BEFORE you may log and claim this EarthCache as a find, you must first send a message or email me through my profile your answers to the following questions. Do NOT put answers in your log postings! (Answers to the questions can be found by visiting the EarthCache site and making observations.)
- What type of unconformity can be seen at Martha Falls? Why?
- What direction do the columns left behind by the cooling lava point? Why?
- Optionally post a picture of yourself and/or your GPSr.
*** All sincere efforts to answer the questions and complete the tasks will be accepted. ***
Please note that Stevens Canyon Road is closed to vehicular traffic during the winter months (though still accessible via a long ski trip beginning at Narada Falls). The closure and opening dates are subject to change from year to year depending on the weather. Information regarding specific road closure dates in the park can be found at the National Park Service website.
Mount Rainier
Mount Rainier is a massive stratovolcano located in the Cascade Volcanic Arc. It has been built from lava eruptions estimated to have begun over 840,000 years ago. Each eruption causes lava flows that carve and deposit new layers of Andesite and ash on top of existing canyons, cliffs, and glaciers.
Glaciers
Glaicers are one of Mount Rainier's most dynamic geologic features. Glaciers are always in motion being pulled down by gravity and retreating when they melt. As a result of this motion, the lava deposits on Mount Rainier are constantly being eroded and shaped.
Martha Falls
You are standing in Stevens Canyon, which was once filled with one of these glaciers. About 90,000 years ago, one of Mount Rainier's eruptions sent an intracanyon lava flow into Stevens Canyon. The lava flowed between the existing glacier and Tatoosh range. As the lava cooled, it formed columns that pointed towards the colder surface of the glacier.
The lava flow today rests diagonally on top of the wall of the old canyon. Martha Falls in front of you cascades over this cliff that was formed during this lava flow.
Note the unconformity between the Mount Rainier lava flow and the older Stevens Ridge formation both in the above image and in the valley at GZ. Unconformities are a point of contact between two rock units of different ages. The upper rock unit is usually younger than the lower unit. In the case of Martha Falls, the cliff rock is the younger unit that was deposited by one of Mount Rainier's lava flows. It covered the existing rock unit that was the much older Stevens Ridge.
Since the lower rock unit is usually eroded by the time the upper rock unit is deposited on top, these unconformities or points of contact represent breaks in the geologic record.
There are three main types of unconformities:
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Disconformity An unconformity between parallel layers of upper and lower rock units. This type of unconformity is often hard to recognize and are usually found when fossil records in both rock units are studied. Gaps in these records can indicate a disconformity. |
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Nonconformity An unconformity where the lower unit of rock is igneous or metamorphic. In other words, if the older rock unit below the young rock deposit is igneous, the point of contact is called a nonconformity. |
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Angular An unconformity between an older angled rock unit and a younger rock unit that was deposited on top. This type of unconformity is much more obvious than a disconformity since the rock units are not parallel with each other. |
Please be respectful of the environment. All answers can be found at the posted coordinates.
Sources: Wikipedia, Imagine a Glacier interpretive sign, Geology of Mount Rainier National Park Washington (geological survey professional paper 444).