TJ Howell Botanical Drive
Get the most out of the tour
The TJ Howell Botanical Tour takes you through plant communities that are influenced by fire, changes in elevation, and other factors but none these have an influence on plant communities equal to the unusual geology seen on this tour. The following summary will explain (Note: numbers in parentheses correspond to the source listed on the reference page).
Island Arc and Back-arc Basins
Most people are familiar with the geologic process of an ocean crust being pushed under the edge of a continent where it melts to form a chain of volcanoes. All of western Oregon sits on one of these and the Cascade Mountains are the result of the melting ocean crust.
The ocean crust being pushed under Oregon is relatively young and the process of them melting and forming the Cascade volcanoes follows the process most of us are familiar with. However, ancient ocean crusts that have been under water for millions of years have a very different process. When these ancient crusts are pushed under the continental edge (subducted) they melt in two separate phases. The first phase forms a typical chain of volcanoes – like the Cascade volcanoes – while the second occurs further inland resulting in a separate phase of volcanic activity.
These two phases of melting are very different in the way they erupt. The first tends to melt and create isolated volcanoes that are much like Mount Shasta, Mount McLaughlin, and Mt Rainier. The second phase tends to erupt in long cracks that might be thousands of feet long. These are called rift volcanoes and often result in the growth of new crust as younger eruptions push the deposits of older eruptions aside and subsequent eruptions push aside the ones that preceded them.
The composition of these two phases of melting are also very different. The volcanic activity that forms the first phase has more glass (silica) while the second has a higher level of iron and is, hence, heavier.
Over the span of millions of years, growth of new crust by the second phase eruptions begins pushing the first phase away from the continental edge and the heavier weight of the second phase lava makes it sink lower than the surrounding terrain. As this new crust grows and pushes the other volcanoes away from the continent, it will eventually open a way for the main ocean to pour in and create a small ocean. A chain of volcanic islands is created that continues to be pushed further and further from the continental edge.
Islands that are created this way tend to be arranged in an arc-shape with a small basin between them and the continent. For this reason, they are called island-arcs and back-arc basins. If you look along the eastern edge of the Pacific Ocean, you can see the result of this two-phase process in a series of arc-shaped island chains adjacent to a deep trench where the ocean crust is being pushed under them.
In this part of Oregon, a similar event happened beginning about 167 million years ago with the development of a small chain of volcanic islands that were pushed out to sea approximately 15 miles over the span of about 12 million years. In geologic literature this island chain is called the Chetco Islands.
The development of the islands and the small ocean basin that pushed them out to sea ended about 155 million years ago when the Atlantic Ocean began to open and pushed the American Continent westward where it bulldozed the basin and islands back into the edge of the continent.
The TJ Howell Botanical Tour starts in rocks that were once part of the molten mantle under the ocean basin and follows a road that climbs up through the continental crust where it ends at what was likely the inner edge of the fragment of continental crust that migrated out to sea with the Chetco Islands. This tour won’t make it to the islands but the general area can be seen from the Babyfoot Lake parking lot.
Oregon’s largest mantle rock outcrop
The beginning of the tour takes you through a small segment of what is definitely the largest mantle rock outcrop in Oregon and possibly the largest in North America. The red line on the illustration below shows where you are in relation to the rest of the outcrop.
After the first half mile, the tour goes through some relatively unaltered mantle rock (peridiotite). If you look at the slope above the road you can see many outcrops of rock. As you continue driving you might notice that this rocky surface changes to less rocky with a noticeable change in vegetation. This change is caused by a transition from relatively unaltered mantle rock to mantle rock that has been altered to serpentine. One explanation for this might be a difference in age of the unaltered rock and the rocks that have been altered to serpentine. The serpentine mantle rocks are likely associated with the 200 million year old continental crust that was pushed out to sea with its chain of volcanic islands. These rocks were likely already altered to serpentine before that happened. Younger mantle rock filled in under the new ocean basin as the continent was pushed aside. This younger rock is about 160 million year old and is most likely the source of the rocky hill side you see at the beginning of the tour.
Serpentine rock migration
If you pick up a piece of serpentine rock and rub your fingers over it, you will notice a slippery, greasy feeling if you rub your thumb and fingers together. This is caused by natural talc in the rock and this slippery characteristic gives these rocks the ability to migrate over long distance between other rocks. As these rocks migrate, their sides get polished by friction with the other rocks around them. When they appear on the surface, the glass-like, highly polished appearance looks a lot like obsidian. This same slippery characteristic is also one of the reasons why exposures of serpentine rock is prone to land sliding.
