Metamorphism means to change form. In the geologist's sense it refers to changes in rocks (protoliths) in the solid state (i.e. not by melting the rock wholesale). One can view metamorphism as similar to cooking. Ingredients are mixed and placed at a different temperature (and/or pressure) and changes occur. What was a batch of goo, turns into a lovely cake!
Change occurs in order to maintain equilibrium conditions with new states of heat, pressure, or fluids. Thus, major changes in any of these three environmental variables can result in metamorphism. Because of the great conveyor belt of plate tectonics, rocks can encounter a variety of environmental conditions riding around in pressure, temperature, fluid space. As they go, they may metamorphose and may have a tale to tell of where they've been.
The fingerprints of metamorphism are growth of new minerals stable at the new PTF conditions and changes in texture reflecting the state of stress.
There are several sources of the thermal energy that drives metamorphism. Obviously, when and igneous body (some 1200 degrees C) intrudes into unsuspecting host rock, the contact zone heats up considerably. This causes a baking of the neighboring rock called contact metamorphism.
The temperature in the Earth goes up with depth. Near the surface in ordinary crust, the temperature rises some 30 degrees per kilometer. This is partly due to radioactive decay within the crust, but also comes from the fact that the Earth is still very hot from its initial formation (recall that the core is largely molten iron!). So if rocks are taken from the surface "down the tubes" either by burial or on a lithospheric nose dive, they will heat up and metamorphose.
Pressure changes with depth in the Earth as a result of the increased weight of the overlying rock. Increased pressure drives minerals to form more compact phases, driving for example coal to change into diamonds, and clay to rubies. Pressure also changes the state of stress. Crystals will grow or deform by cracking or flowing in response to the change in stress and show either preferential alignment, or evidence of squashing that reflects in some way the stress regime of the new environment. Deformation near fault zones, for example, results in cracking and grinding of rocks and is referred to as cataclastic metamorphism, while deformation deep in the crust occurs more plastically over a wide area producing regional metamorphism.
Changes in the chemistry of the fluids in the pore spaces of rocks also induces change. In its lowest temperature/pressure form, this change is called diagenesis, including weathering discussed in the last lecture. Diagenesis is not usually considered part of metamorphism although the distinction is a pretty subtle one if you ask me.
One common cause of changes in the fluid chemistry is the proximity of something hot - take volcanic activity for example. The heat induces convection currents in the surrounding fluids. The heated water reacts locally with the hot rock and carries a load of dissolved matter (rich in metals and sulfer) from the region of high heat into a region of cooler rock (or water). Here, the fluids tend to "dump their load". This form of metamorphism is known as hydrothermal alteration and is the way most metallic ore bodies formed. When the super-charged fluids come out of brand-new ocean crust and hit 2C water, they drop their load and form what is known as a black smoker. Lots of animals (if you can call them that) live off the stuff. Check these out:
Study Table 8.1 in text
Increasing pressure causes a reduction in volume of the rock by first compaction and then recrystallization of minerals to denser forms. Compaction produces a more closely packed arrangement of grains. This is exmplified by the transformation of clay to slate. Recrystallization involves growth of new minerals. The bulk chemistry need not change (unlessfluids are involved).
If the pressure is higher in one direction than the others, minerals will tend to align themselves such that the fabric formsperpendicular to this axis. A planar fabric is known as a foliation while alignment of crystals causes a lineation.
Micas (which are platey) will grow with their plates along the foliation. Increasing metamorphism resultsin different types of foliation.
In its journey around the tectonic merri-go-round, clay will turn firstto shale during diagenesis, then procede to slate. Growth of micas then will contribute a peach fuzz sheen characteristic of phyllite. As metamorphism becomes more intense, schist forms. Schists have large mica flakes in them.
Under the most intense metamorphism, minerals segregrate into bands of light and dark minerals, characteristic of gneiss.
If the rock easily splits along smooth parallel surfaces, it has what is known as fracture cleavage.
Not all rocks are foliated. For example, contact metamorphism does not generally produce foliated rocks. Also, parent rocks (protoliths) that tend to grow minerals that are not platey or elongated, will produce metamorphic rocks that have no foliation or lineation. These include quartz, and calcite. So sandstones make massive quartzites and limestones make marble, neither of which are strongly foliated. Here is an example of what marble looks like.
Porphyroblasts are larger crystals in a finer-grained matrix. The metamorphic rock will bear the name of the dominant porphyroblasts, e.g. garnite schist.
Changing conditions results in phase transformation from one mineral to another. Minerals coalesce or change crystal structure. The particular minerals that form are characteristic of thepressure/temperature conditions. Particularly useful for determining PT conditions are the following metamorphic index minerals
Low metamorphic grade (low temperatures and pressures) - about 200
degrees C
Slates and phyllites are characterized by:
Intermediate metamorphic grade rocks such as schist often have:
High metamorphic grade - 800 degrees C (verging on melting), such as gneiss and migmatite have the high temperature high pressure phase sillimanite.
Staurolite, kyanite and sillimanite all have the same composition but are stable at different PT conditions (like graphite and diamond). Therefore the presence of one particular form documents the PT conditions. A more accurate idea of PT conditions can be gotten by considering a whole suite of minerals. Determining the PT history of a sequence of rocks describes the journey of that particular crustal package up and down the tectonic elevator.
Obviously, the composition of the protolith plays a strong role in which minerals will grow. Thus basalts, granites and carbonate rocks each develope into the different metamorphic mineral assemblages leading ultimately to amphibolite, gneiss or marble respectively.
Consider Figure 8.2 in the book. Hydrothermal metamorphism occurs near mid-ocean ridges driven by the heat of the volcanic activity there. Intrusion of igneous rocks drives contact metamorphism anywhere it occurs. Both of these sorts are metamorphism with high temperatures and low pressures.
Faults associated with plate boundaries create cataclastic metamorphismin the shallow crust.
Here is an example of: cataclastic metamorphism
Cataclasis grades into totally pulverized minerals that are streaked out in bands characteristic of mylonites.
Finally, burial of sediments in a sedimentary basin takes the rocks down the PT road characteristic of the crust, the so-called geotherm . They respond to this by developing the characteristic mineral phases of burial metamorphism. The minerals are a guide to just how deep and hot sediments got.
Lisa Tauxe
