G101/201                                                        Name_______________________________________

Igneous rock textures and composition          Class time (circle one)      MW       TTH

 

For each of the following pictures, pick the textural description that matches the picture and write the picture’s letter next to the description.  (color pictures can be found on the class web page)

                                                                                                                

Phaneritic:  crystals are visible through out the entire rock

 

Porphyritic:  Large minerals embedded in a rock with base or background made of tiny (non-visible) crystals 

 

Pegmatitic:  VERY large crystals make up the entire rock

 

Vesicular:  Pockets or holes in a rock made of very tiny (non-visible) crystals

 

Flow Banding:  Layering in a rock created by a lava flowing over the earth’s surface

 

Aphanitic:  Very fine grained rock with no really visible minerals

 

Glassy:  Very smooth glossy rock that has the appearance of glass

 

Pyroclastic:  Any rock created by the amalgamation of small fragments of ash, pumice and other rock particles thrown out of a volcano during an eruption.

 

 

 

Text Box: C

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 For each of the rocks named below, fill in the table as to their composition (felsic, mafic, ultramafic or intermediate) and if they are intrusive or extrusive.

 

Rock Name

Rock Composition

Intrusive/extrusive

Granite

 

 

Scoria

 

 

Welded ash tuff

 

 

Gabbro

 

 

Peridotite

 

 

Basalt

 

 

Pumice

 

 

Andesite

 

 

Ash tuff

 

 

Amygdaloidal basalt

 

 

Palagonite

 

 

Diorite

 

 

Rhyolite

 

 

 

For each of the following rocks, answer the questions and determine their compositions (felsic, mafic, intermediate) based on the color or minerals that are given.  Hint, Bowens reaction series could be VERY helpful here.

 

 

What is the texture of this rock?___________________________

 

The dark mineral you are seeing in this sample is the mineral hornblende. 

Hornblende is an amphibole (group of minerals).  It can often be

identified in rocks by its color (black), habit (elongated crystal), and

cleavage (often visible when you catch it in the light).  Based on the

texture, the large amount of hornblende in this sample and the medium

grey color of this sample what composition is this rock?

 

______________________

 

Based on your two previous answers what is the name of this rock?

 

__________________________________________

 

 

 

What is the texture of this rock?________________________

 

The minerals in this rock include Quartz (often appears as a clear

grey mineral), hornblende (notice the elongated habit), and a very pale

white/pink microcline.  Based on the minerals, what is the composition

of this rock?______________________________

 

Based on your two previous answers, what is the name of this rock?

 

 

 

 

 


What is the texture of this rock?___________________________

 

The white mineral you are seeing in this rock is plagioclase feldspar. 

Often times, feldspar appears as elongated crystals with good

(easy to see) cleavage.  Based on there being a plagioclase feldspar

in this rock and look at the dark color of the very fine background rock, what is the composition of this rock?

 

__________________________

 

Based on your two previous answers, what is the name of this      

rock?_________________________________________

  

This rock is full of fragments of other rock particles (pumice and lapilli)

and is made primarily of ash.  There are no minerals visible in this

sample, volcanic rock fragments. 

 

What is the name of this rock?______________________________

 

Based on what we have talked about in lecture, what is the

composition of this rock?__________________________________

 

 


 

This is an extrusive felsic rock. You cannot see this in the picture

but there are a few crystals of quartz visible in this sample. 

What is one texture that is obvious from the photo?

 

______________________________________

 

What is the name of this rock_______________________

 

 

Explain how a porphyritic texture forms.  Give the setting and the mechanics as to HOW you get a fine grained rock with larger crystals embedded in it.

 

 

  

 

 

Explain how an amygdaloidal texture forms on a rock.  Explain the setting and the mechanics as to HOW you get a vesicular volcanic rock with secondary minerals forming in the vesicles.