[MUSIC] One key observation that comes from trying to make life and trying to identify conditions where life might originate is that the viability of a biosphere on Earth is wholly dependent on the environment, essentially on the composition and the physical characteristics of Earth. What is incredibly intriguing is that as far as we can tell, Earth has been habitable for at least the last 3.8 billion years, in spite of incredible changes in the energetic and geologic forces that determine the conditions of surface environments. In the final two lessons this week, we will look at evolution of the Earth from core to mantle to crust, to hydrosphere and atmosphere, with the aim of giving you a foundation for understanding the Earth's system as a whole, before you delve forward in discovering the next major emergences along our own evolutionary path. To do this, let's consider what drives Earth's evolution. All the geological processes are the physical or chemical response of Earth to internal and external energetic forces. The internal sources of energy being the primordial and radiogenic heat in the Earth's core and mantle, and the external source being primarily the heat radiating from the sun. In this episode, we'll consider the internal energetic forces. We'll address the external in the next episode. Internal heat in the Earth is derived from two sources. Primordial heat obtained during the amalgamation of the Earth due to the energy released during impact of protoplanets to form the final product. And radiogenic heat which is produced by the decay of radioactive nuclides into their daughter products. Since the moon forming impact, the Earth has been steadily cooling, there was a finite amount of energy produced during the origin of Earth. And there are finite number of radioactive nuclides inside the Earth, steadily decaying to their daughter products, so that the Earth is producing less heat over time. The Earth is following the second law of thermodynamics, it is cooling down because it is approaching thermal equilibrium. That is, heat radiates from the very hot core to the relatively cool surface and then escapes to space, and it will continue to do so, until the whole planet is a uniform temperature, just as a hot glass of water dissipates its heat and cools to room temperature after sitting there for a while. The questions that we need to address are, how much has Earth cooled? Or how hot was it at the beginning of geologic time? And how has it cooled? Or what is the mechanism that facilitates Earth's approach towards thermal equilibrium? Now the first step to answering these questions is to look at how the Earth is now, and how, or excuse me, how hot the Earth is now, and how it cools now. Modern Earth is compositionally structured. It has a very thin layer of crust, surrounded by a liquid hydrosphere and a gaseous atmosphere and inhabited by a biosphere, all the living things, including us. All that material, crust, hydrosphere and biosphere and atmosphere to ozone represents only about 2% of the thickness of the Earth. The rest is comprised of the mantle, composed of dense magnesium and iron-rich rocks, and the even denser metal core, which makes up the innermost layer of the Earth. The core is partly molten. Its outer layer is liquid, and the turbulence of this layer is what gives Earth its magnetic field. Now in order for that material that makes up the core, which is almost entirely iron, to be liquid at the extremely high pressures in the center of the Earth, temperatures at the core have to be between about 5,000 to 6,000 degrees Celsius. In contrast, the Earth's surface, on average, is about 14 degrees Celsius. So in its attempt to reach thermal equilibrium, heat has to radiate from the core and become effectively carried through the mantle, and that happens through a process called convection. The mantle solidified within about a hundred million years after the moon forming impact. But temperatures are so high still in the mantle that it actually even though it's solid, it can deform plastically. Now conceptually, the behavior of the mantle is similar to that of a chocolate bar in a warm room. It can bend and deform, because heat makes it soft even though it is still a solid. The mantle convects, or flows, by hot and buoyant material at the bottom of the mantle rising. Colder, denser material, material from near the surface of the mantle sinks downward, taking its place and then slowly warms up and rises again. This is how heat gets from the core to the top of the mantle. The way that heat gets from the top of the mantle out to the Earth's surface and finally released to space is via a process called plate tectonics. Now, you've probably heard this term before. It's a theory that describes how modern Earth works on a larger scale. How heat and matter are cycled between Earth's surface and its interior. And it is driven by a mantle convection. Briefly, plate tectonics theorizes the Earth's outermost layer, about the upper most 100 to 150 kilometers collectively described as the lithosphere, is a semi-rigid shell that is divided into discreet pieces or plates. These plates move around the surface of the Earth relative to one another, floating on the convecting part of the mantle, which is called the asthenosphere. Now as hot material rises upward from