What is the difference between MAGMA and LAVA?
The answer is relatively simple. Both are molten rock associated with volcanoes. Both are considered to be IGNEOUS rocks. The difference is where they live. Molten rock BELOW the surface is called MAGMA while rock that is ABOVE the surface is called LAVA. Even though magma and lava may have the same chemical structure, the rock that is formed when they cool has very different properties. The rock that forms from magma is called INTRUSIVE igneous rock while rock that cools on the surface is called EXTRUSIVE igneous rock.
You would think that they would look very similiar but you would be wrong. The length of time it takes for laval to cool above the surface is relatively short but the length of time for magma to cool is much longer. Lava can sometimes take minutes to solidify while rock insulated by the earth can take centuries. Slow cooling gives the distinct minerals in the rock time to consolidate into crystals. Those crystals are the teltale proof that a rock is an intrusive igneous rock. Granite is the intrusive rock that many of us are familiar with. It most often has pink, white, and black crystals. Those crystals are formed from the distinct potasium feldspar, plagioclase feldspar, quartz and biotite and/or amphibole crystals that are present. The potassium feldspar gives granite its distinct pink color.
Rhyolite, on the other hand, has a very similar chemical makeup. It often has the same pink tinge as granite. The difference is that rhyolite is an extrusive rock. It cooled quickly above the surface. It has crystals so small that you would need magnification to see them.
Gabbro, on the other hand is a coarse grained dark-colored rock. Much of the deep oceanic crust is Gabbro. Look at the following picture. Is it an intrusive or extrusive rock?
If you said, "intrusive," you got it right. The mottled appearance formed by the large crystals is the give away. Gabbro is chemically similar to basalt. Take a look at the following picture:
The lack of distinct crystals in this fine-grain basaltic rock identify it as being extrusive.
This is a simple earth cache. Go to the rocks at ground zero, Spend a few minutes examining them. You will find that most of the rocks are grey. There used to be a very unique intrusive granite rock here. This has been moved. Here is the information you need to send to the cache owner:
- How many people are in your party?
- Describe the rocks that you find at ground zero. Are they intrusive or extrusive? How do you know?
You will need to travel a short distance to answer the next question. You will find a field of rocks at N 39 33.021 W 119 50.938.
- How would you describe the rocks here? Intrusive or extrusive? How do you know?
Be sure to mail the answers to these questions to randallh@outlook.com. You may go ahead and log the cache but your log will be deleted if I don't get your answers within a few days.
References:
Geoscience News and Information - Geology.com, 2016, See http://geology.com/rocks/igneous-rocks.shtml
Tarbuck, Edward J., Lutgens, Fredrick K., Earth Science Thirteenth Edition, 2012, Prentice Hall, Pages 55-61