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Geology/beach rocks


Hi Mr.Patton, I was wondering if you could help me figure something out. I was recently in Ecuador on a study abroad trip and while we were there I took time to walk down the beach and collect some rocks, well as I looked on the ground I saw lots of blue-green almost teal rocks. after I set them down a while in the sun, i came back to get them and the color had kind of faded, how ever it seems the water intensifies the color. one rocks is half light green and half dark green very distinctly separated you can almost feel the line of change in colors. I have found similar rocks in Oregon, both of which are very volcanic areas as far as the ocean is concerned, dos this have any reason behind why the rocks of colored?


Hello.  I spent some time in Quito back about 24 years ago.

Okay, first if you look at a geologic map of Ecuador you will notice that there are linear bands of different rocks east and west of the Andes.  The Andes gave their name to a particular form of extrusive igneous rock, Andesite.  It is a white-ish rock with black specks (phenocysts).  It is high in quartz and plagioclase feldspar (both white, feldpar can be pink. with pyroxene and hordblende(black minerals accounting for the specks.

The other bands of rocks to the west or seaward were acreted.  By that I mean they were slapped onto the Andes range by the movement of the south american plate against the Pacific plate.  This movement also caused the uplift of the Andes.

Any sediments on the sea floor were bulldozed onto the side of South America by the Pacific plate goint under the S. American plate.  What happens to the sediments, whether they are solidified into rocks or not, is they get heated and mashed up at terrific pressure changing them in a process called metamorphism.  Limestone changes to marble.  Shale into slate, granite into Charnokite, and so on.  A lot of recrystallizaion takes place and new minerals form.  Minerals are not rocks, they are the building blocks from which rocks are made.  So in the example of Andesite, its a rock, but quartz, feldspar, hornblende and pyroxene are minerals.  On occassion large bodies of minerals form but the do not usually survive the weathering process.  There are lots of green minerals, but what you have indeed is a rock.

Since the rock was smoothed it was transported to the coast in a stream. If the size of the smoothed rock was fairly large, it indicates that it did not travel far enough for stream transport to break it down to sand.  It indicates that it was deposited in an area from a young stream with a high gradient.  The beach processes also are working on it as well.

The green color in rocks and minerals comes about from the addition of various metals.  Copper accounts for green coloring in a lot of minerals.  Without a picture I cannot but guess on what your stone might be.  

Know this, Andesite is an intermediate volcanic rock. If you take Basalt, and modify the chemical composition while the rock is melted it becomes Andesite if some of the darker minerals settle out.

If you take basalt and it undergoes metamorphism as seen in the acretion of rocks to a continent, they form what is known as greenstone belts, because low-grade metamorphism of basalt produces chlorite, actinolite, epidote and other green minerals.

So what you have is some of that.  

The color intensity change with wetness has to do with the way the small crystals of minerals and water interact with the rock.  You can oil the rock to keep that look.

The difference in the rock bands is due to the original rock layers before metamorphism.  It could also be a response of the minerals to the tectonic pressure, layers in metamorphic rocks tend to align perpendicular to the direction of the pressure.  So if the tectonic pressure was West -> to East, the layers in the rock would align | North to South.


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Keith Patton


I can answer questions concerning physical and historical geology, environmental geology/hydrology, environmental consulting, remote sensing/aerial photo interpretation, G&G computer applications, petroleum exploration, drilling, geochemistry, geochemical and microbiological prospecting, 3D reservoir modeling, computer mapping and drilling.I am not a geophysicist.


I have 24 years experience split between the petroleum and environmental industries. I have served as an expert witness in remote sensing, developmental geologist, exploration geologist, enviromental project manager, and subject matter expert in geology and geophysical software development.

American Association of Petroleum Geologists
American Association of Photogrammetrists and Remote Sensing

Bachelor and Master of Science
Registered Geologist in State of Texas

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