Volcano Exercise    Homework #3     Due on Friday, Feb 13, 1998.

Instructions:  Answer (typed!) all the questions using your notes, textbook, lab book and any other references you can find (if needed).  Look at the igneous rocks in your labs this week with this problem in mind. You may work with a classmate on the problems to help understand things clearly, but you must do your own writing (if necessary, reread the syllabus section on Plagiarism). The purpose of this assignment is to increase your ability to use observational evidence combined with reading materials to draw conclusions.  Use the descriptions and pictures of igneous rocks in your textbook to help you solve this problem.....

Homework #3: Answer questions 1-4. Due on Friday, Feb 13, 1998.
 While on a scuba diving vacation to a remote and very beautiful island in the South Pacific, you take a break from diving and backpack for a couple days back into the mountainous region of the island's interior.  You happen upon a small village located near the base of a 1000 meter tall, very steep volcano which appears (to you) to be dormant.  The village is located along the bank of the main stream draining the south flank of the volcano.  The villagers inform you that last week one earthquake knocked them out of bed and the volcano smoked for two days with a number of smaller earthquakes.  After revealing your geological background (only this one course) you are feasted regally, treated to the local fermented beverage and escorted up the volcano the next day with the village elders.  The volcano is emanating a thin ribbon of smoke at the top.  You have a headache and feel nauseous from your royal treatment the night before.  Some of these are 1-2 word answers; others require sentences.

1.  You find many fragments of a light gray colored rock that is very light weight and has a glassy texture with many, many small holes (or vesicles) in it, essentially a "frothy" texture, lying on the volcano's slope.
 A.  What specific type of igneous rock is this?
 B.  Why does it have holes in it?
 C.  What does the color of the rock suggest about its composition (mafic or felsic)?  Why?

2.  The slope of the volcano is so steep that little vegetation grows there.  You sink into the loose, gray, powdery slope (B) up to your ankles, making the Vibram soles of your hiking boots useless compared to the bare feet of your companions.  The surface also contains abundant marble-sized rocks that are fairly rounded (C) and a few oblong, streamlined rocks as much as 50 cm in length (D).  These rocks are light gray to rusty red in color (from iron weathering).
 A.   What is a general term for these three particle types (they are all ________________)?
 B, C, D.  Provide names for these three features (the particles described).

3.  Digging deeper into the slope, you find a layer of hard, cemented rock, light gray in color with a very fine-grained texture that has a bedded appearance (layering).  It looks like it is mainly composed of volcanic ash and shards of glass, with a distinctly glassy, fused texture to it that reflects the sunlight well, and seems to hold the rock together (almost as the ash was “welded” together).  There are also a few larger (5-10 mm diameter) pieces of dark red to black, glassy rock fragments (C) stuck in the rock along with everything else.
 A.  What is the name of this rock?
 B.  What process resulted in this rock's deposition on the mountain side?
 C.  What are the black pieces in the rock called (C)?

4.  The top of the volcano contains a funnel-shaped hole approximately 200 meters in diameter and 100 m deep.  At the base of the hole there is a 50 meter high, dome-shaped, steep-sided, accumulation of steaming, fresh, light gray, lumpy-surfaced lava flows.
 A.  What is this hole in the volcano called?
 B.  What is the composition of the fresh lava flows (mafic or felsic)?
 C.  What is the dome-shaped accumulation called?