"Most of the time the surface of our world gives no hint of what lies below" "but there are a few places on earth where we can get a glimpse of what's happening inside the planet." "That is just one fantastic sight." "You can really see how much of this cone has fallen in" "These taller slopes are all very recent." "Like all the loose boulders we see down here have fallen off the crater walls since March." "This is Mount Kilauea on the island of Hawaii." "Scientists from the US Geological Survey regularly fly up here to take the pulse of the volcano." "They need to get as close to the active crater as possible." "It's not a place where you want to hang around if you don't have to." "It's the pond of lava at the bottom of the crater which they need to keep an eye on." "Any change in level could be warning of an imminent eruption." "We're standing on the crater rim of Pu'u'O'o cone." "This is the active vent on the eastern slope of Kilauea volcano." "Potential hazard here is the, when the pond is so low there's not much supporting the inside of the crater so we'll have large chunks of the crater rim that fall and if that happens pond level would then rise and fall, rise and fall" "and you'll also get spatter and, and of course you don't want to be standing on a chunk of rim that might actually go down in." "Mount Kilauea is the world's most active volcano." "Geologists suspect that all this vulcanism reflects some remarkable phenomenon deep within the planet." "But the problem scientists face is that the Earth's interior is totally inaccessible." "Yet in a sense scientists have managed to descend to the very centre of the Earth." "In a remarkable series of experiments they've probed our planet to its core." "They've shown how the same process that created this volcano also keeps the entire surface of our planet in constant motion." "There's no better place to see that motion than Iceland." "The island sits astride an extraordinary break in the crust, a rift that marks the boundary between two of the vast plates that make up the planet's outer skin." "Up close this rift appears as a series of deep and narrow cracks in the surface." "Here geologists like Bob White can study at first hand the way the plates move." "This fissure here is one of the, the many swarm of fissures across about a 50 kilometer-wide zone where the motion between the American plate on that side is moving away from the Eurasian plate on that side at a rate of about 20 millimeters a year," "on average, about that much per year." "The movement that's so visible in Iceland is in fact happening all over the world." "By the late 60s scientists had worked out that beneath the crust the entire surface of the planet is divided up into a small number of pieces - plates." "These plates are all slowly moving carrying the continents with them." "The theory that described this motion was called plate tectonics." "It was the biggest step forward scientists had ever made in understanding the Earth." "But there was something missing." "They knew the plates were moving, but scientists had no idea why they were moving." "Nobody was more worried about this problem than one of the architects of the theory:" "Dan McKenzie." "By the beginning of the 70s we had some understanding of what was going on the surface 'cos we'd been able to map the motions of the plates really in quite some detail all over the, the world," "but what we really didn't have at that stage was any decent understanding of how these motions were maintained, and the obvious place to look is, is underneath the plates in the mantle 'cos it clearly, they clearly weren't being driven" "by the winds from the outside, right." "It had to be an internal process." "The first hint of what lies inside our planet came from scientists like Bryndis Brandsdottir, who spends much of her time monitoring the many earthquakes which shake Iceland." "We use different kind of sensors to monitor different things within the Earth." "For instance you can see earthquakes which occurred in the region late last night and early this morning." "But with the right equipment scientists can do far more than locate and record local earth tremors." "With better senses we can record earthquakes from all over the globe and if they're big enough to send waves through the earth those waves can give us information about various places within the Earth" "By putting together information from thousands of earthquakes, scientists built up their first complete picture of the Earth's interior." "At its centre lies the core, an immense ball of liquid iron, the size of the planet Mars." "Beyond the core is the vast bulk of the planet, the mantle, which is composed of solid rock." "The plates are simply the upper layer of the mantle with the crust just a thin veneer on top." "But this picture gave no hint of any movement in the mantle that could be driving the motion of plates." "To make progress scientists needed a different approach." "Here in Lovgrun," "Swedish island in the Baltic, scientists have long been intrigued by a mysterious phenomenon." "Canadian geophysicist Dick Peltier has made the journey from Toronto to meet Martin Eckman