An igneous rock with large crystals embedded in a matrix of much finer crystals is indicative of a two-stage cooling process, and the texture is porphyritic Figure 3. If that magma is then involved in a volcanic eruption, the rest of the liquid will cool quickly to form a porphyritic texture.
The rock will have some relatively large crystals phenocrysts of the minerals that crystallized early, and the rest will be very fine grained or even glassy. Using Figure 3. Large waves crashing onto a shore bring a tremendous amount of energy that has a significant eroding effect. Several unique erosion features commonly form on rocky shores with strong waves.
When waves approach an irregular shore, they are slowed down to varying degrees, depending on differences in the water depth, and as they slow, they are bent or refracted. In Figure That energy is evenly spaced out in the deep water, but because of refraction, the energy of the waves—which moves perpendicular to the wave crests—is being focused on the headlands Frank Island and Cox Point in this case.
On irregular coasts, the headlands receive much more wave energy than the intervening bays, and thus they are more strongly eroded. The result of this is coastal straightening. An irregular coast, like the west coast of Vancouver Island, will eventually become straightened, although that process will take millions of years. Wave erosion is greatest in the surf zone, where the wave base is impinging strongly on the sea floor and where the waves are breaking. The result is that the substrate in the surf zone is typically eroded to a flat surface known as a wave-cut platform or wave-cut terrace Figure A wave-cut platform typically extends across the intertidal zone.
Relatively resistant rock that does not get completely eroded during the formation of a wave-cut platform will remain behind to form a stack. Here the different layers of the sedimentary rock have different resistance to erosion. The upper part of this stack is made up of rock that resisted erosion, and that rock has protected a small pedestal of underlying softer rock. The softer rock will eventually be eroded and the big rock will become just another boulder on the beach.
Note that this is a somewhat unique situation. Most stacks do not show that nature of differential erosion. Arches and sea caves are related to stacks because they all form as a result of the erosion of relatively non-resistant rock.
An arch in the Barachois River area of western Newfoundland is shown in Figure This is easily seen in igneous rock, which may cool at variable rates.
What is the most common type of extrusive rock? Often, lava cools over a few days to weeks and minerals have enough time to form but not time to grow into large crystals. Basalt is the most common type of extrusive igneous rock and the most common rock type at the Earth's surface.
Which mineral is the last to crystallize from a melt? What does Bowen's reaction tell us? Bowen's reaction series is a means of ranking common igneous silicate minerals by the temperature at which they crystallize. Minerals at the top have a relatively high crystallization temperature, which means that they will be the first minerals to crystallize from a magma that is cooling.
How is pyroxene formed? Diopside, which forms in metamorphosed carbonates, is the primary metamorphic pyroxene.
The two mineral groups are related to one another, as during metamorphism water reacts with pyroxene minerals to convert them to amphibole. What is mafic lava? Minerals near the bottom, such as quartz and muscovite, crystalize at lower temperatures. Norman L. Bowen was an early 20th Century geologist who studied igneous rocks. He noticed that in igneous rocks, certain minerals always occur together and these mineral assemblages exclude other minerals.
Curious as to why, and with the hypothesis in mind that it had to do with the temperature at which the rocks cooled, he set about conducting experiments on igneous rocks in the early s. He conducted experiments on igneous rock—grinding combinations of rocks into powder, sealing the powders into metal capsules, heating them to various temperatures, and then cooling them. The consequences of these characteristics are twofold:.
Volatile content of melts. Which leads to another topic — volatiles. Volatiles are elements that dissolve in magmas but transform to gas as magma reaches the surface and thus are depleted in lavas. Note that minerals high on BRS contain no volatiles, while those toward the bottom amphiboles and micas contain volatile elements. This tells us that mafic magmas tend to be poor in volatiles although not always , while felsic magmas tend to be volatile-rich.
This is another reason why basaltic magmas tend to erupt passively while felsic magmas tend to erupt more explosively. Classification of igneous rocks. Igneous rocks are classified on the basis of 1 chemical composition and 2 texture. Composition is controlled by. Compositional ranges are shown in Box 5.
Compositional classification may also be done by characteristic minerals found in different rocks, as illustrated in Fig. Textures are controlled by conditions of crystallization and cooling.
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