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Vesicular basalt EarthCache

Hidden : 04/27/2013
Difficulty:
2 out of 5
Terrain:
2.5 out of 5

Size: Size:   other (other)

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Geocache Description:



Vesicular basalt

Basalt

When a friend and I visited this beach I was wondering about strange stones on that beach. There were dozens and hundreds of black stones with lots of holes. How could that be?

The answers can be found in the geology of the earth.

The black stones are basalt, a volcanic rock which is formed by rapid cooling of basaltic lava or magma. Normally the basalt rocks are very compressed and hard, so they can be used very good for cobblestones for building streets and paths. Lots of us know basalt by its typical six corner columns.

The basalt rocks at this location are special. They show lots of holes which have been created through gas bubbles. When magma is active in a volcano many gases are released from the inside of the earth due to changes in the pressure from the magma rising. When finding such vesicular basalt (=basalt with lots of pores) you know that this is special. The gas bubbles from inside the earth could not make it to the surface of the lava this time. The lava cooled down too fast before the gas bubbles had the chance to make their way out of the lava into the atmosphere. So they had to stay in the cooled down lava - the basalt rock.

At this beach area you find really strange and weird rocks with lots of pores and pore lines which look like real streets or tunnels through the rocks.

Together with all the other pebbles at the beach also the vesicular basalt is very old. It was created millions of years ago and eroded into rocks of different sizes. These rocks have been rounded through the erosion by the wind and the sea. So they have the form of pebbles but often the size of rocks.

In my opinion that location is very special and I enjoyed looking at the rocks very much. You can see the creation of the earth in them.

Hopefully you enjoy your visit as much. To log this EarthCache please send me the answers for the logging tasks via my profile:

1. Look around and find some vesicular basalt. Have a closer look at the pores. How big are they? Do they always have the same form or can you see different forms? Why?

2. Try to find a rock with "tunnels". What may have developed that tunnels? You can find the answer with a closer look.

You are allowed to log directly after sending me the answers of the logging tasks. I will contact you if the answers are completely wrong to correct them together.

Enjoy your visit!

Quotes:
- Analysis of Vesicular Basalts and Lava Emplacement Processes for Application as a Paleobarometer/Paleoaltimeter, Dork L. Sahagian, Alexander A. Proussevitch and William D. Carlson


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