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Video:  Rocks that form Underground

1.    How do Geologists unravel the history of a rock?

By comparing the information contained in a rock with a modern environment, searching for an environment that provides the same sort of clues.

 2.    What is a crystalline substance?

Most solids are crystalline.  A crystalline substance must have an internal structure in which atoms are arranged in orderly arrays.

 3.    What is cleavage?

A plane of cleavage represents a plane of weakness in the crystal structure, a weakness in the bonding of the atoms.

 4.    What is the difference between a solid, a liquid and a gas?

A solid has atoms arranged in a rigid structure.  In a liquid the atoms are able to move about more freely, but are confined to a given volume.  In a gas there is no volume or shape and the atoms are free to move about in the space available.

 5.    What effect does the rate of cooling have o the formation of crystals in a material?

Typically the slower the cooling rate the larger the crystal size.

 6.    What are the characteristics of an igneous rock?

In igneous rocks the crystals are arranged in a random manner, called interlocking arrangement.

 7.    How does a glass form?  Give an example of an igneous rock that is a glass.

Glasses form by cooling a liquid melt so rapidly that the atoms are not able to sort themselves out into a structural array.  Obsidian is an example of an igneous rock that is a glass.

 8.    To produce visible crystal surfaces, what conditions are necessary?

Significant temperature change between liquid and the surrounding environment so that rapid cooling can occur.

 9.    If an igneous rock has large crystals visible without magnification, where did this
       rock form (on or near the surface of the earth or at depth)?

Such a rock formed at depth.

 10.  What is a plutonic rock?  What are the characteristics that indicate a rock is
        plutonic?

A plutonic rock is one that formed underground, has crystals large enough to see with the naked eye and whose crystals are arranged in a random fashion.

 11.  Characterize the crystalline structure of an igneous rock.  Are the crystals
        arranged in a random or preferred orientation?

Random

 12.  If a rock has a fine-grained texture, under what conditions did this rock
        probably form?  What is a volcanic rock?

Such a rock cooled more rapidly, rapidly enough so that the crystals did not have enough time to grow large enough to be visible with the naked eye.  Such a rock is considered to be a volcanic rock, a rock associated with a volcano and/or volcanic eruption.

 13.  Can a volcanic rock have crystals that are large enough to see with the naked eye?
        If so, how did this rock form?

This condition indicates two stages of cooling.  The first stage allowed enough time for the crystals of one mineral to become large enough to be seen with the naked eye.  Then the remaining melt, along with the large crystals already solidified, was erupted from a volcano and cooled rapidly so that the remaining mass developed crystals too small to be seen with the naked eye.

 14.  What do you think the porosity and permeability of igneous rocks will be?
        Will there be directional differences in strength?

Generally the porosity and permeability will be quite low as rocks that cool from a melt do not typically have pore spaces in them.  Some volcanic rocks do have pore spaces in the form of vesicles but such rocks typically have low permeability, the pore spaces are not connected and they float.

 15.   What are the characteristics that may allow the identification of a rock as a
        metamorphic rock?

One type of metamorphic rock is foliated or banded.  This is a phenomenon that does not happen with rocks cooled from a melt.  There are some nonfoliated metamorphic rocks that exhibit large crystals of one mineral type, also not characteristic of rocks that form from a melt.

 16.    How do the properties of an igneous rock differ from a metamorphic rock?

An igneous rock tends to be massive with no preferred orientation of mineral grains.  Metamorphic rocks on the other hand do have a preferred orientation of grains caused by their response to the pressure under which they formed.

 17.    Under what conditions do metamorphic rocks form?

Metamorphic rocks form at depth, frequently around a cooling body of magma or with deeper burial.

 18.    Describe how one rock gives rise to another.

We are talking about the rock cycle.  Rocks are weathered, breaking down into sediment.  Sediment can be formed into sedimentary rock, which can be heated to form a metamorphic rock, which can be heated enough to become molten and thus forms an igneous rock.

 19.  What do you think the porosity and permeability of metamorphic rocks will be?
        Will there potentially be directional differences in strength?  What is foliation?

As with igneous rocks, the formation of metamorphic rocks does not typically cause porosity or permeability to develop.  The pressure under which these rocks form causes mineral grains to develop perpendicular to the direction of pressure.  Thus there is a preferred orientation to the crystals in the rock and that will affect its strength.  This preferred orientation of crystals is the foliation.