East Orange is home to volcanic outcrops of ancient rumblings. Some of which are a few millions of years old. The three main types of volcanic rocks made here are Basalt, Basaltic Andesite, and Tuff. These three rocks are considered to be igneous because they formed on the surface when when lava cooled rapidly. Depending on the depth, speed of cooling, viscosity (how slow a liquid moves), and temperature of the water (since this formed underwater), they can form various types of igneous rocks. So how did these rocks form, and why is there a volcano in the middle of Orange County?
The rocks formed when the Farallon Plate had a divergent boundary on it's western end. A divergent plate boundary is what happens in Iceland. It's literally when two land masses are splitting apart from each other. When this happens, magma from below has a strait shot up to the surface and volcanoes start to form. Sometimes, these channels of molten rock occur a few miles out from the plate boundary and form a small, temporary, hot spot. In this case, this is what formed the El Modena Volcanics.
Eventually, this plate got swallowed up by the now current San Andreas Fault, transforming it into a transverse plate boundary that we know of today. A transverse plate boundary is when two plate slide past each other. Because this hot spot was west of the convergent plate boundary, or subduction zone, it did't get destroyed. A convergent plate boundary happens when two plate collide into each other. On land, they form mountains such as the Himalayas. underwater, they form deep see trenches such as near Japan or Chile. So now that you know how this volcano formed, lets discuss how lava formed these different rocks.
Basalt
Basalt is the most common rock in the ocean and is one of the most common igneous rocks in the known universe. If it cools in water such as divergent plate boundaries at mid ocean ridges, it forms bubbles as the water around it superheated from about 3º C to about 540º C in a matter of seconds. If it cools on land, such as divergent plate boundary in Iceland or hot spots in Hawaii, it can form more slowly. This can allow time for larger crystals to form inside the rock, making it darker and more mafic.
Basaltic Andesite
Basaltic andesite tends to have larger crystals and is more green than regular basalt. This is due to the fact there is more olivine crystals in the rock. Olivine is the first mineral to harden when getting from hot to cool. It usually forms in explosive eruptions, unlike regular basalt that forms as it flows out. Again, the same rules apply when there are holes or not, but more than likely there won't be any holes. Even though this volcano formed underwater, it was millions of years ago. When it happened, there was some basaltic andesite formed with holes. But over millions of years, these holes weathered, making the rock smooth. When basaltic andesite formed on land, it become a solid rock. It's hard to tell if it originally formed underwater or not.
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Tuff
Tuff is a white volcanic rock, made from ash fallout. Overtime, when a volcano erupts, the ash can harden and form rocks. The rate at how fast or slow it can harden depends on how much ash was expelled and how far it was distributed from the volcano. Tuff is usually white, due to the fact that it is grounded up rock in powder form. Tuff is usually thickest near the volcano's crater and as you get farther away from it, it weakens. Tuff is usually only formed on land. If it to form in water, the ash would be swept away by the ocean's currents and making the tuff unable to harden because there isn't enough of it.
Works Cited
https://www.ucl.ac.uk/earth-sciences/impact/geology/london/glossary/rocks/igneous/basalt
http://maurice.strahlen.org/Kamchatka/eruptive_rocks.htm
http://s17.photobucket.com/user/Michiezz826/media/Igneous%20Rocks/VolcanicTuff.jpg.html
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1. Ancient Rumblings on the first line of your email.
2. How many millions of years old are these rocks?
3. How thick is this volcanic outcrop?
4. Using the plaque to help you, find one of each of Basalt, Basaltic Andesite, and Tuff. Describe the textures and colors for each rock.
5. Which rocks, in order of most to least, have crystals AND why do you think some have more than others?
6. Based on your answers above, which rocks formed on land AND which ones formed underwater? Explain how you came to your conclusion.