"Rock Hound's" Webquest
Mystery Rock 4

What kind of a rock is this?

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



Where did I find these rocks?

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

Polychrome Mountain
5,750 ft
Alaska Range
7500 ft
Alaska Range
7500 ft



What rock formations are in the area where I found these rocks?

In Denali National Park, I did a lot of hiking and found a lot of evidence that volcanoes were once active here.  The colorful volcanic rock patterns of Polychrome Mountain, the odd bulbous rocks near Little Stony Brook, and layers of volcanic ash near the East Fork of the Tolklat River.

Lava Flows and Ash

Volcanic Ash and Volcanic Flows 
of Polychrome Mountain
(Denali N.P.)


Between 40 and 60 million years ago, lava blasted from the Cantwell Volcanoes.  The lava that was blown from these volcanoes was very viscous, bubble gum-like.  Much of this lava landed on other hot lava and began to flow downhill.  This lava eventually formed the igneous rocks that can be seen throughout the central area of Denali National Park.  Layers of Rhyolites (pinkish/ reddish rocks) and Basalts (darker rocks) now give Polychrome Mountain its multi-color look.



Pillow Basalt

Pillow Basalt  Near the 
Little Stony Creek 
(Denali N.P.)


When these lava flows hit water, they cooled quickly into bulbous shapes.  The hot basalt hits the water creating lots of steam.  The basalt, steam, and water mixes with sand and mud to create pillow basalt. 



Volcanic Ash

Volcanic Ash Along the 
East Fork River
(Denali N.P.)

The Cantwell volcanoes sent ash flying into the air.  This dark ash fell along the outer reaches of the volcanoes.   This ash never formed into solid rock but over time, it mixed with water to form a very clay-like substance.




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 these rocks?

Tests What to do What I found out about 
Mystery Rock #4
General Description What is the grain size and texture of your rock.  Are there flecks of shiny material?
Found in a bulb-like mass.  The grain size is large, the texture is very rough, and the rock crumbles apart with little effort.
Color Look at the sample and determine its color - white, black, green, clear, etc
The rock has a very dark brown color that is made up of black, gray, and brown flecks.
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 has many flecks of shiny material.
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 easily with a knife but cannot be scratched with a penny so I would say its hardness is around 4 on the Mohs hardness scale.
Rub your rock across  a piece of  tile (unglazed side) and determine the color of the powder. 

The sample makes a brown streak.


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

Where the rock was broken, it crumbled to make an irregular break.





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.


Pillow Basalt

Color Luster Hardness Streak Fracture
Brown Many Shiny Flecks 4 Brown Irregular

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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