Question answered by Dan Moore, Ph.D., Professor of Geology, BYU- Idaho
The Menan Buttes are tuff cones, a type of volcano that forms when hot, basaltic magma interacts with water near Earth’s surface.
Basaltic magma, produced in Earth’s mantle, travels towards the surface through magma-filled cracks called dikes. As the magma in a dike nears the surface, it can interact with water. Usually, heat from the magma warms surrounding rocks that vaporize water before it nears the magma. However, water reaches the magma where it is abundant and flows easily. Water and magma interact explosively. Heat from the magma causes the water to vaporize.
These explosions break the magma into BB-sized pieces called ash and throw these fragments into the air. Accumulating ash from an ongoing eruption forms a growing cone. The ash in the cone solidifies to form a rock called a tuff. Thus, the Menan Buttes are a tuff cone.
The sediments beneath the Menan Buttes consist of gravel deposited by the Snake and Henrys Fork rivers. Water fills the large pore spaces in these gravel deposits. The Menan Buttes are part of a string of at least six volcanoes, including the Annis Buttes (southeast) and Sand Hill (northwest). These volcanoes formed when magma traveling through a dike reached the water in the gravel deposits. Each volcano formed at a different location along the dike.
Today, visitors to the Buttes observe volcanic cones composed of tuff (welded ash). In places, the tuff contains rounded river cobbles and angular pieces of lava flow from the underlying deposits.
Tephra and Tuff
When a volcano erupts it will sometimes eject material into the atmosphere. This material is known as tephra. As long as the tephra lies loosely in cones or layers on the ground, it is still called tephra.
Once tephra consolidates into a more or less firm rock, it is called tuff. As Dr. Moore stated above, the Menan Buttes are tuff cones that were formed when tephra (ash) was thrown into the air and welded together.
As the aerial image below shows, the Menan Buttes are not perfectly circular. The cones are stretched to the northeast, a sign that winds blew consistently from the southwest throughout the eruption.
This experiment will illustrate how the stretched cones were formed.
Materials: A fan, plastic tablecloth, masking tape, marker, spoon, 1 cup of each tephra sample (cocoa, cornmeal, oatmeal, and rice), yard stick.
- Lay table cloth or paper flat across your work area. Tape the corners down, if necessary.
- Place the fan at one end of the tablecloth.
- Turn on the fan to the low setting. Use a spoon to gently sprinkle one ingredient at a time in front of the fan, as if releasing it into the wind. Place your hand to the same position relative to the fan each time.
- Turn off the fan between releases of each ingredient.
- Use a washable marker to outline the area of the deposit where the majority of the ingredients fell.
- Measure the farthest distance traveled by each ingredient.
- Repeat until all tephra samples have been used.
Once ejected into the air, the wind carries tephra particles. How far a tephra particle travels depends upon wind speed and size of the particles. Coarse and heavy particles fall on or near the volcano; fine-grained, lightweight particles travel farther.
At high wind speeds the tephra layer is long and narrow. At lower wind speeds the tephra layer is shorter and wider. When there is no wind, the tephra deposit may be circular around the vent.
You can repeat the experiment with the fan on medium and high speeds. How fast do you think the wind was blowing during the Menan Buttes eruption?