Mission Briefing: Ground Test Sites for a Survey Satellite

Explain why underground heat and diamonds are found in some places and not others.

Answer Key

Where the ground is hot

Read the briefing and Table 1, then answer the questions.

Mission Planning Briefing: Checking the Satellite on the Ground

1A space agency is getting a new Earth-observing satellite ready for launch. From orbit, it will measure heat coming out of the ground and the colors of minerals in bare rock. To check its readings, the mission team needs test sites on the ground where scientists already know what lies below. Mission scientist Dr. Ayumi Ferraro briefed the team, along with Noor Petrakis and Tadeo Marchetti, seventh graders in the agency's summer intern program. She described two resources the satellite should help find: heat for geothermal power plants and diamonds.

2Earth's interior is hot. In most places, rock gets about 25 °C warmer for each kilometer you go down, and the ground surface averages about 15 °C. Where magma rises close to the surface, the heat is much closer, and groundwater heated by it can be over 200 °C just a few kilometers down. Most magma rises where plates pull apart, where one plate sinks beneath another, and over hot spots. A geothermal power plant that spins turbines with steam usually needs water of at least 150 °C at a depth of 2 km.

3Diamonds are made of carbon, and they form only under enormous pressure, about 150 km or more below the surface. The oldest parts of continents, which formed more than 2.5 billion years ago, sit on cold, thick roots of solid rock that reach down 200 km or more. Diamonds can form and stay unchanged in these roots for billions of years. Under younger parts of the plates, the cold, solid rock does not reach that deep, and few diamonds are kept. Rarely, a very fast, explosive eruption from deep in the mantle has punched up through these roots and left a carrot-shaped pipe of rock called kimberlite. Such eruptions are extremely rare, and most known pipes are tens of millions of years old or older.

4Table 1 lists six possible heat test sites. Table 2 lists five kimberlite pipes that have been tested for diamonds. Gems are weighed in carats, and one carat is 0.2 g.

Table 1. Possible heat test sites
Site	Setting	Temp. 2 km down (°C)
A	Chain of volcanoes above a sinking plate	240
B	Middle of an old continent, no volcanoes	65
C	Rift valley, crust pulling apart	210
D	Wide coastal plain with thick sediment	70
E	Volcanic island over a hot spot	260
F	Worn-down mountains, volcanoes long gone	60

Use paragraph 2. Suppose the rock warms by the usual amount as you go down from a surface at 15 °C.

The usual temperature 2 km down would be about 65 °C.

Which statement best describes the pattern in Table 1?

A. The sites with thick sediment or old mountains are the hottest 2 km down.
B. The sites near volcanoes or pulling-apart crust are far hotter than the rest.
C. The temperatures 2 km down span about 20 °C from coolest to hottest.
D. The sites in the middle of continents are the hottest 2 km down.

Use paragraph 2. Circle every site in Table 1 that could supply a steam power plant.

ABCDEF

Site F once had volcanoes, but its rock 2 km down is no warmer than usual today. Which explanation fits the briefing best?

A. Its mountains are worn down, so the Sun heats less of the rock below.
B. Its old volcanoes used up most of Earth's inner heat under that region.
C. It is no longer above a place where magma rises, so its old magma has cooled.
D. Thick sediment on top of it traps cold groundwater under the old mountains.

The satellite will later scan a region the team has not visited. It has a line of young volcanoes along a coast where an ocean plate sinks beneath a continent. What should the team predict?

A. It may have diamonds near the surface, like the old pipes.
B. It may be colder than usual, since the ocean plate chills it.
C. It may have hot water near the surface, like Site A.
D. It will be about as cool as Site B, since it lies on a continent.

Where the diamonds are

Use Table 2 and paragraph 3.

Table 2. Kimberlite pipes tested for diamonds
Pipe	Crust age (billion yr)	Depth of root (km)	Carats per 100 t of rock
K1	2.9	210	60
K2	2.6	190	45
K3	1.8	160	8
K4	0.9	120	0
K5	0.4	90	0

Which statement best describes the pattern in Table 2?

A. Pipes through older crust with deeper roots held more diamonds.
B. Pipes through younger crust held more diamonds than pipes through older crust.
C. Each pipe held about the same number of carats, whatever the crust's age.
D. Pipes through crust between 1 and 2 billion years old held the most diamonds.

Pipe K4 is made of kimberlite, like the others, but it held no diamonds. Which explanation fits the briefing and Table 2 best?

A. Its eruption was too slow, so its diamonds turned back into soft carbon on the way up.
B. Its crust is younger, so its diamonds have not had time to grow to a size worth mining.
C. Its diamonds were washed out of the pipe by rivers soon after the eruption ended.
D. Its root reaches only 120 km, so there was likely no cold, deep rock where diamonds last.

Sort each resource by the process that mainly put it where it is.

The 240 °C water under Site AThe diamonds in pipe K1Steam for a power plant on the island at Site EThe diamonds in pipe K3Hot springs in the rift valley at Site C

Magma rising at plate boundaries or hot spots	Great pressure under old continents, brought up by rare eruptions
The 240 °C water under Site A Steam for a power plant on the island at Site E Hot springs in the rift valley at Site C	The diamonds in pipe K1 The diamonds in pipe K3

How long will a pipe last?

Answer the questions about a mine at pipe K1.

A mine at pipe K1 has found about 50 million tonnes (t) of kimberlite that is worth digging. It digs and crushes about 2.5 million t of rock each year.

At this rate, the mine will use up the kimberlite in about 20 years.

10.

Which statement about diamonds is best supported by the briefing and your answer to question 9?

A. They are limited, because they formed and erupted long ago and are dug up in decades.
B. They will grow back in the pit within a few hundred years once mining stops.
C. They are spread evenly through the crust, so a new mine can open almost anywhere.
D. They are renewable, because the mantle keeps pushing new diamonds into old pipes.

11.

Tadeo says, "If you drill or dig deep enough, you can find hot water and diamonds almost anywhere, so it does not matter where the test sites go." Do you agree? Make a claim, use evidence from both tables, and explain your reasoning with the geologic processes in the briefing.

I disagree. Heat and diamonds are found only where certain geologic processes happened. In Table 1, Sites A, C and E, near sinking plates, pulling-apart crust and a hot spot, are 210 to 260 °C at 2 km, while Sites B, D and F are only 60 to 70 °C, close to the usual 65 °C. Magma rising in those places brings heat near the surface, so drilling the same depth elsewhere does not find steam. In Table 2, only pipes through crust older than about 1.8 billion years with roots 160 km or deeper held diamonds; K4 and K5 held none. Diamonds need the pressure and cold of deep roots under old continents and a rare eruption to bring them up, so the test sites must be chosen in those places.