Viscosity:
After magma rises from the mantle and passes through the crust, its behavior depends upon the viscosity (thickness) of the molten rock. High viscosity lavas flow slowly and typically cover small areas, creating stratovolcanos such as Mt St Helens. Explosive eruptions occur where cooler, more viscous magmas such as those that create andesite, reach the surface. Dissolved gases which are unable to escape the thicker lava, build up pressure until explosions occur, blasting lava and rock fragments into the air. Lava flows formed from this type of thick and sticky lava do not flow downhill as easily and tend to build steeper sloped volcanoes.
Non-viscous (runny) magma produce volcanoes characterised by effusive eruptions. Lava flows generated by effusive eruptions pour large amounts of lava onto the surface of the Earth. These types of flows vary in shape, length, thickness and width depending on the type of lava erupted, slope of the ground traveled upon and duration of eruption. Effusive eruptions occur when hot (1200°C) low-viscosity magmas reach the surface of the Earth's crust. This type of magma allows for dissolved gases to escape more easily as eruption occurs, creating lava that flows downhill quite quickly and easily. Effusive eruptions build up gently-sloping volcanoes such as the ones in Hawaii.
Vesicles:
A Vesicular texture is characterized by a rock being pitted with many cavities (vesicles) on its surface as well as in its interior. The texture is often found in extrusive aphanitic, or glassy, igneous rock. Aphanitic describes rocks which are so fine-grained that their component mineral crystals are not visible with the unaided eye. Phaneritic describes rocks that contain minerals that "are" visible to the unaided eye. Vesicles form during the extrusion of magma upon the surface of the Earth as pressure decreases. Dissolved gases within the magma come out of solution, forming gas bubbles. As the magma cools and begins to solidify, these gas bubbles are preserved within as vesicles. Rock types that display a vesicular texture include, but not limited to, pumice and scoria.
Amygdaloidal:
Amygdaloidal is a term that describes a volcanic rock, usually basalt or andesite, which has vesicles that have become filled with secondary minerals, such as calcite, quartz or chalcedony. Individual cavity fillings are termed amygdules. Sometimes these can be sources of semi-precious stones such as agate or jasper. Amygdules form when fluids containing dissolved minerals flow through the rocks and deposit the minerals as solids within the vesicles. Lunar basalts are not amygdaloidal because the Moon contains no fluids (and never has).
Tephra:
Tephra is the term used to describe solid or molten rock fragments of any size ejected from a volcano. The smallest fragments (less than .002 inch in diameter) are considered ash. Mid-sized fragments (up to 2 1/2 inches in diameter) are called lapilli. Anything ejected from a volcano that is larger than 2 1/2 inches in diameter is called a block if it was ejected as a solid and a volcanic bomb if it was ejected as a liquid or semi-solid form.
The Iron King Volcanics, which overlie the Spud Mountain Volcanics, are the youngest formation in the Big Bug Group. These Volcanics consist mainly of basaltic flows that contain pillow structures.
Use the rock found just behind and to the left of the information kiosk for answering the questions. Email your answers to the questions, to me, using the link in my profile only. If your answers are not received by me, your log will be deleted. Photos are accepted and appreciated as long as the answers are not pictured. You do not have to wait for confirmation from me before logging this cache as completed. Most of all……learn……and enjoy the view.
1. Are vesicles present in the rock?
2. Was this rock formed from viscous or non-viscous magma?
3. Is this rock phaneritic or aphanitic?
4. Does this rock contain amygdules?
5. What is the size of this piece of tephra? (technical term
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