and see things for himself." "The sea has always been important here." "People notice when it changes." "In 1940 Lisa Nygren was a young woman." "Lisa tell us that she is about 35 years old here and you can see that in front of the boathouse here there is a lot of water." "Nowadays there is no water here in the long ridge." "It's completely dry, just gravel and grass." "It seemed to the people here that the sea was slowly ebbing away." "She tells me that when she was young there was water here, it was filled with water, a small inlet from the bay." "You mean you could bring boats right up into what are now fields." "Yes, rowing boats." "Peltier and Eckman are not the first scientists to study the phenomenon." "Two and a half centuries ago the famous Swedish physicist Anders Celsius noticed the same thing." "This is the famous rock of Lovgrun and here is the sea-level mark made by Celsius in 1731, so 250 years ago sea-level was here." "Today it's down there." "Thanks to Celsius' foresight, Peltier and Eckman can calculate the rate of apparent sea-level drop here - over one centimeter each year." "The problem is there's absolutely no evidence of any significant change to global sea-levels for 10,000 years, since levels rose at the end of the Ice Age." "Perhaps it's not the sea that is falling, but Scandinavia which is rising." "On the Swedish mainland Peltier and Eckman have found evidence that this is indeed what's happening." "It's astonishing." "Believe it or not, this is a beach." "We are now 200 meters up a mountain and 30 kilometers from the sea." "9,000 years ago this was at sea-level and this was the seashore." "You can see the stones here have been reworked by the sea waves." "Over the years this has been lifted up nearly 200 meters and this is the result." "All the evidence suggests that since the ice melted 10,000 years ago" "Scandinavia has slowly but steadily been rising out of the sea." "It may seem completely bizarre but when you think about it there's a very simple and wonderful explanation that has to do with the Earth's mantle and with Ice Ages." "20,000 years ago the Earth was in the midst of an Ice Age." "At that time all of Scandinavia was covered by ice." "Over central Sweden the ice was about 3 kilometers, almost 2 miles deep." "The great weight of the ice load forced Scandinavia to sink." "Just like loading a boat in the sea, the greater the weight of the load put in the boat the deeper it sinks into the sea." "But by 9,000 years ago the ice had melted." "As time passes the land begins to rise rebounding back to its original outline, just as a boat bobs upwards when load is removed." "So what Celsius first measured 21/2 centuries ago was not the sea ebbing away but the land bobbing up as the great weight of the ice disappeared." "However, this explanation, neat as it is, requires that beneath Scandinavia the earth's solid mantle is acting very strangely." "Like the water beneath a boat it must be flowing." "What sort of stuff is rock solid yet flows?" "Understanding that would turn out to be the key to the mystery of why the plates move, but to get that key scientists would have to perform an ingenious series of experiments which would take them deep into the planet's interior." "The first step was to get hold of some rock from the mantle." "It's rare stuff but just occasionally volcanic eruptions bring to the surface small unmelted fragments of the mantle." "It's a green crystalline material called olivine, but to tell how this stuff behaves in the mantle scientists need to simulate the enormous temperatures and pressures of the Earth's interior in the lab so a tiny sample of olivine is squeezed" "between the jaws of a specialized vice called a diamond anvil." "To understand how pressure affects the sample," "Eckhard Salje needs to look deep to the structure of the crystal." "The way we are doing that is to bounce off the sample here X-rays, so radiation is coming in here, it's bounced off here into a detector and that will tell us what the actual atoms are doing." "As Eckhard uses these techniques to descend ever deeper into the Earth, he's revealing how the behaviour of our planet is governed by what happens at the atomic level inside the mantle." "Although the sample always remains solid the intense pressures can disrupt the structure of the crystals" "and that means the solid material can slowly flow." "Suppose this is a piece of the Earth's mantle." "Seismologists tell us, based on earthquake observations, that this material is solid." "It's elastic, it's resilient." "What could be more elastic or more resilient." "However on a longer timescale its behaviour is completely different." "So the mantle of the Earth is both a solid and a fluid." "It's all a question of timescale." "So over long periods of time the solid mantle is flowing, but what does this flow have to do with the motion of the plates?" "Scientists would find the answer to that when they voyaged even deeper, down to the iron core." "We have here a piece of iron in which we're going to re-create ignitions at the centre of the earth." "And we have to do that by generating very high pressures and we have to melt it