GEOB
Read the Lab 2 Tapestry page, then work through the following questions on volcanism in Mount Rainier and the Tahoma watershed. After completing this lab, you should be able to:
· Summarize the tectonic processes (plates and directions of motion) that generate the Cascade volcanic arc
· Outline how lava flows have contributed to the Tahoma watershed topography
· Explain why lahars are the major geological hazard of Mt. Rainier
· Describe what hydrothermal alteration is and how it contributes to lahars
Q1
Using Fig. 1, describe in two or three sentences the tectonic processes responsible for the Cascade Volcanic Arc. Be sure to reference the relevant tectonic plates and their directions of motion.
Q2
Magma composition dictates the shape and eruptive style of volcanoes. The cascade volcanoes depicted in Fig. 1 are primarily of intermediate composition, so their eruptions tend to be explosive and lavas do not flow very readily: Select all of the evidence visible on the Cascade volcanoes in Fig. 1 that supports these statements.
Lightly-coloured parent material
Lava domes and calderas
Dark-coloured parent material
Summit glaciers separated by exposed ridges
Flanks dissected by fluvial channels
Relatively steep slopes
Q3
Judging from Fig. 2(b), approximately how thick are individual strata (in meters) on average at Willis Wall below "Gun Sight", near the summit of Mount Rainier?
Q4
The exposed strata on Willis Wall below "Gun Sight" on Fig. 2(b) were deposited by lava flows in the period between 40 ka and 15 ka ago (remember that 1 ka = 1000 years) [1]. Given this time interval and any characteristics you can estimate from Fig. 2(b), what was the average rate of major lava flows occurring down the North flank of Rainier in this time period? You can assume each stratum was formed by one major lava flow. Report your answer as a number of lava flows per ka.
Q5
Figs. 3 and 4 indicate that the rock forming the ridges above Tahoma creek (Emerald Ridge and Success Cleaver) is younger than the bedrock in the valley bottom where Tahoma creek flows today. This scenario is surprising since the ridges at Tahoma watershed are formed by lava flows, and we expect flowing lava to take the path of steepest descent to the valley bottom -- not to flow along the ridges. We expect the oldest rock to form the ridges.
Two competing hypotheses have been put forth to explain why ridges are younger than valleys at Tahoma watershed:
1. Rainier's flank was initially a mixture of relatively hard and soft lava flows high above contemporary Tahoma creek. Glaciers preferentially eroded through Rainier's softer lava flows, leaving the more resistant lava flows behind as the contemporary ridges
2. Lava flows from Rainier's summit tracked through thin areas in much larger ancient glaciers, building ridges adjacent to deep valley-filling glacial ice which subsequently melted away
The next several questions will guide you toward the evidence to reject one of these hypotheses.
First, using the topographic and geological maps Figs. 3 and 4 (and Google Earth, if needed), estimate the elevation difference between Emerald Ridge (on the North Tahoma watershed margin) and Tahoma creek. Note that the contour interval in Fig. 4 is 50 m. Report your answer in meters.
Q6
Assuming the area where Tahoma creek flows today was once buried under lava flows of similar age and elevation as Emerald ridge on the Tahoma watershed boundary, how fast at minimum would have glaciers needed to incise downward to account for the modern topography of Tahoma watershed? Report the minimum required glacial incision rate for hypothesis #2 in mm/yr.
Q7
Glaciers in similar watersheds as Tahoma typically incise at rates between 0.05-0.5 mm/yr [1]. Given these values, which hypothesis (#1 or #2) seems more plausible to account for the relatively young Tahoma watershed ridges? Explain your reasoning in two to three sentences.
Q8
Lahars are most likely wherever steep gradients and hydrothermal alteration coincide. Given the Fig. 7 hydrothermal alteration map and the Fig. 4 topographic map, the next Rainier Lahar is most likely to flow predominately down which watershed?
Pullayup
Tahoma
Nisqually
Mowich