What is the history of this rock?
The rock in this photo is a banded gneiss. The bands are made of different minerals. The light bands are more felsic, and the dark bands are more mafic. The minerals separated due to heat and pressure. The waviness of the bands also shows how the rock was hot enough to alter. It was not hot enough to melt all the way.
Metamorphic rocks start off as some kind of rock. The starting rock can be igneous, sedimentary, or even another metamorphic rock. Heat and/or pressure then change the rock into a metamorphic rock. The change can be physical, chemical, or both.
During metamorphism, a rock may change chemically. Ions move in or out of a mineral. This creates a different mineral. The new minerals that form during metamorphism are more stable in the new environment. Extreme pressure may lead to physical changes. If pressure is exerted on the rock from one direction, the rock forms layers. This is foliation. If pressure is exerted from all directions, the rock usually does not show foliation.
There are two main types of metamorphism: contact and regional.
Contact metamorphism results when magma contacts a rock. The rock changes because of the magma's extreme heat (Figure below).
(A) Hornfels is a rock that is created by contact metamorphism. (B) Hornfels is so hard that it can create peaks like the Rocky Mountains.
Regional metamorphism occurs over a wide area. Great masses of rock are exposed to pressure from rock and sediment layers on top of it. The rock may also be compressed by other geological processes. If the rock is buried deeply, it may experience high temperatures.
(A) Regional metamorphic rocks often display layering called foliation. (B) Regional metamorphism with high pressures and low temperatures can result in blueschist.
Metamorphism does not cause a rock to melt completely. It only causes the minerals to change by heat or pressure. If the rock melts completely, it will cool to become an igneous rock.
Hornfels is a rock with alternating bands of dark and light crystals. Hornfels is a good example of how minerals rearrange themselves during metamorphism (Figure above). The minerals in hornfels separate by density. The result is that the rock becomes banded. Gneiss forms by regional metamorphism from extremely high temperature and pressure.
- Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock.
- Contact metamorphism occurs when a rock is altered by heat from a nearby magma.
- Regional metamorphism occurs over a large area when a rock is buried or compressed.
- Compare and contrast the two types of metamorphism.
- Under what conditions does a rock become foliated?
- Describe how and why rocks are altered by regional metamorphism.