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Rock On ~ A Hunt for Gneiss Rock

A cache by Cedar & Co. Send Message to Owner Message this owner
Hidden : 3/27/2015
1.5 out of 5
1.5 out of 5

Size: Size: other (other)

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Geocache Description:

This Earthcache is part of a rock series that will take you to some interesting raw rock specimens to learn about each one.  Inside Rockwood Park is where this rock lesson takes you.

Please stay on the trails at all times, CITO – Cache In & Trash Out and respect all plants and wildlife.

Rockwood Park and trail information:
- free admission & parking.

Thank you to Rockwood Park for permission to place this Earthcache.


Our planet has 4 layers:
1- the outer crust: the solid outer layer we live on
2- the mantle: where magma, semi-solid rocks and minerals are
3- the outer core surrounding the very middle: made of a mixture of iron and nickel
4- the inner core: the very center of the earth, a ball of mostly solid iron

Simply there are 3 types of rocks: Igneous, Sedimentary and Metamorphic.

The rock cycle: (see diagram) Igneous rock is created from cooling magma or lava. It can then be altered by heat and pressure changing it into metamorphic rock or uplifted, weathered and eroded into sediments. Sediments are compacted and cemented together to form sedimentary rocks. Sedimentary rocks if uplifted, weathered and eroded again will form more sedimentary rocks or under heat and pressure will be altered to form metamorphic rocks. Metamorphic rocks can be uplifted, weathered and eroded into sediments and then when compacted and cemented will create sedimentary rocks or if melted will produce more magma.





= rocks formed from the cooling and hardening of magma (hot liquid rock)

Rocks in the earth, deep inside the mantle, are under a constant change of high pressure and high temperatures causing them to melt. Others are in a semi-liquid state. Melted liquid rock is called magma. When magma is exposed on the surface of the earth it is called lava or molten rock. Igneous in Latin means fire-formed.

When magma cools into solid rock then crystals are formed. There are different kinds of igneous rock depending on the rate the solid rock had cooled, where it cooled, and the differences in the chemical compositions. 

Igneous rocks are created 2 ways:

Intrusive Igneous Rock: forms when the rock cools slowly while still under the surface of the earth. Visually it has large coarse-grained mineral crystal specks of different colours.

Extrusive Igneous Rock: is created when liquid magma (forced up out of the earth through volcanoes) cools very quickly on the surface of the earth because it is exposed to air or water. The rapid cooling process does not give enough time for crystals to grow very large. This rock is therefore made of very fine grains and crystals.

Textures of Igneous Rock (6 main ones)

Aphanitic: extrusive (formed on the surface) / fast cooling / grains are too small to be seen with the unaided eye. Examples of stony and dull aphanitic rocks are: felsite (light coloured and fine grained) and basalt (dark coloured and fine grained)

Glassy (volcanic glass): extrusive (formed on the surface) / near instant cooling / no or very few crystals have time to form during solidification of lava. Example: obsidian (dark silver gray to black, shiny)

Pegmatitic: intrusive (formed under the surface) / rocks who’s crystals grow to large sizes ranging from 2.5 cm to a few meters

Phaneritic: intrusive (formed below the surface) / slow cooling / grains are large enough to be seen with the unaided eye. Example: granite, gabbro, diorite

Porphyritic: having a mixture of large grained crystals amongst a finer grain groundmass. Initially slow cooling deep down, grows large crystals and the final cooling (at a shallower depth or as it erupts from a volcano) is rapidly. It is possible the groundmass has an aphanitic quality. Example: Basalt is an aphanitic porphyrite rock; granite is a phaneritic prophyritic rock

Pyroclastic (fire fragments): extrusive (formed on the surface) / lava solidifies almost instantly during the eruption of a volcano. Occasionally, tiny holes form when gas bubbles are trapped and expand (or water is trapped) during this process creating a frosty texture. Examples: pumice (vesicular rough textured), scoria (cinders)


SEDIMENTARY Rocks (known also as Secondary Rocks)

= rocks formed from compacting & cementing together sediments

Sedimentary rocks are made from layers of broken sediments (sand, pebbles, rock, minerals, plant, animal, etc.) that have compacted and cemented together over billions of years at the earth’s surface or in water. These rocks can contain fossils.

Layers of sedimentary rock are formed mostly parallel to the earth’s surface. If the layers are on an angle or broken this indicates a movement of the earth has occurred.

