Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful.
The Earth is divided into several layers which have distinct chemical and seismic properties depths in km :. The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents.
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The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum.
We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz silicon dioxide and other silicates like feldspar.
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Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below.
Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries i. There are at present eight major plates:. There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates.
Earthquakes are much more common at the plate boundaries. Did you know that dinosaurs lived on the other side of the Galaxy? Christiansen said it took her about four hours to make the film using timed animations in PowerPoint. She also noted a couple minor corrections to the text in her video: plesiosaurs are not dinosaurs, and we complete a galactic orbit every million years not million years. Galactic movement is more complicated than the video shows, though. The other stars and planetary systems in the galaxy are also moving, at different speeds and in different orbits.
The inner portions spin faster than the outer regions. What's more, the galaxy itself is moving through space, slowly approaching the nearby Andromeda galaxy.
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As the whole galaxy's moving and we're rotating around the centre, it kind of creates this spiral. So in the Solar System's rotation around the galactic centre, we're not returning to a fixed point. The neighbourhood is different from the last time we were here. Earth, however, is not drastically different; it still supports complex life. That's partially thanks to the path of our sun's galactic orbit.