We call this purified material felsic because of the relatively high percentage of silica and oxygen. Basalt is extrusive. The magma from which it cools breaks through the crust of the earth and erupts on the surface. We call these types of events volcanic eruptions , and there are several main types. The volcanoes that make basalt are very common, and tend to form long and persistent zones of rifting in nearly all of the ocean basins.
We now believe that these undersea volcanic areas represent huge spreading ridges where the earth's crust is separating.
It's a lot like a cut on your arm, which will bleed until a scab forms. Basaltic magma is like the blood of the earth - it's what comes out when the earth's skin is cut the whole way through.
As an eruption ends, the basalt "scab" heals the wound in the crust, and the earth adds some new seafloor crust. Because the magma comes out of the earth and often into water it cools very quickly, and the minerals have very little opportunity to grow. Basalt is commonly very fine grained , and it is nearly impossible to see individual minerals without magnification. Basalt is considered a mafic silicate rock.
Among other characteristics, mafic minerals and rocks are generally dark in color and high in specific gravity. This is in large part due to the amount of iron, magnesium, and several other relatively heavy elements which "contaminate" the silica and oxygen. This steaming can persist for decades, long after the lava has solidified, depending on the thickness of the flow and the temperature of its interior.
Because the equation only looks at cooling of the lava flow's upper crust, the basal crust thickness is assumed to equal 70 percent of the upper crust according to this study. However, because the core of each flow type should cool at similar rates, we are basing cooling rates on the study. Also, of note, the flows studied in were much thinner with shorter cooling rates and may not account for long-term changes in wind and rain patterns. Preliminary analyses of the LERZ eruption flow thicknesses, suggest that the average flow thickness is around 10—15 m 33—50 ft.
Based on the cooling rate calculation, it could take roughly 8 months to 1. Solidification of flows ranging 20—30 m 65— ft thick could take about 2.
The thickest LERZ flows on land, which are approximately 55 m ft thick, may take roughly 20 years to reach a completely solid state. Because flow thickness, wind speeds, rainfall amounts, air and ground temperatures, and other factors all affect lava cooling rates, there is a range of uncertainty on how long the interior of a flow remains liquid. With flow crust being such an efficient insulator, it can take years to decades for lava within thick flows to solidify.
It takes much longer for the flow to cool to ambient temperatures. Imagine a huge flow of hot, liquid magma that is settling. The outer layer is starting to cool and darken in color from orange to black.
As it cools, it needs to shrink a bit, as hot materials usually take up more space than cooler ones. Because of this shrinking, the surface of the lava starts to crack. In this case, the lava starts to crack into regular shapes. Those shapes are forming because of how the lava cools. The more uniform the material of the lava is or basically how smooth and well mixed it is , the more evenly those centers pull.
That means it is more likely it will cool into hexagonal chunks. Scientists also think that faster cooling, like when lava is exposed to water, may also help with the formation of these columns. These chunks begin to form at the top layer of the lava, which is cooling the fastest.
As lower levels of the lava start to cool, they are also pulled into the shape under each center. In this way, you get lava cooling and contracting down into these cylinders. It usually begins to shape from the top down into the middle or even bottom of the lava flow. This is how really tall cylinders can form. And even when the material is a bit irregular, these cylinders can still form.
They just might not be hexagons… instead of 6, they might have 5 or 7 sides, or a more irregular shape.
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