Deformation behavior of rocks and resulting structures

Physical Geology Lecture Outline

The picture to the right was taken from a helicopter and is from merging glaciers in Svalbard Norway. Glaciers are rock bodies that deform both ductilely and brittlely at the earth's surface, and hence give insight into this behavior. The various fold forms in the glacial ice give evidence as to the nature of the ductile deformation, while the crevasse field represents brittle deformation. The dark stripes are moraines, formed when rock debris from the mountain The key is that the bonds that hold the ice together are much weaker than those which hold silicate minerals together, and the melting temperature of H2O is much less than that of silicate material, and so ice deforms readily at the earth's surface, whereas silicate rocks do not.

Important concepts in deformational behavior of rocks:

What are important factors in deformation behavior and what is their effect?

Types of brittle structures:

Types of ductile structures:

This is a beach cobble of some of the Fransciscan melange found along California's coast, in this particular case at Patricks Point. Note the white features - these are brittle extensional features known as veins. High fluid pressures help open up these cracks, and then quartz minerals grew out of those waters to form the white veins.

This beach boulder is also from Patricks Point, and shows chert layers folded in a complex manner. These chert layers were orginally deposited as deep ocean silicious oozes (from the skeletal elements of some types of plankton). These melange rocks are characterized by a very complex history of deformation that combined brittle with ductile deformation.

Brittle-ductile transition in the crust:

This is a view looking along the San Andreas fault, where the y axis is depth below the earth's surface in km, and the x axis is distance along the fault in km. Each dot represents an earthquake. Note how the earthquakes occur mostly above 15 km or so. That is because at lower levels the rocks are two hot and ductile to break, and instead the fault movement is by steady, ductile creep. Here you can 'see' the seismogenic part of the earths crust. One can also note some sesimic gaps in this data. Image from: .

Migmatites: this represents a special rock a mixture of metamorphic rock and often irregular veins of melt (usually granitic) that occurs at deep levels in the crust, usually when the crust has been thickened beyond normal. Migmatites deformed in very complex and arguably chaotic way.

Migmatite is often used as a building stone. This is a slab of migmatite from the Adler planetarium along Chicago's shoreline (near the aquarium and Field museum). The horizontal field of view is several feet. The darker material (also known as the melanosome) is the older meatmorphic rock, in this case amphibolite. The lighter and reddish material is granite, and was magma when this rock was at its peak metamorphic conditions down in the earth's crust. You can see how the older metamorphic amphibolite has been folded and also intruded by irregular veins of the granite magma. Some of those granite layers are in turn folded, clearly indicating deformation and intrusion were happening at the same time. Sometimes the granites formed from the partial melting of the adjacent metamorphic rock, and sometimes it is from injection of magma from elsewhere. This is the geologic record of a juicy deep interior of a mountain belt.

This is an image of a crater on Venus that has been heavily dissected by a series of faults. Many other planetary bodies show faults and folds and other features that result from tectonism. Image source:

In class structure lab - inferring deformation conditions of select specimens.

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