Earth Science Themed Essays
The rock specimens that I collected were found along a 15-mile stretch of road north of Huntington, Oregon, on the west side of the snake river. They all came from the Huntington Formation in the Izee and Olds Ferry terraines, both of which are in the Blue Mountains Region.
Both the Izee and the Olds Ferry terraines, along with nearby Wallowa and Baker terraines, all formed as a sequence of volcanic islands during the Late Triassic, although the Izee terraine is a sedimentary basin created primarily from the erosion of the other three terraines. Their accretion to the North American front can be explained through a hypothesized series of events.
Approximately 370 to 270 million years ago (MYA), between the Late Devonian and Early Permian, the Wallowa terraine formed at an ancient subduction boundary west of the North American craton. Based on current paleomagnetic data, the island arc began its accretion onto the North American western front at about 11° to 19° North between 270 and 260 MYA. This process continued for several million years, until the first third of the Mesozoic, when the subduction zone beneath the island arc shifted, leaving the volcanoes forming the Baker terraine lying dormant and creating the new volcanoes of the Olds Ferry terraine. On the intra-arc region between the Olds Ferry terraine and the Baker terraine formed the sedimentary deposits composing the Izee terraine. Also about that time, 190-155 MYA, the Blue Mountains island arc, as it has come to be called, and the North American continent begin moving northward together. Finally, about 155-115 MYA, from the beginning of the Kimmeridgian of the Jurassic to midway through the Aptian of the Cretaceous, the entire Blue Mountains island arc collided with western North America. During the Tertiary, 65-5.3 MYA, the new front was covered by other volcanic rocks, and since the Late Miocene, 6 MYA to the present, has been uplifted and eroded.
Modern-day evidence of this model of continental accretion and subduction zone movement can be seen in the Aleutian Islands of Alaska and their neighboring seamounts.It was during the Late Cretaceous as well that the western end of the continent saw the onset of plutonism; the Idaho batholith is one of these types of intrusive deposits prominent in this area. The onset of plutonism itself could have been due to the change in crustal plate movement. Since the subduction zone did not just disappear after the entire Blue Mountains arc was accreted, it can be assumed that it went on to form the Cascade volcanic mountain range.
The Olds Ferry terraine formed approximately 226-222 MYA, during the Late Triassic. The terraine mostly consists of volcanic (flow) and volcaniclastic (pyroclastic and epiclastic) rocks, although only volcanic (flow) rock specimens were collected on my expedition. In addition, siliceous plutonic bodies occur locally. The volcanic (flow) rocks range in composition from basalt to rhyolite.The Izee terraine, as stated earlier, is composed of clastic sedimentary rocks of Jurassic age. The sedimentary rocks formed primarily from igneous rocks of the Olds Ferry and Wallowa terraines.
The accretion process caused severe metamorphism, as did the more recent volcanic activity in the region. Therefore, almost all the specimens collected that originally appeared as igneous rocks in the Olds Ferry terraine, and as sedimentary rocks in the Izee terraine, were metamorphosed in one way or another.
The first specimen, a slightly metamorphosed porphyritic basalt, shows oxidization and another form of slight discoloration on the external side, while large white phenocrysts are visible on the inward side. The phenocrysts, which are quartz and plagioclase crystals, range in size from 1 millimeter in diameter to microscopic grains of an indeterminable size. Their presence indicates two-stage cooling in the original rock. The basalts of the Huntington Formation are distinctly fine grained, with an aphanitic texture. Grains of interbedded fossilferous marine sediment on specimens collected from the Huntington Formation by other parties indicate that these lavas were erupted in a submarine environment.
The second rock is an aphanitic-textured basalt porphyry. It has mostly phenocrysts of quartz and plagioclase, but a few unknown types of minerals are also present. The largest phenocryst is 4 millimeters in width, while the smallest are no larger than the surrounding matrix itself. This specimen is an extrusive deposit covering the Huntington Formation, coming from the well-known Columbia River basalts. The date of intrusion is somewhere in the Miocene, approximately 23 to 5.3 MYA. This specimen also shows signs of severe mechanical weathering. Judging by the way the outcrop that the specimen came from was cracked, and by the fact that local weather conditions make for frequent freeze/thaw, ice wedging is the most obvious agent.
The third specimen collected is a weakly contact metamorphosed rhyolite. Much of the parent rock is still visible in this sample in the form of phenocrysts. The majority of phenocrysts are only about 5 millimeters in size. The texture is fine-grained crystalline. The rock is nonfoliated, with patches of biotite mica and potassium feldspar present. Slight oxidization on the outer edge provides evidence of chemical weathering.
The fourth rock is an unaltered fragment of porphyritic basalt, identical in all respects to the first specimen. The basalt contains relatively few phenocrysts that range in size from 1 millimeter to 3 millimeters. As with the earlier specimen, the phenocrysts are quartz and plagioclase.