within the mantle to the surface, it melts, forming a magma that rises into the crust and erupts along plate boundaries in the oceans, which are called spreading ridges. The magma crystallizes to form new basaltic ocean crust, which makes up the top part of the ridges' lithosphere. The oceanic lithosphere then spreads laterally away from the ridge, creating space for new rising magma. As it spreads away from the hot ridge, the lithosphere thickens and cools, becoming denser with age until it is actually more dense than the mantle supporting it. And as it spreads away from the ridge, it is also drifting towards another plate, which it meets at a convergent plate margin. Now the oceanic plate is the denser of the two plates. It sinks back into the mantle, or it subducts, carrying cold material down to the deep mantle to be warmed up again and continue the mantle convection cycle. Now when an oceanic lithosphere subducts, it's not the same rock that crystallized at the spreading ridge. It has since been hydrothermally altered by sea water due to water circulation near the mid-ocean ridge. And it's been weathered to clays due to low temperature, seawater rock interactions away from the spreading ridge. And it has been capped by layers of sediments that have precipitated from seawater or have been deposited by river runoff. So when this weathered, water ridge ocean lithosphere subducts, it is actually heated up by the surrounding asthenospheric mantle, and it begins to degrade, releasing volatiles into the overlying mantle. Volatiles, especially water, have the effect of decreasing the melting temperature of the mantle, by making it easier to dissolve the bonds between atoms in a solid structure. And so the release of this material from the subducting slam causes the mantle above to partially melt. That melt rises though the crust of the overriding plate as magma, and it forms a volcanic arc. Now, while the earliest products in such a volcanic arc are usually basaltic, as ocean crust continues to subduct, newly formed melts from the mantle rise through the arc and partially melt the pre-existing basalt, forming new, more silica-rich volcanic rocks. If subduction occurs beneath existing continental crust, the rising mantle-derived magma will partially melt the surrounding continental crust, forming silicic volcanic products like granite. As a result, just as ocean crust is made predominantly of a thin and dense layer of basalt, continental crust is composed of granitic type rocks, much more buoyant and forming a much thicker crust. Plate tectonics satisfactorily describes what drives nearly all phenomena we observe on Earth today. Thick mountain ranges like the Himalayas are formed when two plates of continental crust collide, and neither can subduct. They just stack up onto each other. Most volcanoes form from devolatilization of the downgoing slab in a subduction zone environment. They form island chains of volcanoes when subducting under ocean crust, like in Japan. Or they form mountains of volcanic chains like the Andes when they subduct under continental crust. Earthquakes are the physical response to tectonic plates sliding against one another. Locations where the largest earthquakes occur are along boundaries between plates where lithosphere is sliding past, over or under one another. We can actually measure the direction and the speed at which the plates are moving. And we can use paleomagnetic data to calculate where the plates have been, so that we can track the plates as oceans grow and push continents together into one giant, super continent and then break it apart again. So now that we know how the layers of the Earth work together, let's go back to our original questions. How hot was the mantle, and has it always released the same amount of heat to the surface as it does today? Has it always cooled in the same way, by plate tectonics? Models for how average mantle temperatures have changed over Earth history can only work backwards from the modern parameters. They estimate the modern rates of heat flow out of the mantle and the modern rates of heat being produced in the mantle by radioactive decay, and then they calculate backwards. And variations in these models are a result of uncertainty of the efficiency and the continuity of mantle heat loss via plate tectonics and on uncertainty about the amount of radioactive nuclides in the mantle. Now in the case of the first variable, if heat flow is greater than heat production, the mantle cools. On modern Earth, heat flow is limited by the efficiency of plate tectonics to conduct heat from the top of the mantle to the Earth's surface. So heat flow should be comparable to today's for as long as plate tectonics operated the same way as it does now. But what evidence do we have for how plate tectonics operated in the past? Or if it even existed? Well, the most clear cut evidence of plate tectonic processes are the rock products that made at tectonic margins. Slices of oceanic crust that are formed at mid-ocean ridges and subsequently smooshed up or accreted onto continents and exposed. Or metamorphic rocks called blueschists and and eclogites that are formed only in subduction zone settings where pressures are very high, and temperatures are low in the cold subducting slab. The oldest unequivocal examples of these