under those very high pressures." "Then we're going to stick it in this - the hydrogen gas gun." "Now Keith is loading the gun now with about 7lbs of gunpowder." "When the gun is fired it pushes this heavy piston down this hydrogen-filled tube which starts out at 10 times atmospheric pressure, but the time we arrive here it's about 2,000 times atmospheric pressure." "Takes this disc, bursts it, hydrogen comes out of here and pushes on this projectile accelerating it down this tube up at, at about 10,000 Gs up to a velocity of about 8 kilometers a second which is why we call it the fastest gun in the West." "Once it gets here it impacts on the target generating very high pressures, melting the, melting the iron target and has about 50 billionths of a second to measure that temperature." "I'll zero the counters." "Neil Holmes is going to all this trouble because he wants to answer a very basic question about the Earth's core." "How hot is it?" "Yeah, everything looks good over here..." "Since scientists are pretty sure that the core contains molten iron, the thinking is that its temperature must be close to the melting point of iron." "Calibrate target chamber..." "But not the melting point at the surface." "Bringing up X-rays..." "The melting point under the stupendous pressures at the centre of the planet." "Whenever you're ready." "Ready to fire." "Let's go ahead, Yeah." "Well, there's not much left in here." "That used to be a part of some of it." "I don't know what." "So the whole target, everything we started with, is destroyed, but when the projectile hit the iron, the iron made a brief flash of light." "The colour of that light tells us what the temperature was." "What it tells us is that melting temperature is 6,200 Centigrade" "This is over 1,000 degrees hotter than the surface of the Sun." "What this means is the core of the Earth is a glowing ball of hot iron," "3,000 miles in diameter and slowly over the aeons that, that heat is gradually moving out to the surface of the Earth." "So at the Earth's centre beneath the mantle there's a heat source hotter than the surface of the Sun." "Of course, as anyone who's ever placed a pan of water on a stove will know, when you heat a fluid from below it starts moving." "A little bit of dye shows what happens." "Hot material comes up from the bottom, rises to the surface where it cools and sinks back again." "This is the process known as convection." "Could this be happening in the Earth's mantle?" "To answer this question scientists have had to put together everything they've learnt about the Earth's interior:" "the temperature of the core size of the mantle" "the properties of mantle rock and the rate at which they can flow." "Adding all this together scientists came to an amazing conclusion." "Like water in a gigantic saucepan, the planet's interior is ceaselessly churning away as it convects heat from the core towards the surface." "So the situation now is really very much more interesting because we have this mobile Earth which is mobile because it's transporting heat from the interior towards the surface both upwards by hot material rising and downwards by cold material sinking" "and the thing acts really just like a, an engine." "It's converting heat, right, the heat which it's transporting, into some sort of work and it's this work which is driving the plates round on the surface." "It's not difficult to see how the motion of the plates relates to convection in the mantle." "At the mid-ocean ridges hot mantle is flowing up and as it cools it forms new plate which moves away from the ridge." "Eventually it's cold and dense enough to sink back down again into the planet's interior." "So the complex motion of the plates is actually part of a simple process by which the Earth is slowly losing its massive store of heat." "But what about the vulcanism in places like Hawaii?" "How did that fit into mantle convection?" "The answer to that started to become clear when Dan McKenzie began experimenting with convection." "His idea was to simulate the behaviour of the Earth's interior in the laboratory." "The heat source, the core, was represented by a waster bath." "For the mantle above McKenzie chose a rather surprising material:" "a layer of golden syrup." "What we wanted was to look at convection in something which had a very large range of viscosity like the Earth does because we, we knew that, that plates on the Earth's surface are really very viscousb and strong" "because they move as units over huge areas, but underneath that we know from the way in which Scandinavia recovers after the ice melted thatÿÿ ÿÿ97 00:2ÿÿ05,277 -00ÿÿÿÿ24:07,268 that :2ÿÿviscosity underneath" "so what we wanted was a material which had a huge range of viscosity and Lyle's golden syrup is ideal." "But when McKenzie ran the experiment the result was a tremendous surprise." "Instead of large plates he found that the convection organized itself into an interlocking pattern of small cells each centered around a rising plume of hot material surrounded by sheets of cold sinking treacle." "We could heat it in different ways, right, we could do all kinds of different things." "We could