The fifth sample of rock is a plutonic granite. It is one of the siliceous plutonic bodies that invaded the Olds Ferry terraine between the end of the Jurassic and the beginning of the Cretaceous periods. The specimen has a rough, phaneritic texture, coarsely crystalline and indicating the slowest rate of cooling. Large grains of pink potassium feldspar, along with smaller, darker grains of mica and some amphibole are visible.
The sixth specimen, which is also one of the most interesting, is a pyroclastic tuff. It obviously came from a felsic to intermediate eruption, the latter being the more likely cause. The ash and debris would have to have traveled a considerable distance, as the site of discovery is located approximately 240 miles from the nearest volcano, Mt. Jefferson. The rock is finely grained and flakes off at the touch. There are several pieces of pyroclastic debris visible, the largest 1.5 centimeters in diameter. After settling, this layer of ash was covered with sediment, and then hardened. Since radiometric dating was not a realistic option for this writer, it is impossible to give an estimate as to the time of arrival. My best hypothesis is that it came from one of the Tertiary eruptions that blanketed the area before the last ice age.
While conducting my rock hunt, I repeatedly observed a series of quartz bands running through the exposed Huntington Formation basalts. Extracting one, though, was difficult. I finally succeeded in finding one weathered enough to remove. The bands are a post-arrival addition to the basalts. They are hydrothermal and formed when superheated water, saturated with silica, flowed through the cracks in the basalts and rhyolites caused by the regional metamorphism. Then, in a geologic blink that lasted approximately 10 to 20 thousand years, the western coast of North America expanded far enough, the subducting plate feeding the hot water moved farther and farther out, and the water quit flowing. The silica that had been collecting in the veins finally hardened into what it is today. The texture of the quartz is a typically fine-grained crystalline. Shades of color in the quartz vary, but white, clear, and light gray are its primary colors.
After journeying approximately seven miles down the road, I left the exposure of the Olds Ferry terraine and began encountering the masses of sedimentary rocks present in the Izee terraine. The first, and most common, was a rough, terrigenous breccia composed primarily of angular chunks of the metamorphic and volcanic rocks previously encountered. The Olds Ferry rhyolite appears in the form of several pieces each 2 centimeters long by 1 centimeter wide. There is one grain made of granite approximately 3 centimeters long by 1.5 centimeters wide. The grains of rhyolite and granite, along with an unknown and extremely oxidized mineral, surround a much larger matrix of unaltered basalt from either the Baker terraine or the Olds Ferry terraine. Slight banding in some of the minerals is evidence of the metamorphism caused by the arrival to the front.
The next specimen collected in the Izee terraine arrived at an unknown time. It is an arkosic siltstone, slightly metamorphosed and featuring a series of thin quartzite bands. The unmetamorphosed particles are less than 0.06 millimeters in size and very smooth. The round grains provide insight that this rock formed as a result of the depositing of terrigenous sediments. This further complicates the matter of deciding on the origin of the stone. The silicate sand grain could have formed early on, weathered from the Blue Mountains island arc's rhyolites, or at a later date, brought in by the Snake River that flows close by. It is this writer's opinion that this rock had indeed formed elsewhere. I base this on the fact that this rock specimen is rounded in a way that points to a long period of time spent in a stream or river, and that no other specimens of an arkosic siltstone have been documented in the Izee terraine. I believe it was transported to the terraine by the Snake River after the Blue Mountains region formed.
The explanation for the metamorphosed quartzite bands in that scenario could stem from one of two places: either from exposure to the formation of the Idaho batholith, or, more likely, exposure to the formation of the Snake River plain.
The tenth specimen is a cataclasite that originally formed at a normal fault in a layer of metabasalt in the Huntington Formation. When discovered, though, the hanging wall had slid far enough down that the footwall metabasalt was grinding against an unknown rock type, igneous in origin. The cataclasite is extremely flaky and shows over 17 different layers, most of which display oxidization and hydrothermal buildup. The fault that the cataclasite was found in ran for a considerable distance before disappearing back into the hillside. The fact that the rock was eroding more rapidly than the surrounding igneous rocks suggests that it does not weather well and was only recently exposed.
The area in which I collected my samples is very rich in geologic history yet is undocumented and lacks detailed geologic maps. The rock types found within are mostly igneous types that would be formed in an oceanic environment, along with sedimentary rocks that formed as other igneous rocks weathered. Most of these rocks were then metamorphosed as they accreted to the western front of the North American continent and exposed to the extreme volcanic activity that ensued. Some are post-arrival intrusive deposits, either the igneous plutons that caused the metamorphism, the sedimentary rocks moved possibly hundreds of miles by the Snake River, or ash deposits from the many active volcanoes in the region.
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Less than 1 period
Supplement a study of Earth science with an activity drawn from this winning student essay.
As a class, go on a field trip to collect rock specimens common to your area.
Divide the class into small groups, assigning one of the specimens you collected to each group.
Challenge students to research the geologic history of the specimens and then report their findings to the class.