18.1: Plate tectonics - what governs Earth's surface dynamics today

As we have covered elsewhere, the way the world works today is largely governed by plate tectonics: the differential movement of large plate-like slabs of the planet’s lithosphere, mainly horizontally, but with significant vertical motion too at sites of subduction and continental collision. The fathomless deep sea trench or a towering Himalayan-style mountain range are ephemeral topographic features, however. While they allow us to spot plate boundaries at a glance today, they don’t last over geologic time: the trench gets filled in with sediment; the mountains are eroded to their roots.

Figure \(\PageIndex{1}\): A key feature of plate tectonics is the subduction of cold oceanic crust, leading to a high-pressure & low-temperature metamorphic regime. Blueschist is evidence of those conditions, which we would not expect in the absence of plate tectonics. (Callan Bentley photo)

So how can we figure out when plate tectonics began on this planet? We must look to other aspects of the geologic record. This is a vibrant area of research in geology now, and there is genuine scientific controversy about how best to interpret a patchy record. This much is agreed upon, however: Plate tectonics results in several features that are decisive “signatures” of its operation, and are potentially “durable” enough to persist in the geologic record through time. First is the production of blueschist as an example of a high-pressure but low-temperature metamorphic rock, only results when you have the subduction of old, cold, dense, oceanic crust. The oldest known blueschist on Earth is merely Neoproterozoic in age (Condie and Kröner, 2008, and references therein).

Second, blueschist is usually associated with other subduction metamorphic rocks, such as eclogite. Distinctive minerals, including glaucophane (a blue amphibole mineral that makes blueschist blue) and lawsonite mark these rocks. But the belt of “subduction metamorphic rocks” is paralleled by a second belt of metamorphic rocks, with a higher temperature signature, the rocks forming in association with the accompanying volcanic arc. These include classic Barrovian metamorphic minerals (sillimanite, staurolite, garnet, etc.) in rocks such as phyllite, schist, and gneiss.

The set of two parallel belts with different temperature/pressure regimes is the key observation: that’s how it works today, with plate tectonics. And unlike the deep trench or the towering volcanic arc, the paired belts of rock with distinct metamorphic signatures stand a good chance of being preserved in the geologic record.

But as we go further and further back in the geologic record, we see fewer and fewer blueschists, and fewer and fewer paired metamorphic belts, forcing us to conclude that plate tectonics was not always the way the Earth’s outer rocky layer behaved.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Though some manifestations of plate tectonics are topographic (e.g., trenches, collisional mountain belts), others are signatures more likely to be preserved through geologic time. Which of the following is considered a durable signal of plate tectonics we might expect to "read" from the ancient geologic record?

a. Blueschist, a high-pressure, low-temperature metamorphic rock

b. Granite, a coarse-grained, felsic-composition igneous rock

c. Basalt, a fine-grained, mafic-composition igneous rock

d. Hornfels, a high-temperature, low-pressure metamorphic rock

Answer

a. Blueschist, a high-pressure, low-temperature metamorphic rock

Exercise \(\PageIndex{2}\)

Why are paired metamorphic belts considered significant?

a. Different settings in plate tectonics produce different combinations of temperature and pressure conditions. These result in different suites of metamorphic rocks forming at the same time, in adjacent settings. In plate tectonics, we would expect a high-pressure, low-temperature belt associated with subduction to be paired with (not too far away) a parallel higher-temperature belt associated with a volcanic arc.

b. Because over time different belts of metamorphic rock tend to pile up together, starting off in wildly different locations at different times, and plate tectonics eventually brings them together in pairs.

Answer

a. Different settings in plate tectonics produce different combinations of temperature and pressure conditions. These result in different suites of metamorphic rocks forming at the same time, in adjacent settings. In plate tectonics, we would expect a high-pressure, low-temperature belt associated with subduction to be paired with (not too far away) a parallel higher-temperature belt associated with a volcanic arc.