You’re Standing on a Floating Plate This just in—ice floats in water! Unlike most materials, as liquid water cools to near its freezing point, its density decreases and then expands as it freezes. Thus, colder water and any ice float on the warmer liquid water below.
Seems anticlimactic, doesn’t it? However, if water did not possess this unusual property, Earth’s habitability would dramatically decrease. Ponds, lakes, streams, rivers, and possibly even oceans would freeze completely solid—not just on the surface—more regularly and take far longer to thaw after temperatures rise.
Amazingly, a similar phenomenon occurring deeper in the Earth may be responsible for enabling our planet to maintain the long-standing plate tectonics so critical for enduring life. Recall that Earth consists of a shallow crust, mantle, and an outer and inner core. Moving from the crust toward the core, most of the relevant materials in Earth’s interior absorb water more readily (because both temperature and pressure increase with depth). One material, a mineral called aluminous orthopyroxene found throughout the Earth, exhibits peculiar behavior in the upper mantle (just below the crust) called the asthenosphere. In this region, aluminous orthopyroxene’s capacity to dissolve water drops dramatically, but increases again at greater depths. Therefore, the materials in this region of the mantle absorb less water than those above and below with the consequence that a large abundance of “hydrous” melt exists in the asthenosphere. Think of a cracker sandwich with jelly (as the asthenosphere) in the middle.
Why does all this matter? The melt provides two important functions. First, it significantly weakens the asthenosphere such that it becomes more malleable and fluid. Second, the melt absorbs a tremendous amount of water compared to the other mantle components and, consequently, it dehydrates the region above the asthenosphere called the lithosphere (or crust). The lithosphere is comprised of the crustal plates that migrate over the surface of the Earth. Taken together, these two effects lead to a process—otherwise known as plate tectonics—where the rigid crustal plates “float” on a weakened and malleable asthenosphere.
Another mineral called olivine constitutes the dominant component of Earth’s mantle, and the solubility of water in olivine and orthopyroxene is similar—at least in the absence of aluminum. Until recently, scientists believed that olivine controlled the water storage capacity of Earth’s interior. However, the addition of aluminum increases the solubility of orthopyroxene nearly one hundred times. Consequently, scientists now recognize aluminous orthopyroxene, with its unusual solubility characteristics, as the controlling material responsible for Earth’s ideal tectonic activity.
This discovery also implies constraints on planet sizes where plate tectonics can occur. On planets that are too large (or small), the location of the asthenosphere will be too deep (or shallow) to permit the necessary crustal plate movement. As the authors of the article conclude, the existence of plate tectonics “is possible only in a planet with a water-bearing mantle” (that also contains sufficient aluminum). Such results echo the words of the Creator who fashioned Earth not as “a waste place, but formed it to be inhabited. (RTB,JZ)
In the beginning God created the heaven and the earth.
And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.
And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.
*** Will Myers
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