Laboratory Manual for Introductory Geology 4e

1.2.4 Temperatures and Pressures in the Earth’s Crust
Both temperature and lithostatic pressure increase with depth in the Earth, and
the temperature in the core is estimated to be about 5,000°C. The increase in
temperature with depth, called the geothermal gradient, ranges from 15°C to
50°C per kilometer in the upper 10 km of the crust, with an average beneath
the continents of about 25°C/km. The increase in pressure with depth, called
the geobaric gradient, is about 1 kbar for every 3.3 km of the crust. Exercise 1.5
explores conditions at varying depths in the crust based on the geothermal and
geobaric gradients.
EXERCISE 1.5
Temperature-Pressure Conditions in the Crust
Name:
Course:
Section:
Date:
(a) Draw lines representing the minimum (15°C/km), maximum (50°C/km), and average (25°C/km) geothermal
gradients on the diagram below, using a different color for each. Assume for this exercise that the temperature at the
Earth’s surface is 0°C.
Pressure (kbar)
0 1 2 3 4
0
1
2
3
Depth (km)
4
5
6
7
8
9
10
0
50 100 150 200 250 300 350 400 450 500
Temperature (˚C)
(b) The deepest mine on the Earth penetrates to a depth of about 2 km. Using the geothermal gradients you just drew,
what range of temperatures would you expect halfway down the mine shaft?
At the bottom of the
mine?
(c) Using the pressure scale at the top of the diagram, draw the average geobaric gradient in a fourth color, with an
increase of 1 kbar per 3.3 km.
(d) What will the pressure be at the lowest level of the mine?
(e) What will the pressure be 8 km below the surface?
(f) What will temperature and pressure conditions be at
●
5 km, assuming an average geothermal gradient?
●
4 km, assuming the minimum geothermal gradient? The maximum geothermal gradient?
kbar
kbar
1.2 AN INTRODUCTION TO THE EARTH SYSTEM
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