Gabbro's density ranges from 2. The day after the lab is a good time to question the students about the significance of rock densities. Here are some questions for discussion: 1. What do you think causes some rock types to have higher densities than others? Assuming there are no air pockets in the rock, the types of minerals will determine the density.
Granite contains lots of quartz and feldspar - both fairly light minerals, whereas basalt and gabbro are made of heavier minerals.
Consider a zone of subduction. Why is it that when ocean crust and continental crust collide, the ocean crust always goes under the continental crust, and not vice versa? Ocean crust is made of basalt and continental crust is mostly granite.
Which would be less dense - granite, or magma that contains the same minerals as granite? The molten minerals would less dense for the same reason hot water is less dense than cold water, and hot air is less dense than cold air. Molecules that are moving faster tend to be farther apart. This is why molten material tends to melt its way to the surface. It's a density thing! I have a large scrap of gabbro counter-top , and I ask students, "How would you determine the density of this piece of gabbro?
The oceanic crust is relatively thin and lies above the mantle. The cross section of oceanic crust in Figure 2 shows the layers that grade from sediments at the top to extrusive basalt lava, to the sheeted dikes that feed lava to the surface, to deeper intrusive gabbro, and finally to the mantle.
Continental crust is made up of many different types of igneous, metamorphic, and sedimentary rocks. The average composition is granite, which is much less dense than the mafic rocks of the oceanic crust Figure 3.
Because it is thick and has relatively low density, continental crust rises higher on the mantle than oceanic crust, which sinks into the mantle to form basins. Improve this page Learn More. These differences help define and explain how the earth works. Granite is great stuff! Not only is it my personal favorite, it is without a doubt the most common rock type on the continental land masses.
Yosemite Valley in the Sierra Nevada and Mt. Rushmore are two notable examples of granitic rocks. But granitic "basement rock" can be found just about everywhere east of the Rockies if you're willing to dig through the dirt and sedimentary rocks at the surface.
Granite is intrusive, which means that the magma was trapped deep in the crust , and probably took a very long time to cool down enough to crystallize into solid rock. This allows the minerals which form plenty of time to grow, and results in a coarse-textured rock in which individual mineral grains are easily visible. Granite is the ultimate silicate rock.
Relatively small amounts of iron and magnesium occur, but since they have generally higher densities it's not surprising that there isn't very much in the granite. Due to the process of differentiation , most of the heavier elements are moving towards the core of the earth, allowing the silicon and oxygen to accumulate on the surface. Tile 10 Description. Tile 11 Description.
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