2.5 Direct observations of the Earth's composition

Question 1

Approximately how deep are the deepest mines (gold mines in South Africa)?

Answer 1

c. 4 km

Question 2

Which are able to go deeper: mines or boreholes?

Answer 2

boreholes

Question 3

Approximately how deep is the deepest borehole (an oil well)?

Answer 3

c. 10 km

Question 4

How deep is the Russian Kola Deep Borehole in Siberia?

Answer 4

c. 13 km

Question 5

How can volcanoes provide us with information about the lower crust/upper mantle?

Answer 5

Magma that feeds volcanoes originates in teh lower crust or upper mantle and carries up samples of the rocks from these layers to the surface.
We can also estimate the composition of the upper mantle from the chemical analysis of lavas produced at mid-ocean ridges (they are formed by partial melting of the upper mantle).

Question 6

Where do diamonds form?

Answer 6

in the high-pressure conditions of the upper mantle (depths of up to 250 km)

Question 7

What is the name for an igneous rock containing diamonds?

Answer 7

kimberlite

Named after the Kimberley diamond mine in South Africa

Question 8

What is the name for an igneous rock containing diamonds?

Answer 8

kimberlite

Named after the Kimberley diamond mine in South Africa

Question 9

Define the term xenolith.

Answer 9

a fragment of ‘foreign’ rock included in an igneous rock, which has come from a different source

Question 10

What is the composition of xenoliths found in kimberlites?

Answer 10

peridotite (mantle rock)

Question 11

Define the term peridotite.

Answer 11

an ultramafic igneous rock composed of the minerals olivine and pyroxene

Question 12

What is an ophiolite?

Answer 12

a section of oceanic crust and upper mantle broken off and attached to the edge of a continent during plate movement

Question 13

What information can be gained by studying ophiolites?

Answer 13

Ophiolites allow us to examine the structure and composition of oceanic crust without the need to drill a borehole through the ocean floor.

Question 14

What was Project Mohole (1961-66)?

Answer 14

a project that aimed to drill through the thin oceanic crust into the mantle

Question 15

How deep did the Project Mohole drilling reach?

Answer 15

c. 183 m

Question 16

How deep is the Kola Superdeep Borehole?

Answer 16

c. 12 km (a third of the way through the continental crust)

Question 17

Why could the Kola Superdeep Borehole not reach any deeper?

Answer 17

at c. 12 km deep, the temperature (c. 300°C) was much higher than expected and the rock was too plastic for drill bits to be effective

Question 18

What is the composition of oceanic crust?

Answer 18

• rich in Fe and Mg
• basalt (pillow lavas)
• dolerite (dykes)
• gabbro (in layers)

Question 19

What is the composition of continental crust?

Answer 19

• rich in Al and Si
• granitic rocks
• igneous, metamorphic and sedimentary rocks

Question 20

What is the density of oceanic crust?

Answer 20

2.9 g cm^-3

Question 21

What is the density of continental crust?

Answer 21

2.7 g cm^-3

Question 22

What is the maximum age of oceanic crust?

Answer 22

c. 200 Ma

Question 23

What is the maximum age of continental crust?

Answer 23

c. 4000 Ma

Question 24

What is the average thickness of oceanic crust?

Answer 24

7 km

Question 25

What is the maximum thickness of oceanic crust?

Answer 25

10 km

Question 26

What is the average thickness of continental crust?

Answer 26

35 km

Question 27

What is the maximum thickness of continental crust?

Answer 27

c. 90 km

Question 28

What is the composition of the lithospheric upper mantle?

Answer 28

peridotite

Question 29

What is the composition of the asthenosphere?

Answer 29

peridotite

Question 30

Define the term kimberlite.

Answer 30

an ultramafic igneous rock with fine crystal size