The learning point of this EarthCache is to get the geocacher to become familiar with the Igneous rock Basalt.
Igneous Rocks
Let`s do a little bit of review before we look into the igneous rocks. There are so many different types of rocks to be found around the world. And they all belong to one of the three types: Igneous, Sedimentary or Metamorphic rocks. The igneous rocks are formed from volcanic activity. This means they are either created from cooled magma or cooled lava. The difference between magma and lava is its location - interior versus exterior. (See illustration)
Magma is the molten rock found inside the crust of the earth. Pockets of magma come up from the mantle/main vent and push their way through weak spots and vents in the earth's crust, often under volcanoes. When magma is pushed up through a volcano that's erupting, the magma becomes lava as it exits out of the crater.
The distinction is important since there is a main difference between if the rock cools from lava or from magma, even that both are volcanic. The formation and difference we are talking about here is important because the rocks that come from magma become intrusive igneous rocks, and the rocks that come from lava become extrusive igneous rocks. The difference is due to how fast the material cools.
The cooling rates/process.
Since magma is located inside the earth, the overlying rock and sediment act as a blanket that keeps the magma warm and allows it to cool slowly. Lava, however, exits the volcano and cools much faster.
An easy way to see and understand this better is to explain the cooling rates by using baking as an example. After baking bread, you turn off the oven and allow the bread to cool before slicing. If you leave the bread inside the oven to cool, it does so very slowly. If you take the bread out of the oven and sit it on the counter to cool, it does so more quickly.
In rocks, the rate of cooling allows mineral crystals to grow, which make up the igneous rocks, to grow. The longer time the rock is allowed to cool, the bigger mineral crystals and grains we get.
Because the magma cools slowly, the ions have time to move around and form mineral crystals. As a result, when you have rocks that cooled from magma, you can easily see the different mineral crystals in the rock. These rocks are intrusive igneous rocks because they cooled slowly 'inside' the earth.
Because lava cools more quickly, there is not enough time for mineral crystals to form and grow, so you have rocks with very tiny mineral crystals that cannot be seen without the aid of a magnifier. These rocks are considered extrusive igneous rocks because they cooled quickly 'outside' the Earth.
Basalt
Basalt is actually the most common type of rock in the earth's crust, and it also makes up most of the ocean floor.
The basalt rock is made of different dark colored minerals such as pyroxene and olivine. The volcanic rock is low in silica content and comparatively rich in iron and magnesium. Basalt also contains some light colored minerals such as feldspar and quartz, but the amounts are very small, and by that don’t “shine” through the stone. Typically, you can't see most of the mineral crystals without using a microscope in the basalt stone because the quick cooling that prevents large crystals from forming. This makes the crystals tiny and unseen by the human eye.
Black color is given to basalt by pyroxene and magnetite. Both of them contain iron and this is the reason why they are black. So this is iron again which is responsible for the coloration of basalt. Plagioclase, volumetrically usually the most important constituent, is mostly pale gray in color.
Basalt forms when lava reaches the earth's surface at a volcano or a mid ocean ridge. The lava is between 1100 to 1250° C when it gets to the surface. It cools quickly, within a few days or a couple weeks, forming solid rock. While very thick lava flows may take many years to become completely solid.
Basalt in the field
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There are two types of volcanic basalt, described under in words to easier understand and remember the two different types:
Two Hawaiian words are used to describe the two types of volcanic basalt: 'A'a and pahoehoe. 'A'a basalts have rough surfaces (that make barefoot people cry, "Ah! Ah!" as they walk across it). This type of lava surface is rough with pointing and sharp edges. 'A'a is cooler, slower-moving basaltic lava. Pahoehoe basalts have a smooth glassy surface that looks like many ropes. The "ropes" form when the surface cools, becoming solid rock while lava flows beneath it. Pahoehoe is a hotter, faster-moving basaltic lava.
Basalt has a strict chemical definition. It is defined in the TAS diagram shown above. Basalt is an igneous rock that contains more than 45 and less than 52% of *SiO2 and less than five percent of total alkalies (K2O + Na2O) (*SiO2 = Silicon dioxide, also known as silica, silicic acid or silicic acid anydride is an oxide of silicon with the chemical formula SiO2, most commonly found in nature as quartz and in various living organisms.)
As you can see from the diagram, the neighboring rock types like basaltic andesite, basanite, picrite (picrobasalt), trachybasalt and even more distant rocks like phonotephrite or andesite may have very similar look and can be easily mistaken for basalt in many cases.
Igneous_rocks diagram
To log this cache.
To get to log this cache you will have to visit and answer the questions which are related to the coordinates given the earthcache.
When answers are collected, send them to CO for verification.
You can log immediately after answers are sent CO. If there are any questions about your answers CO will contact you.
Logs without answers to CO or with pending questions from CO will be deleted without any further notice.
Please do not include pictures in your log that may answer the questions.
Questions:
1. Answer the questions under by visiting the Coordinates.
A. Use the TAS diagram, and study the basalt balls at the coordinates. Would you say that the samples here is a “true and clean art” of basaltic rock? Or outside the given terms? (that contains more than 45 and less than 52% of SiO2 and less than five percent of total alkalies)
B. Study the white balls next to the basalt stones here at the location. Are the white balls an intrusive or extrusive igneous rock. Describe.
C.Study the igneous rock diagram! The basalt stone is a mafic stone that is extrusive. Where would you place the white balls in the diagram? Describe where, why and what type of rock we have!
2. (It’s voluntary to post a photo in your online log of your visit)