alter the top temperature and alter the viscosity contrast." "Really doesn't make much difference, right." "You always get this pattern, lots of little cells, not really at all like what we wanted for plates, but this then gave us the idea that, that maybe on Earth the same thing was going on" "and that underneath these big plates there were perhaps lots of little cells like this which were convecting." "Could one of Dan McKenzie's convecting cells lie beneath Hawaii?" "If so, then all this lava is coming from a plume of mantle rock that has risen from deep within the planet." "Why don't you stand about here and give me enough coil..." "Lava flows in underground channels down from the main vent." "That gives Carl Thornber and Dave Bercovici a chance to go fishing for a sample." "Perfect, absolutely perfect shot." "OK, now we are ready to go ahead with this sample?" "OK, take care to hang on to the end of that." "OK, looks good." "We're at the top of that lava fall and it's in." "Oh Jesus, that's moving." "OK, I'm trying to get a good sample." "Man it's hot." "Come on baby." "OK, we're coming up." "We've got a big one." "OK we're ready to pull and clinch." "OK, here we go." "1, 2, 3." "Let's go." "That was a good sample." "Yeah." "Anyone for tea?" "Routine analysis of fresh lava samples allows us to predict what the volcano might do next, but perhaps more significantly - some of these are still hot - more significantly in terms of, of where all these basalts come from" "this stuff contains an extraordinary proportion of helium 3 which is relatively rare in volcanic rocks." "Now one might wonder where this helium 3 comes from." "One source where helium 3 is, is still present within the Earth today is at the core mantle boundary and if that's the case then one might expect that this stuff came from way down there." "The composition of the lava indicates that it's come from mantle rock that has risen rapidly from deep down in the Earth, and there's another line of evidence that Hawaii sits above a plume." "This evidence comes not from the erupting lava, but from the shape of the Hawaiian archipelago." "One of the major clues that we are standing above a mantle plume is that the Hawaiian islands themselves form an almost perfectly linear chain that goes off to the north-west." "The plume is sitting under the Pacific plate which has been moving across it for 100 million years, so the volcanic islands the plume created in the past have been carried away by the moving plate." "In time an entire line of volcanic islands was effectively burnt into the Pacific plate." "In fact this island and Kilauea volcano are moving off the centre of the plume and giving way to a new volcano Loihi which is forming under water about 15 miles to the south-east of this island and in about 200,000 years Loihi will breech sea-level" "and become the next Hawaiian island." "Once scientists realised that mantle plumes really do exist, they began to see them everywhere." "Where the plumes are coming up they push up the surface a little bit and so what we've been able to do, quite recently actually, is to, to map the whole of the, the convective circulation underneath the plates" "and you can see that very clearly here in the Pacific." "Here's South America NorthAmericaAustralia and Japan and Hawaii in the middle and you can see these red patches where plumes are coming up and the biggest of these is Hawaii and what you're looking at here essentially the fact that the sea floor's" "all being pushed up, right, by the plume coming up." "So Hawaii owes its existence to the fact that the plume beneath the island pushes up the overlying plates into a large dome and where this happens on land, the effects can be unexpected and far-reaching." "These patterns make sort of dimples and even though they're very small they control the, the, the flow of the, of the largest rivers on Earth and probably one of the best examples is, is the Nile" "where it flows out from Lake Victoria here, it flows out through the depression, the dimple between these two plumes here and then goes northwards and it goes again through exactly between these two and then goes off into the Mediterranean," "so the convective circulation from the plumes is actually much more important in terms of how rivers flow than we thought before and we can see this pattern of, of, of rising plumes all over the Earth's surface everywhere we look." "So beneath the crust, beneath the plates the rocks of the mantle move to their own rhythm." "Rising from deep within the planet's interior plumes of hot rock push up towards the surface." "Less than 20 years ago plumes were no more than a theoretical possibility." "Today they are recognised as major features of the planet which can remain stable for tens, perhaps hundreds of millions of years, and it's becoming clear that over Earth history they've played a major role in shaping the surface of the planet." "These are the Kanheri Caves in western India, a large temple complex carved by Bhuddist monks out of a single vast lava flow." "This flow is the result of a volcanic eruption far bigger than anything humanity has ever witnessed." "Yet this is