When on Earth did modern plate tectonics begin?

地球是一颗充满活力的行星，原因很简单，因为它仍在从吸积热和随后的放射性元素衰变中冷却下来。它散热的主要机制是板块构造，这一理论自20世纪70年代以来一直被广泛接受。地球由一个致密的金属核心组成，周围环绕着部分熔融的硅酸盐地幔，地幔本身被一个浮力地壳覆盖，无论是大陆上的还是海洋上的。我们生活在主要存在于海平面以上的大陆地壳上。洋壳构成了海洋的底部，很少露出水面。洋壳是由大洋中脊的地幔熔融形成的，例如位于大西洋中脊的冰岛火山岛就坐落在那里。新的地壳不断形成，迫使较老的地壳向外扩散，海洋变得更大。随着洋壳从海脊向外扩散，海壳变冷，密度变得更大。最终，它与由密度较低的物质组成的大陆相互作用。洋壳在大陆下方被驱动回到地幔，这一过程被称为俯冲。火山沿着俯冲带上方的大陆边缘形成，其中至少有一部分活动是新大陆地壳的补充。这可能是导致我们的大陆最初形成和随后演变的主要过程。现在可以在太平洋边缘观察到这种现象，那里广泛存在的火山活动被称为“火环”。然而，并不是所有的海洋都能继续增长！作为对太平洋持续增长的回应，大西洋已经停止变大。最终，海洋将完全闭合，周围的大陆将相撞，形成线性山链。一个很好的例子是喜马拉雅山，印度在那里与亚洲相撞。这个被称为板块构造的整个过程对我们的星球产生了深远的影响，为我们提供了赖以生存的土地、捕鱼的海洋、饮用的淡水以及复杂的气候模式。它也是我们气候的调节器，因为大陆岩石的风化导致二氧化碳下降到深海，在那里储存二氧化碳。了解板块构造对地球和环境科学家来说至关重要。还有一些我们知之甚少的重要细节，比如它是如何以及何时开始的。这项提议试图通过一项新的对关键岩石的研究来研究这一点，这些岩石表征了板块构造，特别是那些由俯冲造成的板块构造。当洋壳被俯冲时，不断增加的压力和温度将其变成更致密的岩石。随着地球的演化，这种“变质”的确切压力和温度条件也发生了变化。我们建议通过使用俯冲过程中在洋壳中形成的矿物来研究这一点。岩石本身经常被侵蚀破坏，但一种名为金红石(二氧化钛)的坚固矿物的微小晶体可以幸存下来，在从它们衍生的沉积物中找到。通过测定它们的年代，并使用它们的化学成分作为指纹，我们可以计算出被侵蚀的俯冲带内的压力和温度。同样，在俯冲过程中形成的火山岩也随着时间的推移而变化。这些记录也经常被侵蚀破坏，因此曝光的记录可能不具有代表性。另一种坚固的矿物称为锆石(硅酸锆)，通常在风化过程中幸存下来，最终与金红石一起出现在较年轻的沉积物中。使用与锆石类似的方法，我们还可以研究地球历史上不断变化的岩浆作用类型。。目前的岩浆记录表明，现代俯冲开始于约2.5亿年前，而变质记录表明，现代俯冲开始的时间较晚，约为7亿年前。我们的新方法将检验这一点。我们将能够说出较年轻的日期是否正确，较老的日期是否标志着不同类型的板块构造，或者较早的日期是否确实代表了现代板块构造的开始，暴露的岩石记录是有偏见的。

Earth is a dynamic planet, for the simple reason that it is still cooling down from the heat of accretion and subsequent decay of radioactive elements. The main mechanism by which it loses heat is plate tectonics, a theory that has been widely accepted since the 1970s. The Earth is formed of a dense metallic core surrounded by a partially molten silicate mantle which itself is capped by a buoyant crust, either continental or oceanic. We live on the continental crust which largely exists above sea level. The ocean crust forms the floors of oceans and is only rarely exposed. The ocean crust forms by mantle melting at mid ocean ridges, such as the mid Atlantic ridge upon which sits the volcanic island of Iceland. New crust is constantly formed, forcing the older crust to spread outwards and oceans to grow larger. As the ocean crust spreads away from the ridge, it cools and becomes denser. Eventually it interacts with a continent, made of less dense material. The ocean crust is driven beneath the continent back into the mantle, a process known as subduction. Volcanoes form along the continental margin above the subduction zone and at least some of this activity results in addition of new continental crust. This may have been the main process responsible for initial formation and subsequent evolution of our continents. It can be observed now around the margin of the Pacific Ocean, where widespread volcanism is known as the "Ring of Fire". However, not all oceans can continue to grow! The Atlantic Ocean has stopped getting bigger as a response to the continued growth of the Pacific. Eventually, an ocean will close completely and the surrounding continents will collide, resulting in a linear mountain chain. A good example is the Himalaya, where India has collided with Asia. This whole process known as plate tectonics has a profound affect on our planet, providing us with land on which to live, seas in which to fish, freshwater to drink and our complex weather patterns. It is also a regulator of our climate since weathering of continental rocks results in drawdown of CO2 to the deep sea where it is stored. Understanding plate tectonics is central to Earth and Environmental Scientists. There are still important details that we know little about, such as how and when it began. This proposal seeks to investigate this by a novel study of critical rocks that characterise plate tectonics, in particular those that result from subduction. When ocean crust is subducted, increasing pressure and temperature change it into denser rock. As the Earth has evolved, the exact pressure and temperature conditions of this "metamorphism" have also changed. We propose to study this by using minerals that form within ocean crust during subduction. The rocks themselves are often destroyed by erosion, but tiny crystals of a robust mineral called rutile (titanium dioxide) can survive to be found in sediments derived from them. By dating these and using their chemical composition as a fingerprint, we can work out the pressure and temperature within the eroded subduction zone. Similarly, the volcanic rocks that form during subduction have changed through time. These are also often destroyed by erosion so that the exposed record may not be representative. Another robust mineral known as zircon (zirconium silicate) often survives the weathering and ends up alongside rutile in the younger sediments. Using similar methods with zircon we can also investigate changing styles of magmatism throughout Earth's history. . Currently the magmatic record implies that modern subduction began around 2500 million years ago, yet the metamorphic record implies a later start of around 700 million years ago. Our novel approach will test this. We will be able to say whether the younger date is correct and the older marks a different kind of plate tectonics, or whether the older date does indeed represent the onset of modern plate tectonics, and the exposed rock record is biased.