<p>These plates spread out and issue new crustal material (consider the Mid-Atlantic Ridge), slide against one another (the San Andreas Fault), head-butt (forming the Himalayas), or dive beneath each other (consider the subduction zone along the Pacific Northwest, where oceanic plates dive beneath the North American continental plate, generating earthquakes and stratovolcanoes). But how long has this process been operating? A very long time, it appears. Earth's earliest crust is believed to have formed 4 billion years ago, not long (relatively speaking) after Earth itself coalesced 4.5 billion years ago. Rather than blanketing Earth with a single homogeneous layer, that crust differentiated into basaltic oceanic crust and less dense granitic continental crust.</p>
<p>As reported in the journal Nature Geoscience, a team led by Alan Hastie (University of Edinburgh, UK) did lab-based high-pressure-temperature melting experiments with materials that likely made up our ancient crust. They found that continental crust formation most likely required exceedingly high pressures, such as that found at a depth of 50 kilometers or more. Thus, as early as 4 billion years ago, some sort of subduction-like processes must have already kicked in, helping to create continental material. Their lab results support evidence of rocks from the field and suggest plate tectonics are nearly as old as Earth itself.</p>

Subduction Goes Back-Way, Way Back!

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