2019-04-15 10:39 三立在线
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Questions 43-52 are based on the following passage and supplementary material.
This passage is adapted from Angus Chen, "The Dawn of Plate Tectonics:'©2014 by American Association for the Advancement of science.
A journey to the Mariana Trench, the deepest crevice on Earth's surface, reveals the great Pacific tectonic plate descending deep into the planet where it Line recycles back into mantle rock. This recycling of old tectonic plate, called subduction, drives plate tectonics and is nothing new to scientists, but exactly when the process got started is a hot debate. A new study may put that to rest by unmasking a sequence of 4.4-billion­year-old lavas as the remnants of the first subduction zone on Earth. If correct, the discovery marks the dawn of plate tectonics and thus several geological processes critical to Earth's environment and perhaps even its life.
In 2008, scientists studying ancient lavas in
15 northern Quebec, known to geologists as the The team says each rock layer in the sequence at the Mariana lavas and the Nuvvuagittuq lavas describes a step in the birth of a subduction zone.ancient subduction zone. "Seeing the evolving chemical signature:'Turner says, "was much more robust than just saying there is or isn't niobium:'now, the Nuvvuagittuq rocks have been modified by intense heat and pressure "which can mask and modify geochemical signal" through contamination from nearby rocks. Furthermore, while Pearce geochemical fingerprint much, and Sleep says this is reasonable. Despite all of the difficulties of studying such timeworn rocks, he says "the work done in this paper is very valuable:'