Requirements to Log This Earthcache:
In your message with the answers, list all members of you group (only one set of answers need to be sent)
You should be near the informational area of this rest stop. All around this area you will find various large rocks along the side of the walking path. Many of these rocks will be covered in pits, known as vesicles, these are the rocks you are looking for. Find one you like and answer your questions based on observations of that rock (you’ll have plenty to choose from).
QUESTIONS:
- Describe the rock you are making observations of, shape, size, color, texture (smooth, soft, course, rough, etc.)
- Look at the vesicles in the rock, locate the largest vesicle you can see. How large is this vesicle?
- How do you suspect the vesicles formed in these rocks?
- At you chosen observation rock, take a picture with your GPS device, a selfie, a personal item, or more ideally a scrap of paper with your Geocaching name. Just get a pic from the location.
LESSON:
Igneous Rocks in Arizona
Arizona’s oldest volcanic rocks are 1.8 billion years old. Some of Arizona’s most spectacular geologic features and scenery are volcanic in nature, e.g., Flagstaff’s San Francisco Peaks. The result of millions of years of volcanic activity leaves a landscape rich in mineral and volcanic igneous rock. Arizona has a rich geology where a wide variety of different types of stones can be found. It is home to igneous stones, sedimentary stones, and refashioned metamorphic stones with stones dating from the Holocene, all the way back to the Jurassic. Basalt stones are found in Arizona and hard to the touch, and gray to black in appearance. Granite stones can be found in Arizona composed of quartz, feldspar, mica, and hornblende. Diorite, an intrusive igneous formed of plagioclase feldspar, amphibole, and pyroxene. Other igneous stones that can be found Arizona include rhyolite, pumice, and Obsidian.
Today you will be looking at some igneous rocks off of I-17 between Phoenix and Flagstaff, in the transition zone on the map, between the Colorado Plateau and the Range and Basin.
Igneous Rocks – Definition & Formation
Igneous rocks are “fire-born,” meaning that they are formed from the cooling and solidification of molten (melted) rock. The word igneous derives from ignis, the Latin word for “fire.”
The igneous rocks are one of the three main types of rock found on earth. They are also called the ‘new’ rocks or magmatic rocks. Sometimes they are referred to as parent rock because all other rock types are formed from them.
Igneous rock is formed when liquid rock freezes into a solid rock, the process of cooling and solidification of hot molten magma. It is this magma that becomes the source for volcanoes and igneous rocks. When hot, molten magma at 600 to 1,300 °C (1,100 to 2,400 °F) cools and crystallizes at the earth’s surface or inside the crust, they solidify into igneous rock. Lava cools quickly on the surface of the earth. These are known as fine-grained extrusive, or volcanic, igneous rocks. Extrusive rocks are often vesicular, filled with holes from escaping gas bubbles.
Volcanic rocks often have holes or cavities in them because of the way they are formed. Igneous rocks like pumice have holes because the maga/lava cools quickly, trapping air and gas bubbles inside. That's what those holes actually are - bubbles trapped in rock. This usually happens during explosive eruptions, when the force of the eruption aerates the lava, making it frothy and bubbly.
All magmas contain gases dissolved in solution called volatiles. As the magma rises to the surface, the drop in pressure causes the dissolved volatiles to come bubbling out of solution. The gas bubbles become trapped in the solidifying lava to create a vesicular texture, with the holes specifically called vesicles.
When magma (molten rock beneath the Earth's surface) rises to the surface and erupts as lava, it contains dissolved gases like water vapor, carbon dioxide, and sulfur dioxide. As the lava reaches the Earth's surface and pressure is released, these gases begin to escape from the magma. If the lava cools quickly enough, these gas bubbles become trapped in the solidifying rock, forming vesicles.
The size and abundance of vesicles in volcanic rocks can vary depending on factors such as the viscosity of the magma, the rate of cooling, and the gas content of the magma. For example, basaltic lavas, which are more fluid and have a lower viscosity, tend to have more abundant and larger vesicles compared to andesitic or rhyolitic lavas, which are more viscous.
Over time, these vesicles can be filled with secondary minerals like quartz, calcite, or zeolites, forming what is known as amygdaloidal texture in the rock. These holes and vesicles give volcanic rocks a distinctive appearance and can provide valuable information about the conditions under which the rock formed.
Example of Basalt with vesicle features. Exaple below is from Basalt in the Bonito lava flow. This basalt stone is filled with hollow vesicles. This is similare to some of the vesicle feautres you might observe at ground zero for this earthcache.
SOURCES:
Looking at pictures from the site with a science teacher friend/colleague who helped steer me in the direction of vesicles for questions.
https://www.quora.com/Why-do-volcanic-rocks-have-so-many-holes
https://opengeology.org/textbook/4-igneous-processes-and-volcanoes/
https://www.sciencefacts.net/igneous-rocks.html
https://www.sciencefacts.net/igneous-rocks.html
https://sciencing.com/interesting-quartzite-7320506.html