Radioactivity dating to materials
This debate, though unresolved, emphasizes the substantial knowledge of the thermal structure of Earth and plate-tectonic processes that can be obtained from the study of metamorphic rocks.
Depending on the original geometry of Earth’s lithospheric plates, subduction of oceanic crust beneath continental lithosphere may result in complete consumption of an ocean basin and subsequent collision between two continents.
Thermal modeling studies suggest that blueschists will generally undergo heating and be converted to greenschist assemblages if exposure at Earth’s surface does not occur within 100 million to 200 million years after high-pressure metamorphism.
Early exposure at the surface also increases the chances for removal by erosion, however, resulting in a low probability for preserving blueschists greater than 100 million to 200 million years old.
They are the rocks involved in the cyclic processes of erosion, sedimentation, burial, metamorphism, and mountain building (orogeny), events that are all related to major convective processes in Earth’s mantle.
Three-dimensional diagram showing crustal generation and destruction according to the theory of plate tectonics; included are the three kinds of plate boundaries—divergent, convergent (or collision), and strike-slip (or transform).continent-continent collision and to collision between oceanic and continental plates.
As a result, young metamorphic belts aligned roughly parallel to the present-day continental margins (e.g., the Pacific margin) as well as older metamorphic belts are used to infer the geometries of the continental margins at earlier periods in Earth history.
Some geologists have argued that the lack of well-developed high-pressure belts formed during Precambrian and Paleozoic time (4.6 billion to 252 million years ago) indicates that plate-tectonic processes have changed significantly throughout geologic time.
Specifically, they claim that greater heat production in Archean time (about 4 billion to 2.5 billion years ago) would have produced hotter crustal geotherms, resulting in thin hot lithospheric plates whose mechanical behaviour may have been quite different from that of the present-day plates and hence may not have permitted formation of subduction zones.
The weight of the subducted slab may drag the rest of the tectonic plate toward the trench, a process known as slab pull, much as a tablecloth will pull itself off a table if more than half of the cloth is draped over the table's edge.