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Through the Mountain: Rock Cycle Clues From Westbury's Water Tunnel

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Through the Mountain: Rock Cycle Clues From Westbury's Water Tunnel

Use a tunnel rock log and old headstones to model how rock changes and what drives it.

Inside the water tunnel

Read the passage and study Table 1. Then answer the questions.

A Tunnel, a Plan and a Cemetery

1The city of Westbury is growing, and its planners have two big projects. A new 12 km water tunnel will carry water from a reservoir on the far side of Pike Mountain to the city's treatment plant. The planners also hope to use the warm water that seeps into the tunnel to heat buildings near its entrance. As the boring machine cut through the mountain, geologist Dr. Ottilie Grunwald logged the rock and measured the temperature of the tunnel wall (Table 1). The east half of the tunnel passes through the same rocks in reverse order. Before the water was let in, seventh graders from Westbury Middle School toured the tunnel with her.

2Dr. Grunwald explained that the shale near the entrances formed from mud that settled on an ancient sea floor, and some of its layers hold fossil shells. Farther in, the rock changes. Slate, schist and gneiss are metamorphic rocks: they formed when older rock was heated and squeezed deep underground but did not melt. Lab tests show that shale usually turns into slate at about 250 to 300 °C, slate turns into schist at about 450 to 500 °C, and schist turns into gneiss at about 600 to 700 °C. As the rock changes, new crystals grow, and they often line up in bands. At about 700 °C or more, rock that holds a little water can begin to melt into magma.

3The granite in the middle of the mountain cooled slowly from magma several kilometers underground. Today all of these rocks lie at or near the surface. Forces powered by Earth's hot interior pushed this region upward, and over millions of years, rain, ice and streams wore away the rock that once covered them. The temperature of rock goes up with depth, so the tunnel wall is warmest where the most rock lies above it.

4The city also has a 150-year-old cemetery with headstones made of four kinds of stone (Table 2). Marble is a metamorphic rock that forms from limestone. Westbury's rain is slightly acidic, so it slowly dissolves marble, while frost and rain break the other stones into tiny grains. Rainwater washes the worn bits into Pike Creek, which carries them, some as solid grains and some dissolved, all the way to the sea. In the sea, animals such as clams use dissolved minerals from the water to build their shells.

Table 1. Tunnel log, west entrance to the middle
Distance in (km)RockWhat it looks likeRock above (m)Wall temp. (°C)
1ShaleGray, soft, thin layers, fossil shells20014
2SlateDark gray, hard, splits into flat sheets40018
3SchistShiny mica flakes in wavy bands80026
4GneissLight and dark bands, large crystals120034
5GraniteLarge crystals fit together, no bands160042
1.
Use Table 1 and paragraph 2. Which statement best describes how the rock changes from km 1 to km 4, moving toward the granite?
Question 1 options
2.
Use Table 1. From km 1 to km 5, the rock above the tunnel increases by 1,400 m.

On average, the wall gets °C warmer for each extra 100 m of rock above it.

3.
The planners want to heat buildings with water that seeps into the tunnel. Water dripping in near km 5 is much warmer than water dripping in near km 1. Which explanation fits the passage best?
Question 3 options
4.
Suppose rock keeps warming with depth at the rate you found in question 2, starting from 10 °C at the surface. (Table 1 fits this rule.) Paragraph 2 says slate turns into schist at about 450 °C. (1 km = 1,000 m)

About how many kilometers deep would rock need to be to reach 450 °C? km

5.
Use your answer to question 4. The schist at km 3 formed at about 450 °C, yet today only 800 m of rock lies above it. Which explanation fits paragraph 3 best?
Question 5 options
6.
The slate at km 2 formed from the same shale layers that hold fossil shells at km 1, yet the slate has no fossils. Which explanation fits the passage best?
Question 6 options

Stone in the city

Use Table 2 and paragraph 4. Every headstone in the table has stood in the open for 150 years.

Table 2. Lettering worn away on headstones in 150 years
StoneKind of rockWorn away (mm)
MarbleMetamorphic (from limestone)4.5
SandstoneSedimentary2.4
SlateMetamorphic (from shale)0.6
GraniteIgneous0.3
7.

On average, how many millimeters of lettering did the marble headstones lose every 100 years? mm

8.

The marble lost how many times as much as the slate? Do not round. times as much

9.
Rain and frost wore away the lettering on the headstones. Which energy source mainly drives the rain and frost?
Question 9 options
10.
Pike Creek carries worn bits of marble, some dissolved, to the sea. Which path could this material follow next in the rock cycle, based on the passage?
Question 10 options

Modeling the rock cycle under Pike Mountain

Use the whole passage and both tables.

11.
Use paragraphs 2 and 3. Match each rock in the tunnel to what it formed from.
12.
Circle the three steps that are driven mainly by heat from Earth's interior.

13.
A city council member says, "The rock inside Pike Mountain and the stone in our cemetery have nothing to do with each other. Rock just stays the kind of rock it is." Do you agree? Make a claim, use evidence from Table 1 and Table 2, and explain your reasoning with a model of the rock cycle. Name the energy source for at least two steps.