"Rock Hound's" Webquest
Mystery Rock 3

What kind of a rock is this?

Where Found?  |  Rock Formations?  |  Layers?  |  Smushed Together?  |  Geology Tests? | More Information

 

 

Where did I find this rock?

In Wrangell St. Elias National Park, I was hiking in an area where there were lots of dormant volcanoes.

 
Mt. Blackburn
16390 ft (4996 m)
Mt. Sanford
16237 ft (4949 m)

 

 
Mt. Drum
16390 ft (4996 m)
Mt. Wrangell
14163 ft (4317 m)

 

 

What rock formations are in the area where I found this rock?

I did a lot of hiking in Wrangell St. Elias National Park and discovered a lot of evidence that volcanoes were once active there.  Along the Skookum Volcano Trail, I explored lava flows and marveled at towers of layered lava rocks.

 

Lava Flows
 

Lava Flows  on the 
Skookum Volcano Trail
(Wrangell St. Elias N.P.)

 


Lava  poured from the Skookum Volcano about 2-3 million years ago.  This lava flowed slowly from many  vents and fissures.  As the lava flowed further downhill, it began to cool and eventually turned to a dark-colored rock.  Over time this rock was covered by layers of ash and soil.  Now, many years later, erosion has worn away the soil exposing these lava flows.  Here is Christina near lava flows.

 

Layers of Lava


Lava Vents on the 
Skookum Volcano Trail
(Wrangell St. Elias N.P.)

2-3 million years ago, lava flowed from the Skookum Volcano.  This initial lava cooled and formed rock.  Later, more lava flowed over the already cooled rock.  This happened over and over forming layers of igneous rock.  Wind and glacial erosion later shaped this igneous rock into the interesting shapes we see today.

 

 

 

Did my rock sample come in layers?

Sedimentary rocks often form in layers as seen in much of my hiking in the Gates of the Artic National Park.  The rocks that I am trying to identify are not formed in layers.

  

My Rock Does Not Have Any 
Layers Like the Rock in the Picture Above.

 

 

Does my rock sample look like a bunch of rocks smushed together?

Conglomerate rocks form by rocks pressing together to form a new type of rock as seen in much of my hiking in the Denali National Park.  The rocks that I am trying to identify are not formed by having many pieces smushed together.

  

My Rock is Not Smushed Together Like the One in this Picture

 

 

How would a geologist test this rock?

Tests What to do What I found out about 
Mystery Rock #3
General Description What is the grain size and texture of your rock?  Are there flecks of shiny material?
The grain size is so small it can not be seen with the naked eye,  the texture is smooth, and the rock shines like black glass.
Color Look at the sample and determine its color - white, black, green, clear, etc
The rock is black with some light gray flecks
Luster
Look at the sample to determine if the mineral is metallic in appearance (looks like a chunk of metal) or non-metallic (doesn't look like a chunk of metal).

This rock is very shiny.  
Hardness
Scratch the unknown rock with a known hardness to determine which is harder. Continue doing this with harder or softer minerals from the kit until the hardness is determined.
 
The sample can be scratched by pressing down hard with a knife so I would say its hardness is around 5.5 on the mohs hardness scale.
Streak
Rub your rock across  a piece of  tile (unglazed side) and determine the color of the powder. 

The sample makes a white streak.

Fracture


Examine the mineral for areas where the mineral is broken. Is the break irregular or smooth?

Where the rock was broken, it was smooth..  The fracture looked like broken glass.

 

 

 

First, I made an educated guess at what type of rock this was.  Then, I compared the data I collected on this rock to information that I found in National Audubon Society Field Guide to North American Rocks and Minerals.  If I identified the rock sample correctly, I made an identification card.  If I felt that was not the correct identification, I went back and reviewed my data and made another educated guess.

 


Obsidian
g

Color Luster Hardness Streak Fracture
Black Glass Like 5.5 White Like Broken Glass

Submitting "Rock Hound":  Matthew Durant  Boston, MA

 

 

 

I want to learn more about igneous rocks

How Igneous Rock Is Formed  Good description of how igneous rocks are formed.  The site offers several examples of igneous rocks.

Igneous Rocks  Excellent descriptions on how igneous rocks are formed.

Intrusive Igneous Rocks  Great images and descriptions of igneous rocks that are formed intrusively.

Igneous rocks  Great images of igneous rocks.

Kinds of Volcanoes   Great images of the different types of volcanoes.

Volcanoes  A great description of how a volcano works.

Ask-A-Geologist  Ask-A-Geologist questions are answered by USGS employees, retirees, and contractors who have volunteered to participate in the project.

 



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