1、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
2、 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 t
3、hereand 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
4、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 thi
5、cknesses 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
6、 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
7、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 h
8、ave 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
9、 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 evo
10、lve 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 human
11、sleft 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
12、 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 requ
13、iredmanufacturing 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 aro
14、se 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 hug
15、e 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
16、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 he
17、re 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.
18、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
19、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 faul
20、ts. 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 un
21、ique 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 industr
22、ial 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 underl
23、ain 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 o
24、nly 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 landsc
25、ape,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 s
26、plitting 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
27、 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 villag
28、e 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
29、 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.Inst
30、ead, 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. N
31、ick,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 w
32、hen 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
33、 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 actually
