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The term reflection is loose, and considered by some an abuse of language, with inversion preferred; however, point reflection is widely used. Such maps are involutions, meaning that they have order 2 – they are their own inverse: applying them twice yields the identity map – which is also true of other maps called reflections.
The first eon in Earth's history, the Hadean, begins with Earth's formation and is followed by the Archean eon at 3.8 Ga. [2]: 145 The oldest rocks found on Earth date to about 4.0 Ga, and the oldest detrital zircon crystals in rocks to about 4.4 Ga, [34] [35] [36] soon after the formation of Earth's crust and Earth itself.
Earth formed about 4.54 billion years ago by accretion from the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun, which also created the rest of the Solar System. Initially, Earth was molten due to extreme volcanism and frequent collisions with other bodies.
The snowball Earth hypothesis was first posited to explain what were then considered to be glacial deposits near the equator. Since tectonic plates move slowly over time, ascertaining their position at a given point in Earth's long history is not easy.
For the Earth, this could have been an external magnetic field. Early in its history the Sun went through a T-Tauri phase in which the solar wind would have had a magnetic field orders of magnitude larger than the present solar wind. [60] However, much of the field may have been screened out by the Earth's mantle.
The asteroid that killed most dinosaurs 66 million years ago left behind traces of its own origin.. Researchers think they know where the Chicxulub impactor came from based on levels of ruthenium ...
The gravitational attraction between Earth and the Moon causes lunar tides on Earth. [180] The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit Earth. As a result, it always presents the same face to the planet. [181]
Crustal studies – investigation into the structure and origin of the Earth's crust, through to the Moho discontinuity and beyond, at depths of up to 100 km. A method similar to reflection seismology which uses electromagnetic instead of elastic waves, and has a smaller depth of penetration, is known as Ground-penetrating radar or GPR.