Sometimes, the huge forces involved in crustal movement literally tear rocks apart and drag
the broken edges past each other. Near the surface, rock is shattered into angular fragments
along these fault zones and ultimately crushed to powder. Deep down, however, the heat of
the Earth's crust makes rock toO soft and plastic to break. So when rocks are sheared, they
smear out like toffee to form streaky rocks called mylonites. Softer materials recrystallize as
minute grains, while a few more robust larger crystals may survive,
crushed and reduced within this fine matrix. Because the larger crystals
have not recrystallized, they are called porphyroclasts, not
porphyroblasts. It was once thought that the fine grains in
mylonite were simply pulverized; however, it is now known that
they are actually new crystals that form under the strain, in a
process called syntectonic recrystallization. The word
mylonite was coined by Charles Lapworth in 1885 to
describe the streaked rock he found in the Moine Thrust
fault zone of the Scottish Highlands. ow it is used to
describe any rock with a smeared-out streaky texture
like this, and bands can be anything from] -2crnJ
OA-O.8in to 2-3km/1.2-1.9 miles thick.
Sometimes, the huge forces involved in crustal movement literally tear rocks apart and drag the broken edges past each other. Near the surface, rock is shattered into angular fragments along these fault zones and ultimately crushed to powder. Deep down, however, the heat of the Earth's crust makes rock toO soft and plastic to break. So when rocks are sheared, they smear out like toffee to form streaky rocks called mylonites. Softer materials recrystallize as minute grains, while a few more robust larger crystals may survive, crushed and reduced within this fine matrix. Because the larger crystals have not recrystallized, they are called porphyroclasts, not porphyroblasts. It was once thought that the fine grains in mylonite were simply pulverized; however, it is now known that they are actually new crystals that form under the strain, in a process called syntectonic recrystallization. The word mylonite was coined by Charles Lapworth in 1885 to describe the streaked rock he found in the Moine Thrust fault zone of the Scottish Highlands. ow it is used to describe any rock with a smeared-out streaky texture like this, and bands can be anything from] -2crnJ OA-O.8in to 2-3km/1.2-1.9 miles thick.
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