As you travel through Thompson Park, you will see granite boulders, blocks and spires that are the result of mountain building and erosion. Around 70-80 million years ago, buoyant magna (molten rock) rose up through the earth’s crust during formation of the Rocky Mountains. Some of the magna reached the surface of the earth and formed volcanoes, and some of it remained below the surface where it cooled and solidified. The large mass the solidified below the surface in this area is known as the Boulder Batholith. The word batholith comes from the Greek words ‘bathos,’ which means depth, and ‘lithos,’ which means rock. Uplift and erosion over millions of years exposed the granite rock you see around you.
The Boulder Batholith extends from south of Butte to Helena and was named for the area’s prominent boulders. These boulders are primarily the result of weathering. During the mountain-building process, the batholith creates many horizontal and vertical fractures. Ground water in the subsurface moves into the fractures, and the rock begins to decompose into loose fragments.
The rock decomposes more rapidly where fractures intersect, giving the rock a spheroidal shape. When decomposing rock is exposed at the surface, wind, rain, and snow remove loose fragments, and leave the rounded boulders. Where the rock is less weathered, the fractures and rounded corners that begin the process of boulder formation are visible in the outcrop.
Weathering is any reaction between the rock surface and the atmosphere. The idea of surface has to do with chemistry, in that all reactions involving solids are surface reactions; the center of the particle is not involved. But as the surface is attacked and removed, eventually, the center is exposed and subject to reaction. The idea of atmosphere is that agents--gases--are responsible for the destruction of rocks. The idea of atmosphere also implies that the processes of weathering can go on anywhere the atmosphere can penetrate, including underground caves. Thus, the surface of a rock is any surface that comes into contact with the atmosphere, above or below ground.
A second definition states that weathering is any process by which rocks either disintegrate or decompose. Disintegration implies nothing more than a physical reduction in particle size, giving rise to the terms physical or mechanical weathering. Decomposition implies a chemical change that can be total or partial. The only requirement of chemical weathering is that the products are different from the reactants.
What are the agents of physical weathering? The primary agent of physical weathering is the freezing and thawing of water, the process called frost wedging. The 10% expansion as water is converted to ice exerts enormous pressures on any containment. Should that containment be a crack in a rock, the rock will eventually fail after repeated episodes of freezing and thawing. Frost wedging takes place primarily in temperate climates.
Growing plant roots are also effective agents of physical weathering because they grow within cracks and wedge the cracks open as they increase in diameter. In addition, plants are chemically active and thus are agents of chemical weathering. Another effective process is efforescence. This results in the exfoliation of a rock surface through the growth of soluble crystals precipitated from groundwater just beneath the rock surface. Such a process is most effective in desert regions or in outcrops that face the Sun. As groundwater evaporation continues just below the surface and more salts precipitate, the growth pressure exerted by the minerals causes a crack to form and widen parallel to the rock surface. Eventually, portions of the rock will flake off and fall to become part of the talus below.
Another process, exfoliation, can be seen at work in geological features called batholiths. Batholiths are huge masses of intrusive igneous (or
metamorphic) rock created under great pressure by magma solidified deep in the Earth’s crust. The exposure of these masses by erosion results in the development of fractures parallel to the surface of the rock body in a process called unloading, as the rock responds to the decrease in overlying weight. The fractures can then be widened by frost wedging or the growth of plant roots. With time, the outer layers of rock split off in layers that are, in some cases, sizable in thickness.
Once thought to be an effective agent of physical weathering, cyclic expansion and contraction at mineral-to-mineral contacts by daily changes
in temperature are no longer thought to be capable of physically disrupting rock. However, a rapid change, such as cold rainwater falling on a rock
heated by a fire or a lightning strike, could cause a rock to fracture. Although burrowing animals are not capable of directly attacking rocks, their burrows promote weathering by allowing the penetration of the atmosphere below the surface.
To log this Earthcache please e-mail the answers to the following questions. (Please do not post answers in your log.)
1. In front of you is a large outcrop, how do you think water eroded this group of rocks?
2. How do you think weathering formed the large boulders?
Optional: Post a picture of you with the boulders