just one of dozens of such flows which blanket much of western India." "Piled one on top of the other, they form this extraordinary mountain range, the so-called Deccan Traps." "Here is evidence of vulcanism on a truly gigantic scale." "Most geologists assumed that the eruptions which produced this landscape must have continued for tens of millions of years, but in 1985 the French geophysicist Vincent Courtillot came to the Deccan." "He was interested in the way the Earth's magnetic field has changed over time." "When we first came here our purpose was to measure the magnetization of the lava in the Deccan." "When a lava flow cools below a certain critical temperature it will lock in the direction of the Earth's magnetic field." "It has a memory of what the Earth's field was like at the time of cooling and we know the Earth's field has been reversing, flipping many times, approximately once every million years." "So frozen into the lava layers should be a history of these magnetic reversals." "Courtillot was expecting to find dozens of them recorded in the Deccan lava flows." "What he actually found astonished him." "When we came here the estimates for ages of the Deccan lava ranged over as wide as 50 million years, so we expected to have recorded maybe 50/60 reversals of the field." "When we had completed the sampling and the work in our laboratory much to our surprise we found that only 2 reversals had been frozen in the lava." "That only 2 reversals had been recorded was proof that the whole vulcanism could not have lasted much more than say a million years." "This was an enormous out rate indeed." "I could only have been caused by the massive eruption at the Earth's surface of a mantle plume head." "Enough lava was erupted here to cover the United States to a depth of nearly one kilometer." "Vincent Courtillot believes such a stupendous event can only have been the result of a powerful new plume splitting the crust and breaking through to the surface." "When a mantle plume comes to the surface of the Earth the plume head will bulge, deform the crust and then burst out as a lava flow with enormous volumes." "These plumes which are born inside the mantle and erupt to the surface of the Earth will modify the whole landscape of a continent in a very durable fashion." "And it hasn't happened only in India." "Iceland too shows the unmistakable fingerprints of a plume, but here the plume has done more than change the landscape." "There's evidence that it's changed the motion of the plates, reshaping an entire region of the globe." "Iceland occupies a unique position on the Earth." "The fissure that runs across the island is in fact, part of the vast mid-ocean ridge, the great rift that runs up the centre of the Atlantic where the ocean basin is slowly opening up." "Iceland is above water only because it is buoyed up by a mantle plume." "The influence of that plume is felt right across the island in all sorts of ways." "The whole of Iceland owes its existence to the mantle plume." "It's elevating the whole area something like 500 meters above sea-level where we're standing now, but the very hottest part of the plume is right behind me underneath Vatna Jokull, a huge glacier." "Now the glacier's there because that's the very highest point of Iceland." "It's the highest point of Iceland because the hottest material is underneath." "The mantle plume itself is hotter than the rest of the mantle round it so it's a little bit more buoyant and that buoyancy, because it's less dense, pushes up that region and makes the very highest region," "the region where the glacier forms." "Now that particular glacier is the, the largest glacier outside the Arctic or the Antarctic, so it's a big feature and it's another 1,000 meters above us, so it's something like 4,000 meters above its normal level." "It's sort of ironic that the coldest part of the Iceland is sitting right above the hottest part of the mantle." "The hot mantle plume also creates volcanoes, a dangerous combination of fire and ice." "In fact there's 5 major volcanic systems underneath Vatna Jokull and it's those volcanic systems that have been responsible for some of the biggest eruptions in Iceland over the past centuries." "In fact the, the biggest historic eruption in the world occurred from a volcano underneath Vatna Jokull in 1783 and that particular eruption didn't actually kill anybody from the lava that flowed out from it but such a lot of dust and ash" "and other toxic gases were erupted at the same time that the majority of the livestock on Iceland was, was killed and the next winter in 1783 and 1784" "25% of the population of Iceland died as a direct result of that eruption." "In 1996 the volcanoes under Vatna Jokull spoke once again." "This time fortunately the ash fall was mainly confined to the ice cap itself, but as the heat from the eruption melted the ice above, a tremendous deluge of melt water flowed out from under the glacier sweeping all before it." "As roads and bridges were swept away" "Icelanders were once more reminded that the plume that has created their homeland also harbours awesome destructive energy." "It's the power of mantle