rocks about 1 billion years ago. But that doesn't necessarily mean plate tectonics didn't exist before then, it is possible that such rocks simply weren't preserved. They have since eroded away or were recycled back into the mantle. Alternatively, the mantle might have been hot enough that while plate tectonics did operate, the dynamics of it were slightly different. And the mantle may have melted at higher temperatures, forming rocks with different chemistries than today, so ending with different products. Some models suggest that if the mantle were significantly hotter in the Hadean and early Archean, then plate tectonics wouldn't work. A much hotter mantle might melt more extensively, producing very thick, oceanic crust. More than 30 kilometers, as compared to the seven kilometer-thick ocean crust produced today. Additionally, the oceanic lithosphere, which is composed of both the ocean crust and the uppermost mantle, might not even cool down enough to provide the significant density difference that drives modern subduction. If this is the case, some hypothesize an alternative form of heat release from the mantle called vertical tectonics, on which hot magma rises to the Earth's surface primarily in the form of large mantle plumes. Somewhere the plumes that form the Hawaiian islands, for example, and cooler roots of over thick and crust back down into the mantle. The question of when plate tectonics, as we know it today, started remains unresolved. Well, there aren't any type features like complete sections of ocean crust or blueschists or eclogites older than a billion years. Several suites of Precambrian rock, including the one you learned about earlier, the 3.8 billion-year-old Isua Supracrustal Belt, have many characteristics consistent with plate tectonics. More recently, small fragments of the kind of mantle rock that forms in subduction zones were found as inclusions in 3.2 billion-year-old diamonds, again suggesting plate tectonics might be a very old process. Let's now consider the other uncertainty. The concentration of radioactive nuclides in the mantle and their influence on heat production over Earth history. Because radiogenic elements decay, the total quantity of radioactive nuclides in the bulk Earth has decreased over time. But the mantle may have lost its radiogenic heat source proportionately faster than expected from simple element decay rates. Because most significant radioactive elements for heat production, that is 238 Uranium, 235 Uranium, 232 Thorium and 40 Potassium are all incompatible elements. They'd rather be in continental crust than in mantle rock. So if all the continental crust grew in the first 100 million years after the formation of the Earth, there would be less radioactive isotopes to heat the mantle than if continental crust has grown gradually over Earth's history. Well, so how do we know which of these happened? Well, this is another major discussion in geology. The 4.4 billion-year-old Jack Hill zircons indicate that granite existed in the early Hadean, and therefore, there must have been some continental crust, almost immediately at the beginning of Earth history. But the evidence of how much, and the timing of, and the rates at which continents have grown since then, since the early Hadean, remains ambiguous. Now early theories that continental crust has grown gradually over Earth history were inspired by the relative age distribution of continental crust on Earth today. If continental crust doesn't subduct, its growth is cumulative, and the age distribution reflects the volume distribution over time. Now this concept has since been rejected as too simplistic a view of continental growth. It does not account for metamorphic resetting of radiogenic rock ages, it doesn't account for recycling of crustal material into younger rocks, or even recycling back into the mantle, which does happen on a small scale in the form of clastic sediments depositing on ocean crust and then being dragged down into the mantle as the crust subducts. In fact, it is actually entirely plausible that you could get the modern distribution of rock ages, either by gradual and continuous growth of the continents or by growing all the continents at once and then steadily recycling old crusts at the same rate that you make new crusts. And geologists are constantly seeking new geochemical or geophysical proxies to elucidate which of these growth curves is the correct one. And in general, the idea seems to most, or the data seems to most fully support a middle road, gradual and continuous continental growth, but such that it was actually faster in the Archean and Hadean and then stabilized to a slower rate of growth around 3 billion years ago. Around the same time more relatively substantial evidence for plate tectonics is observed. In summary, the estimates we now have on the timing of plate tectonics and continental growth are shaping how we think about mantle evolution. There's a lot more to be done before any of these variables are really constrained for the Precambrian Era. It's important that we keep trying to figure it out though. Not only to understand how the temperature of Earth's interior has evolved over time but also to sort out how the temperature of Earth's surface has evolved. And that is what we will discuss in the next episode. [MUSIC]