Mantle rock can be very unstable on steep slopes, especially the rocks that have been altered to serpentine. In some cases, parts of the slope will slide but it is not uncommon for an entire side of the mountain to slowly slide downward as a single, massive block. Sometimes, the tops of these massive slides tilt back against the mountain and create a small depression that fills with water to create a small pond. These are called sag ponds. There is a good example of this about a fourth of a mile above the road but it takes a cross-country hike on steep terrain to get to it. Another example is a depression seen next to the road about four miles into the tour and is pointed out in the guide. This depression is too porous to hold water but will give you an idea of what these depressions look like.
Landslides on serpentine rock may move at very slow paces that might take months or years to move a few feet down the slope. These slow moving masses of earth are referred to as “soil creep” rather than landslides. As soil moves it typically piles up along the leading edge creating an abrupt rise called the “toe” of the slide. Trees are often pushed over by the moving earth causing them to lean in a down hill direction. Vegetation including shrubs and grass might ride downhill with the moving land leaving behind exposures of barren soil that enables pioneer species of plants to occupy the mountain side where competition from other plants would normally make this difficult.
Mantle rock contains nickel and under the right conditions, the decomposition of mantle rock can create ore deposits that are economically worthwhile to mine. These nickel deposits develop over long periods of time as mantle rock is decomposed by the natural acids in rain water and distinctive layers develop. A fully developed nickel ore body is called a laterite. The nickel laterites in this region developed more than five million years ago before regional uplifting and erosion removed everything but a trace on a few mountain tops in this region, including Eight Dollar Mountain. The reason an example of this mountain-top deposit is next to the road at the foot of the mountain is likely because a large block of the mountain slowly moved down over the span of thousands of years bringing this example of laterite to the foot of the mountain where we see it in a road cut.
Amphibolite is made up of black and white crystals that gives these rocks a salt-and-pepper appearance. This type of rock is created by high pressure and temperature that causes the minerals in sedimentary or volcanic rock to recrystallize into a new type of rock (amphibolites), which generally does not have any resemblance to the original rock. Most of the crustal rock that migrated with the volcanic island chain has been altered to amphibolite, which is also called black granite.
You can only imagine the earthquakes that might have been created with the opening of an ocean basin that pushed a large chunk of continental crust and volcanoes out to sea against a much larger ocean crust going in the opposite direction. As this basin formed, these earthquakes no doubt shook rocks from the continent that rolled into the edge of the basin creating a pile of rocky rubble. This rubble pile is called an olistostrome and makes up much of what is seen along upper part of the tour before arriving at Babyfoot Lake.
Chert is created from the shells of microscopic organisms called radiolaria. All of the chert found in this area are part of the ancient continental crust that was pushed away from the continent with the volcanic islands. The chert developed from radiolaria that lived in a separate and older ocean basin about 200 million years ago. These chert deposits become common in road cuts beginning about half way through the tour.
About half way on the tour, the road passes a deposit of beach sand perched at an elevation of 3,500 feet on the side of Fiddler Mountain. This is a small remnant of an intertidal zone that was under water at high tide and above water at low tide. Fossils found in the same deposits on mountains near Crescent City show that the intertidal flats were slowly uplifted and a forest moved in and grew on top. For that reason, the fossil beds have sea shells in the lower part on plant fossils in the upper part. These old beach sands are between 5-10 million years old and were uplifed from below sea level to where they are found at an elevation of 3,500 during the past five millions years. The uplift is caused as ocean sediments are scraped off the ocean crust at a subduction zone and, as these sediments accumulate, they push up the edge of the continent.
The regional uplift that created the mountainous terrain seen along the botanical drive happened in episodes of relatively fast uplift followed by long periods of stability. When a rapid episode of uplift happens, erosion increases and rivers form narrow, steep-sided canyons. During periods of stability, there is less down cutting in the river but the steep side of the canyons experience increased erosion that result in the development of a wide basin with a relatively flat bottom. The next episode of uplift causes another steep-sided canon to form in the bottom of the basin, which leaves behind a remnant of the basin perched above the bottom of the canyon. As the episodes of uplift and stability continue, a series of terraces from previous periods of stability might be preserved above the bottom of the canyon. The tour will pass through two of these terraces as the road climbs above Josephine Creek.
RGB November 21, 2016