plumes to drive enormous changes to continents and oceans that intrigues geologists." "The material erupted from its volcanoes has built up the crust of Iceland, but scientists now suspect that the plume may have shaped the environment here in a far more profound way." "They've started to ask themselves whether it's merely coincidence that here a mantle plumesits right underneath the great mid-ocean ridge." "Could the Iceland plume have caused he rift in the first place and triggered the opening of the Atlantic 58 million years ago?" "Clues to what happened then can still be found, not in Iceland but on the west coast of Scotland." "Here geologists have found that the Western Isles are largely composed of volcanic rock." "Above the Atlantic surf rise great cliffs of basalt with its characteristic hexagonal columns." "All these formations have been dated to a relatively brief period," "58 million years ago." "Volcanic activity on this scale is evidence for an event similar to that which caused the Deccan Traps, a new mantle plume head which split the crust" "and 2,000 kilometers away to the west," "Greenland too has similar huge basalt formations." "So the rocks on both sides of the Atlantic bear witness to the impact of a plume from deep within the mantle." "As the plume pushed up against the crust it split, rifting Greenland off from Europe." "Once split," "Europe and Greenland continued to move apart creating a new ocean basin in the North Atlantic." "Ever since then the plume has been sitting under the rift producing Iceland, the largest volcanic island on the planet." "The plume under Iceland has produced some enormous changes in the 60 million years or so since it started to rise through the Earth's mantle." "Without that event in the planet's interior the whole of the northern hemisphere would look totally different and North America would still be joined to Europe, but plumes may have even more significant effects than the shaping of the planet's surface." "Some scientists are beginning to suspect that they may change the course of evolution." "The vast Deccan lava flows were all erupted in, geologically speaking, a blink of an eye, but when?" "Usually they may extend up to 100 meters you know and if we look very carefully..." "Paleontologist Ashok Sahni has helped Vincent Courtillot pin down exactly when the plume head hit the surface." "Could this event have had global consequences for living things?" "These here are the teeth of mammals." "These mammalian teeth were recovered in sediments lying in-between the Deccan flows." "They belong to very primitive mammals which are related to the present-day shrews." "These mammals evolved at a fairly fast rate and it's possible to tell from these teeth exactly where we are in time." "They give us a date for the initiation of the Deccan volcanics and this date happens to be 65 million years." "They belong to an era that is what we call the Cretaceous period and surprisingly it is this Cretaceous period, the end of this Cretaceous period which also saw the extinction of the dinosaurs." "If you ask someone what killed the dinosaurs he or she is likely to tell you well, it was the impact of an asteroid and they might well be right." "But that cannot explain all of the extinctions." "Paleontologists tell us that well before the impact hit the earth species had begun slowly disappearing." "When I mean slowly" "I mean in the course of tens of thousands or hundreds of thousands of years." "This is precisely the timescale of the Deccan eruptions." "This is what makes me prefer that explanation." "As the large plume head came from inside the Earth it started erupting lava flow after lava flow." "You should imagine these lava flows pumping into the atmosphere vast amounts of gases" "sulphur dioxide, carbon dioxide, chlorine and also large amounts of dust obscuring the skies." "Obscurity would cause cold, possibly freezing and then acid rains would follow." "Real hell everywhere." "And when the Deccan had finished erupting 60% of the species had disappeared from the surface of the Earth." "Among the survivors were those tiny shrew-like mammals, our own ancestors, who were to flourish in a world freed from the grip of the large reptiles." "If Vincent Courtillot is right, then in a sense we human beings owe our existence to events deep within the Earth." "The link between the Earth's activity and the course of evolution is something we'll be exploring later in the series." "The scientists who've explored the Earth's interior have made a profound discovery." "Beneath all the complex changes we see at the surface there is an underlying simplicity - the vast churning heat engine of the Earth's mantle, an engine which drives the dance of plate tectonics and reforms our world in a myriad different ways." "Here in Iceland scientists can witness the workings of that engine at first-hand." "As the Earth cools by convection so the continents shift over its surface and rift apart, but plate tectonics means that continents don't just separate, they must also collide and it's what happens when two continents collide" "that's the subject of our next programme." "Subtitles:" "Thor"