Sedimentary rocks are created 3 ways:

Clastic Process - eroded, broken and weathered bits of rock transported by gravity to lower areas on earth (example by wind, water, glaciers) where more erosion and chemical changes occur, reducing the particles to much smaller grains, affect the formation of these sedimentary rocks

There are 3 main groups of clastic rocks:

a) those made of gravel fragments called conglomerates or breccias. Example: the rocks can be constructed of quartz or sedimentary, metamorphic or igneous rock fragments bound together with a mixture of sand, mud and chemical cement

b) those made of mud, clay, or silt or a mixture of these called mud-rocks. Example: shale is formed from bits of clay

c) those created with quartz, feldspar or lithic mineral components. Example: sandstone is composed of sand-sized fragments of quartz

2) Chemical Process - when water evaporates leaving behind dissolved minerals affecting the formation of these sedimentary rocks. Example: thick deposits of salt from the repeated action of flooding and evaporation forms rock salt

3) BioChemical (Organic) Process - when the formation of sedimentary rocks is from the accumulation of organic debris such as shells, coral, plants, carbon, etc. Example: the formation of coal is from the accumulation of plant debris and rock sediments 

Texture of Sedimentary Rocks

The grain size and orientation of the clasts (broken fragments that make up the sedimentary rock: minerals, pre-existing rock, shells, etc.) are used to determine how they are formed and what they are formed of. Individual pebbles and/or sand grains can be visible. Colours may also be present.

Sedimentary rocks may break apart or crumble easily.


METAMORPHIC Rocks (meta=change) (morph=form)

= rocks formed by altering the chemical make up of the original (parent) rock with a process of heat or heat & pressure

Igneous or sedimentary rocks that have been buried deep into the earth are squeezed by other rocks on top of them pushing them down further exposing them to high heat. Intense pressure and temperatures transform the minerals and chemicals. This alteration can happen by: changing the mineral configuration - growing larger or breaking down and forming new minerals; or by reacting to fluids that have entered the process. The kind of new rock that is formed depends on which types of rocks are used in the process and how long the process takes. Example: marble is made from limestone makeup and sometimes dolomite

Textures of Metamorphic Rocks

Metamorphic rocks are the hardest type of rock and are therefore more resistant to erosion and weathering. Some show shiny crystals. Foliated textures (those subjected to uneven stresses of pressure) have materials aligned in ribbon-like layers or planes. (Example: slate {fine grained}, gneiss {coarse grained}) Nonfoliated textures show impurities but no visible alignment. (Example: marble, quartzite, hard coal)

Two types of metamorphisms are:

a) regional: occurring over hundreds of miles/km usually associated with the faulting and folding processes, involving intense heat and pressure, that create mountains, foliated textures occur

b) contact or thermal: occurring over a smaller areas where hot magma comes in contact with the surrounding rocks and alteration occurs due to intense heat


Rocks have different colours due to:

a) minerals  - come in a variety of colours (example quartz comes in many colours)

b) temperature - due to the chemical make up of minerals, crystals will form at different temperatures. Example: minerals that are able to crystalize at high temperatures will make those rocks dark in colour.

c) iron (II) oxide - gives the rock a colour of grey or green

d) iron (III) oxide - gives the rock a colour of red to brown

e) oxidization - of organic material in arid climates gives the rock a red to orange colour

f) lack of oxidization - of organic material, in underwater environments, gives the rock a black or grey colour


Gneiss Rock

Gneist is a German word meaning to sparkle. This very hard rock is typically characterized by alternating bands of various widths of light and dark colour but bands are not always present in every sample. The darker bands are made up of iron and magnesium. Lighter bands consist of more silicon, oxygen, aluminum, sodium and potassium. Bands are created when extreme temperature and pressure stretch the rock material in opposite directions.

Gneiss is very common rock close to the middle of the earth making it one of the oldest rocks around.

Many uses for gneiss rock include counters, flooring, face stones on buildings and gravestones. It is attractive for decorating purposes and durable for building.

Why Gneiss?

Why Gneiss (pronounced nice) because it is so different from the pine trees here which are so knotty! Hahaha. Hiking the park we found this striking bolder of many colours. The wide striped band is attractive and very opposite from the marble rock just 500 meters south of it on the other side of the fault line on Frying Pan Lake. What a beautiful specimen.


Please observe the raw rock specimen at this location by finding, if possible, a freshly fractured surface rather than a weathered surface.

a) Are the bands in this specimen wavy or straight?

b) What is the width of the largest band you see?

c) Describe the size and all the colours visible in the multiple stripe bands underneath?

d)Estimate the width and height of the entire exposed rock sample above ground:

e) From your observation and reference to this cache page does the rock you are studying belong to the Igneous, Sedimentary or Metamorphic rock group?

Those who send answers will receive a bonus.

Photos of you in the area are always nice to see and add to the interest of the geocache page. Please consider adding some of your own.
Favourite: If you like this Earthcache, please consider giving it a favourite point so others will know you enjoyed the experience. Thanks a bunch for visiting ~ We hope you have enjoyed learning about Gneiss Rocks. Rock On...


”Cedar & Co.” have earned (GSA) Geological Society of America's
highest earthcache level:

Platimum Earthcache Level is awarded by Geo Society .org for visiting and logging 20 or more EarthCaches in 5 or more states/provinces/countries and have created 3 or more EarthCaches.

”Mica Discovery Award is awarded by
Geo Society .org for visiting and logging
100 or more EarthCaches.

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29 Logged Visits

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