GEOLOGY OF THE WICHITA MOUNTAINS
Based on current scientific theory, around 550 to 600 million years ago, this entire area was under water after the land collapsed into a basin and was flooded by the sea. The floor of the basin was thickly covered in sediment washed into the sea. Hot lava formed under this sedimentary rock and was forced upward into it. Because the lava was insulated by the layers of rock above it, it cooled slowly, forming the dark, finely grained igneous rock called gabbro.
Around 50 million years later, another intrusive sheet of hot granite lava formed under the surface and was forced up through the older gabbro. Although some of the lava reached the surface through vents, much of it cooled below the surface as well, though not as slowly as the gabbro. The granite it formed, called Mount Scott granite, is the most extensively found rock in the Wichita Mountains.
Mount Scott granite is a leucogranite, meaning it is lightly colored and contains few dark specks of materials. The rock mostly consists of microperthite (an alkali type of feldspar, the rock that makes up over 60% of the earth's crust) and contains low amounts of iron and magnesium. Geologists believe that, around 515 to 425 million years ago, the land subsided or sank under sea water, and the sheet of Mount Scott granite was then buried under sediment. Then, around 300 million years ago, there was a massive uplift, and the land was raised up around 20,000 feet. This produced mountains much larger than you can see today.
The limestone that formed from the sediment that covered Mount Scott weathered and eroded away around 250 million years ago, but you can still see remnants in the area: the Slick Hills, the ridge to the north where the wind farm is located, consists of this limestone. The Mount Scott granite also weathered and eroded, and as rocks were stripped off the mountains, they filled in the land around it. Enough of these gravel deposits eroded away that the Wichita Mountains were exposed again.
WEATHERING
The splitting and rounding of the granite around you was caused by a process called weathering. There are three types of weathering: chemical, mechanical, and biological. This earthcache will focus on the last two; to find out more about chemical weathering, stop by our companion earthcache nearby, GC4XGZ8, Cobblestones of Medicine Park.
MECHANICAL WEATHERING
Unlike chemical weathering, which changes the makeup of the rock material itself, physical weathering doesn't change the rock, it just breaks it apart. There are many different processes that can cause physical weathering, including frost wedging, differential expansion and contraction, and unloading.
- FROST WEDGING happens when water seeps into cracks in the rock and then freezes. As water becomes ice, its volume increases by 8 to 11%. This can create up to 30,000 pounds per square inch of pressure and further split open cracks in the rock.
- DIFFERENTIAL EXPANSION is the result of hot/cold cycles. Simply put, rocks expand when they heat up and contract when they cool down. Different minerals in the rock expand and contract at different rates, and this can cause cracks. Fires and lightning can speed up this process from the rapid temperature changes they cause. This is more common in desert environments, where the difference between daytime and nighttime temperatures can be extreme.
- UNLOADING happens when rocks are buried deep below the surface, compressed by the weight of the layers above, and then those layers are removed. This lets the rock expand, and parallel cracks form called joints. The layers of rock then start gradually peeling away like an onion. If the cracks develop vertically, it's called exfoliation; if they develop horizontally, it's called sheeting.
BIOLOGICAL WEATHERING
As the name suggests, biological weathering is done by living creatures, not by geologic processes. Trees can cause weathering -- tree roots can grow into cracks in the rock and further split them apart. Animals can also cause weathering by walking on rock surfaces and wearing them away.
So can lichen. Although it looks like one organism to the naked eye, lichen is fungi and algae, living together in a symbiotic relationship. The fungi release chemicals that break down rock minerals; the minerals thus released from rock are consumed by the algae. As this process continues, holes and gaps continue to develop on the rock, exposing the rock further to physical and chemical weathering.
LOGGING THIS EARTHCACHE
To log this earthcache, email us or send us a message and copy and paste these questions, along with your answers. Please do not post the answers in your log, even if encrypted. There's no need to wait for confirmation from us before you log, but we will email you back if you include your email address in the message. Group answers are fine; just let us know who was with you.
1. The name of this earthcache: Breaking Down Mount Scott
2. Based on your reading and observations, what type or types of mechanical weathering (listed above) affected the granite rocks here?
3. What evidence do you see of biological weathering?
Photos of your visit are not required but always appreciated.
SOURCES
Oklahoma Geological Survey, Geology of the Eastern Wichita Mountains, Southwestern Oklahoma.
Weathering - Dr. John Douglass, Paradise Valley Community College
Rock Weathering - Dr. Lanbo Liu, University of Connecticut
Biological Weathering, Prof. Martha Mamo, University of Nebraska-Lincoln
Fish and Wildlife Service, Geology of the Wichita Mountains
US Geologic Survey, America's Volcanic Past (source no longer available online)
This earthcache was placed with permission of the Wichita Mountains Wildlife Refuge.