What many people don't know is that this sculpture is here along 5th avenue. They walk right passed it. For the few who actually look at it, many people don't realize that this sculpture is millions of years old. The sculpture was carved from a solid piece of black basaltic rock. Better known as basalt, it is one of the most common rocks on earth. Basalt mostly makes up the sea floor, but can be found on land near active and inactive volcanoes. Examples would be inside a solidified lava tube such of that in Hawaii or Iceland.
Basalt Flows are formed from the solidification of lava flows. These are the youngest rocks in the world:
Eventually, erosion takes place and breaks up the flows into basalt rocks. These are new rocks, but aren't the youngest.
Basalt is an igneous rock, meaning it was formed from the solidification of liquid magma. There are two types of basaltic rock: intrusive and extrusive. Intrusive basaltic rocks form usually inside or part of a volcano. This can happen inside the caldera, dikes, lava tubes, etc. There are three ways Basalt can form. One is formed under water either at divergent plate boundaries like the Mid-Ocean Ridge or at volcanic hot spots like Hawaii where lava pours into the ocean. The other way is that is flows out of a volcano on land such as that in the Cascades in the north western United States. And lastly, one that is formed in a lava tube or dike where it never reached the surface. If the basalt solidified on the surface, it's called an extrusive rock. If it solidified underground, then it's called an intrusive rock.
How do we tell the difference if it's formed on land or under water? If it formed in the ocean or on land, then by nature it's an extrusive rock. By looking at the rock, we can classify them by observing the size of the bubbles. If it formed under water, the basalt would have to cool extremely rapidly due to the cool air temperature. The rapid coolness of basalt flow makes it vesicular. Vesicular rocks contain vesicles or tube like structures. They were formed when bubbles inside the liquid magma, from boiling seawater, were captured as the rock hardened. Land formed basalt looks quite different. It too cooled rapidly, but not as fast as basalt formed at mid ocean trenches.
Note that the air temperature around a river of molten rock is near the same temperate of the rock. It's also why you see wavy patterns in the air on streets on hot days. This is because the hotter things are, by nature, are less dense than cooler things. This difference in density refracts light from the sun and you see a wave in mid air because that elements in our atmosphere are a good conductor of heat hence why global warming is such a big issue.
So, basalt that formed on land does cool very fast, but because it's not solidifying as fast as an undersea volcano, it can't create large bubbles. These bubbles are very small and from afar you can't see much, if any, at all. They also create ripples as the lava flowed downhill which are really cool to see in areas that have a lot of volcanic activity such as Hawaii or Iceland. The photo below is a vesicular basaltic rock that formed on land.
We can tell if a basaltic rock formed underground, such as a dike or lava tube, by looking for no bubbles or holes. This is because the rock around the liquid magma acted like an insulator, making the magma cool more slowly, allowing time for the gasses present to dissipate away. This is a more solid piece of rock with little to no bubbles and because it formed underground, is more susceptible for other compounds to be mixed with the basalt. A common element is iron since the lower you go in elevation, the more abundant it becomes. As the magma rises, it brings up elements from deep below the earth's crust.
Another way for elements to be added to the mixture is from fractional crystallization when part of a magma cools based on the Bowen's Reaction Series. This chart, shown below, is based on certain minerals with form in different temperatures. The closer the magma gets to the surface, the cooler it gets. It looses heat when melting the rock above it. When it does, some minerals crystalize, making the rock more mafic, or black, in color. Fractional crystallization can explain why or how other materials have ended up where they did and the Bowen's Reaction Series helps us understand these differences in the rock.
TO LOG A FIND ON THIS CACHE YOU MUST GIVE ME THE CORECT ANSWERS. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER. ANY INCORRECT ANSWERS WILL RESULT IN A DELETED LOG
1. "Unidentified Object" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. Note, this IS cheating as only the person who sent their answers has learned something, while the others get a "free ride". It's not fair to others. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. Describe the (a) size of the bubbles, if any, (b) relative size of the rock, (c) color(s) of the basalt here. Please don't touch the sculpture. You can look at it and know what they are.
3. What are the similarities and differences among the sculpted and original basalt?
4. What does the sculpted areas of the rock show a geologist that we can't see from the surface of the rock?
5. Describe the weathering process of these rocks. Is there any rusting or corrosion? If so, where do you see it AND why is this occurring?
6. In your opinion, how did this rock form? Did it form under water, outside a volcano, or underground? How can you tell? Use examples from the text and outside sources as well as your own personal observations.