Lecture 14 - Deformation, Metamorphism and the RockyMountains

Outline

I.                    Mountain Building and Deformation

II.                 Metamorphism and Magmatism

I. Mountain Building or Orogeny and Deformation

Mountains form by plate tectonic collision through subduction of a dense oceanic plate beneath either another oceanic or continenta lplate. The Rocky Mountains represent the subduction and collision of an old oceanic plate with the west coast of North America. The rather horizontal angle of subduction caused the deformation to extend quite far inland. During the subduction process, sediments that accumulate on the ocean floor at the continental margin are scraped up into a pile called an "accretionary prism". Eventually this margin will encounter some continental crust that is too buoyant to subduct and collision will begin. As plates collide the rocks that make up their crusts deform by folding and faulting. Metamorphism often accompanies mountain building and if temperatures are high enough some rock will melt and produce magmas. Deformation involves both folding and faulting of rock. The folding or bending of rocks is considered to be ductile deformation which implies the flow of material; faulting or fracture of rock is considered a brittle deformation process. Whether a rock will deform ductiley or brittley depends on 3 principle factors:

  1. composition- certain minerals are more brittle than others, quartz and olivine are more brittle than micas and calcite
  2. temperature- higher temperatures favor ductile deformation
  3. pressure- higher pressures favor ductile deformation
  4. rate that pressure is applied- slower applied pressure favors ductile deformation. An example is if I pull apart a piece of taffy slowly, it will flow but if I quickly pull it, it will snap into 2 pieces

Folds

anticlines- oldest rocks are in the inside

synclines- youngest rocks are in the inside

Faults - Fracture across which there is relative movement, rock mass above the fault is called the hanging-wall block and the mass below is called the footwall block

II. Metamorphism- The process by which elevated temperatures and pressures within the Earth alter the mineral content and structure of solid rock without melting it.

Different minerals are stable at different conditions of pressure and temperature so as these conditions change, as they do when you go deeper into the Earth, new minerals grow but the bulk chemistry of the rocks stays the same.

Temperature and pressure ranges for metamorphism are greater than 200 degrees C (400 degrees F), which we encounter below about 10 km depth in the Earth, and greater than about 1 kilobar. 1 bar equals the pressure we feel on our shoulders at sea-level due to the atmosphere. 1000 bars =1 kilo bar (1 kb). Pressure increases by about1 kb every 3 km of depth in the Earth due to the added weight of the overlying rocks.

Pressure is usually applied in a particular direction, for instance pressure that rocks feel due to increased burial is directed vertically downward, perpendicular to the surface of the Earth. During mountain building, pressure will be directed in the direction of plate convergence. This directed pressure causes the new minerals that grow during metamorphism to become aligned in a direction perpendicular to the applied pressure. This mineral alignment gives the rock a particular texture that we call foliation. Rocks that are composed of many different minerals often develop foliation while those that consist of primarily 1 mineral (limestone or pure quartz sandstones) do not develop foliation.

We classify metamorphic rocks based on their texture. Their are 2main classifications:

Foliated Metamorphic Rocks

Non-foliated Metamorphic Rocks

starting as a shale or mudstone

marble- was a limestone

1. slate

quartzite- was a sandstone

2. phyllite

 

3. schist

 

4. gneiss

 

 

 

starting as an igneous rock

 

1. schist

 

2. gneiss

 

The difference between these rock types are the degree of foliation and the size of the new minerals. Slate and phyllites have minerals that are too small to see with the naked eye, but the alignment of the minerals (principally micas) give them a visible foliation. Schists have minerals that are large enough to see and gneisses show compositional banding as well as mineral alignment. The kind of metamorphic rock that develops depends a bit on composition,  but more importantly on temperature and pressure conditions. The progression from slate to phyllite to schist to gneiss reflects an increase in the pressure and temperature conditions that formed these metamorphic rocks.

Magmatism

Igneous rocks form my cooling and crystallizing from a magma. If they crystallize at the surface of the Earth they are called extrusive or volcanic rock, if they cool beneath the surface they are called intrusive or plutonic igneous rock. Intrusive igneous rocks cool more slowly and therefore develop larger crystals than extrusive rock which cools very quickly. Sometimes volcanic rock cools so quickly that its minerals can not form a crystalline structure and itis called a glass, obsidian is an example of a volcanic glass.

Forms of Intrusive Igneous Rocks

All bodies of intrusive igneous rock are called Plutons (afterPluto, the Greek God of the underworld). Plutons are given different names depending on their size and shape. We can only see plutons once the rocks overlying them have been stripped away by erosion.

  1. Batholith- Very large irregularly shaped body of igneous rock (at least 100 km2 but up to 1000's of km2) common in the core of mountain belts.
  2. Stock -smaller irregularly shaped igneous body (<100km2)
  3. Sills- tabular shaped igneous body that parallels rock layering
  4. Dike-tabular shaped igneous body that cuts across rock layering

[return icon]Returnto Earth Sciences 3 Lectures Home Page


This page was last reviewed on 2/11/04.