its a shield volcano.Shield volcanoes dont explode.Instead, the lava is very fluidand flows like blood across the landscape.The truck cant make it to the crater.The gear goes on camels.Nick and Berhe go on foot.Do you get excited every time you come up?Oh yeah, I do. Except the hard shift.Thats amazing isnt it?A great sea of lava.You know now that Dr. Nick,Im going to take you to the old craterwhere still it has smoke.This is all the source ofall the grey lava flows that came out of there.It came out from this one.Its another crater but its dead.So its just de-gassing now, right?Its sitting there doing nothing.It is, yeah.This is the final stage when a volcano dies.So its bringing up sulphursand we get this tremendous out-gassing.These gases are actually cooking the rock.This was hard lava just a few years agobut its now really rotten.And its been rotted by gascoming up through it.So this is a very important stageof volcanic activity because its bringing upprecious minerals.Erta Ales other crater is still aliveand active lava lakes are extremely rare.There are only four in the world.This is the Holy Grail for mebecause Ive never seen fresh lavain a lake before.So this is volcano central.The thirty-minute hike to the lava lakeis dangerous. Berhe wants to avoidthe heat of the day,but still get there before sunset.This is pretty unstable isnt it?Yes it is, it is.Especially at dark its very dangerous.And now were on one of the main lava flows.This is like walking on thin ice.It looks quite solid but its actuallyjust a very thin skinwhich was insulating red hot lavastreaming through as tunnels underneathand feeding the fronts of the flows.So weve got to be very carefulmoving over these in case you dropright through into one of these open tunnels.Holy cow! Will you just look at that!Thats incredible.Thats amazing.It took three days to get here,40 degree temperatures.How many dozen bottles of water?And its worth it.You can see the cooled skinand the upwelling magma between them.Its like plate tectonics.Well geologists are always looking forsmall scale examples to explainthe bigger global pictureand here weve got a perfect exampleof plate tectonics on a small scale.We can see dark grey raftswhich are like continents,separated by bright red riftswhere new magma is coming upand forcing those continents apart.Whats happening every now and againare huge bubbles of sulphur dioxidecoming up from depth.Thats the closest well cometo actually looking at the Earths mantle.Theres something primeval being up thereand looking down into the depthsof the Earth. Its an absolutelyfantastic experience, not just as a scientistbut as a human being.Thats amazing.What Nick saw on a micro scalein the lava lake is writ largeacross the Danakil.Ive seen a lot of rocks and landscapesaround the world but this isthe clearest example Ive ever seenof this sort of tectonic setting.We can see these long dark cracks down hererunning into the distance,through that little volcano-theres a crater there-and then the big guy in the distance.And this is marking the edgeof the African plate on my leftand the Somalian plate on the right.And the two are moving apartas a result of magmacoming up along these fissureswhich is pushing the plates apart.Absolutely fantastic.In 10 million years the expansein front of Nick will be transformedinto an ocean.Its a testament to the powerof plate tectonics.Nick now leaves the Afarand goes north through the Ethiopian Highlands,to explore how the Rifts geologyinspired some of the worldsfirst civilizations.This stream will widen intoone of the most fabled rivers in the world.The Nile River is a direct productof precipitation over the Ethiopian highlandsand thats been likened to the water tankof Africa.Rainfall flows to the West and then North.So without the Riftthere would be no Nile River.From Ethiopia, Nick follows the Nileinto Egypt.To both the East and West is desert.But through this valley oasiscame early humans, migrating out of Africa.Six thousand years agothis valley saw the beginningof one of the worlds first civilizations.It was a society shaped by the river,dependent on its seasonal floods.Over there you see the nileometerfor the measuring of the waterby the steps which are foundin the nileometer going down to the waterand there are marks on the wall.Measuring the height of the riverenabled the priests to predictpotential flood levels and to gaugewhat land could be cultivated.So they could predict the floods from this.If it was high it was going to bea good flood, everybody was goingto make a lot of money and they could tax more.If it was low then they were goingto have a hard time and you couldnt tax themas much./Exactly.Because this is the main source of incomefor the priests and the King.But its from this simple instrument,as crude as it seems,that they got their power.And that power was expressedin huge stone structures.We see this beautiful evolutionin mans relationship with the planetin winning stone, working stoneand building these lasting monuments.So I refer to ancient Egyptas the second industrial revolution.The power of what was called Upper Egyptrested here, the temple of Luxor.This is my favourite statuein the whole of Egypt.Its one massive block of black Aswan granite,depicting Ramses II. Running throughout itare these beautiful dykes of pink granite.Its a wonderful exampleof granite as sculpture.Ancient Egypt was so firmly rootedin the detailed knowledgeof the regions geologythat its been called a state of stone.This sandstone quarryhas been worked for thousands of years.To be honest, as a geologistI find this rather dull and boring.I cant find sedimentary structures,fossils or colour variationthat tells me about how this rockwas deposited. I know it was depositedin a very shallow seawaywith an abundant supply of sand.But thats the point. What Ifind uninteresting as a geologistis a delight to the stone masonsthat could easily carve this into flat slabsfor the front of the temples.And whats more, it was perfectfor developing the art of hieroglyphics.The most prized monumentsfrom ancient Egypt are obelisks.Made of a specific red granitefound near the city of Aswan,everyone from the Romans to Napoleonspirited them out of Egypt.One obelisk, however,never made it out of the quarry.This is the unfinished obelisk at Aswan.And they were going for a world recordwhen they were trying to win thisone out of the quarry.This 1100 tonne piece of red graniteis 42 metres long, a third largerthan any obelisk ever erected.Well the Aswan granite is naturally fractured.And like carpenters working withgrain in wood, Egyptian stone masonswere very good at utilizingthese giant fractures.And this beautiful smooth wallis a fracture that runs the lengthof the quarry. You can still seesurveyor marks on the side walls.This is the reason why the unfinished obeliskis still sitting in the quarry.These are cracks that appearedjust when they were starting tolever off the obelisk. Normallythey were pretty good at winning outhuge blocks between jointsthat affect the whole granite system here.But because it was so long,theyd actually stepped over some jointsand they suddenly opened up.So really they were defeatedby plate tectonics.Whats remarkable about the ancient Egyptiansis not just their expertisein stone building; they were also the firstto develop a sophisticated religionbased on a belief in eternal life.Its an interesting exampleof where a new science, in this case geology,has arisen out of a religionand the need for immortalityand lasting memorials.So it was a profound change in the wayman related to the planet.They were burrowing awayin the universe mentallyin terms of their religion,but they were also burrowing awayin the ground and profoundly changed the wayman views rocks. So they developedthe first geological maps 3,000 years ago.Theyre very accurate. They developed mines.They sent out huge prospecting parties,10,000 people strong, to find minerals.It was all out of the need for immortali