New Rocks from Old

Physical Geology 101

15. Metamorphic Rocks I (p. 223-233)

New Rocks from Old

What is the Greek origin of the word metamorphism?

“meta” means ______; “morphe” means ______.

Definition: metamorphism refers to all the changes that occur in the minerals, chemistry, and structure of solid rocks as a result of the physical and chemical conditions deep in the crust.

So metamorphism involves changing existing rocks to make new ones.

This is different to igneous and sedimentary rock processes:

 Igneous rocks: made from molten magma or lava.

 Sedimentary rocks: diagenesis is the lithification process that binds the particles of sediment together to form rock.

When rocks undergo metamorphism, we say that they have been ______.

Does metamorphism involve any rock melting? YES or NO ?

Changes that occur during metamorphism:

 ______: existing minerals convert into new minerals  ______: minerals change their appearance because of the forces that are involved in metamorphism.

Depth of Metamorphism

Metamorphic rocks form in the Earth's crust underneath where sedimentary rocks form near the surface (i.e. where the process called ______occurs), but above the depths where magma begins to form through the process of ______(where present).

The depth range of metamorphism will thus depend on the local ______.

What is the lower temperature limit of metamorphism? ______

What is the deepest depth (approximately) that diagenesis occurs? ______

Pressures at this depth are more than ______times greater than atmospheric.

We have to have high enough pressures AND temperatures for metamorphism to occur, by recrystallization (the growth of new minerals).

What is the upper temperature limit of metamorphism? ______

What mineral starts to melt at this temperature, if present in the rock? ______

1 Physical Geology 101

The three grades of metamorphism are:

______(only occurs at shallower depths than ______) ______

Do low-grade metamorphic rocks resemble the original rock? YES or NO

Do high-grade metamorphic rocks resemble the original rock? YES or NO

Controlling Factors

The type of metamorphic rock that forms is very dependent on what the original rock is, but the exact changes that occur to the original rock will vary depending on what the metamorphic conditions are. The controlling factors are:

1. ______2. ______

3. ______4. ______

Pressure

Pressure refers to the effect of the increasing weight of the overlying rocks as you go deeper and deeper into the crust. This pressure is referred to as the:

______

This pressure is the same from all directions, just like water pressure deep in the ocean.

If pressure is the same from all directions:

- do objects decrease in volume because of the pressure? YES or NO - do objects change their shape because of the pressure? YES or NO

Under lithostatic pressure, rocks decrease in volume- the mineral grains get more and more tightly packed and they start to ______, forming a metamorphic rock.

If forces are stronger in one direction than other directions (unlike pressure), we use the term ______to talk about the forces acting on the rock. This may happen because of the forces of the tectonic plates pushing against each other.

In particular, if there is a greater push from one direction compared to another direction results in a type of stress called ______.

Because of these unequal stresses, rocks get squished in one direction more than others. This causes platy minerals (e.g. ______), which grow in sheets, to align with each other perpendicular to the direction of the largest stress.

This alignment of minerals in metamorphic rocks is called ______.

Because it is a physical characteristic of the rock, we refer to this foliation as a type of metamorphic ______.

In low-grade metamorphic rocks, the grain sizes are usually very small and the foliation may be difficult to see. A tell-tale clue is that the rock breaks apart easily along the foliation, just like with cleavage in minerals, forming flat, plate-like fragments.

This characteristic is called ______.

In intermediate-grade metamorphic rocks, recrystallized mica and chlorite grain sizes are bigger and the foliation is easier to see and is often wavy in appearance.

This type of foliation is called ______.

In high-grade metamorphic rocks, recrystallized minerals segregate into light and dark bands, forming a ______.

This type of foliation is also called ______.

Temperature

Heat is a crucial part of metamorphism because chemical reactions occur faster as temperatures: INCREASE or DECREASE ?

What are the two sources of heat for metamorphism? 1) ______or 2) ______

Eventually the temperatures will get high enough that a certain rock type can no longer remain solid at the existing pressures and partial melting begins. If partial melting doesn't melt all of the rock, we can end up with rocks that are part metamorphic and part igneous (usually granite). These type of rocks are called ______.

Fluids

What are 2 examples of holes or openings in rock that can contain fluids or volatiles?

1. ______(gaps between grains in sedimentary rocks) 2. ______

What types of minerals contain water in their chemical makeup? ______(e.g., clay)

What types of minerals contain OH in their chemical makeup? ______(e.g., mica and amphibole- can be used to make H2O in chemical reactions)

What are 2 examples of fluids or volatiles that are used in metamorphic reactions?

______and ______.

Fluids are important for metamorphic reactions because they allow existing minerals to dissolve, or they allow new minerals to crystallize from the chemical ingredients present in the fluid.

Do fluids speed up or slow down chemical reactions during metamorphism? SPEED UP or SLOW DOWN

How long does it usually take for one silicate mineral to fully convert into a different silicate mineral (even when the temperatures and pressures are very high)?

______

Types of Metamorphic Rocks

We can broadly divide metamorphic rocks up into two categories. Those that have a foliation and those that don't.

In other words: ______and ______

Foliated

In order from the lowest to the highest metamorphic grade:

 ______: forms when shale undergoes low-grade metamorphism. Develops a foliation called a ______.

What products are made from slate? ______

 ______: slightly higher grade metamorphism of shale and forms an obvious foliation.

 ______: intermediate-grade metamorphism of shale, with big mica crystals that form a pronounced foliation called a ______.

 ______: form from high-grade metamorphism of clay-rich sedimentary rocks and granite. The metamorphic minerals segregate into light and dark bands producing a ______texture.

 ______: forms when ______undergoes low-grade metamorphism. The resultant rock is similar to a schist that develops from a shale, but in the case of basalt, contains mostly green chlorite minerals.

 ______: intermediate-grade metamorphism of basalt. Contains amphibole.

 ______: high-grade metamorphism of basalt. Contains pyroxene.

Non-Foliated

 ______: metamorphic rocks with large recrystallized grains that show no foliation.

 ______: forms when ______gets metamorphosed. The original quartz grains recrystallize into tightly packed, reorganized quartz crystals.

 ______: forms from ______. It is entirely made up of the mineral ______.

All the original mineral grains recrystallize, obliterating all sedimentary features, and forming an often pure white rock with a sugary look to it. The purity of the rock is why it is used for gravestones and for sculptures (calcite is soft and easy to sculpt).

FINAL QUESTION:

What famous and intricate Michelangelo sculpture was made out of marble?

______