2.5 Direct observations of the Earth's composition Flashcards
Approximately how deep are the deepest mines (gold mines in South Africa)?
c. 4 km
Which are able to go deeper: mines or boreholes?
boreholes
Approximately how deep is the deepest borehole (an oil well)?
c. 10 km
How deep is the Russian Kola Deep Borehole in Siberia?
c. 13 km
How can volcanoes provide us with information about the lower crust/upper mantle?
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).
Where do diamonds form?
in the high-pressure conditions of the upper mantle (depths of up to 250 km)
What is the name for an igneous rock containing diamonds?
kimberlite
Named after the Kimberley diamond mine in South Africa
What is the name for an igneous rock containing diamonds?
kimberlite
Named after the Kimberley diamond mine in South Africa
Define the term xenolith.
a fragment of ‘foreign’ rock included in an igneous rock, which has come from a different source
What is the composition of xenoliths found in kimberlites?
peridotite (mantle rock)
Define the term peridotite.
an ultramafic igneous rock composed of the minerals olivine and pyroxene
What is an ophiolite?
a section of oceanic crust and upper mantle broken off and attached to the edge of a continent during plate movement
What information can be gained by studying ophiolites?
Ophiolites allow us to examine the structure and composition of oceanic crust without the need to drill a borehole through the ocean floor.
What was Project Mohole (1961-66)?
a project that aimed to drill through the thin oceanic crust into the mantle
How deep did the Project Mohole drilling reach?
c. 183 m
How deep is the Kola Superdeep Borehole?
c. 12 km (a third of the way through the continental crust)
Why could the Kola Superdeep Borehole not reach any deeper?
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
What is the composition of oceanic crust?
- rich in Fe and Mg
- basalt (pillow lavas)
- dolerite (dykes)
- gabbro (in layers)
What is the composition of continental crust?
- rich in Al and Si
- granitic rocks
- igneous, metamorphic and sedimentary rocks
What is the density of oceanic crust?
2.9 g cm-3
What is the density of continental crust?
2.7 g cm-3
What is the maximum age of oceanic crust?
c. 200 Ma
What is the maximum age of continental crust?
c. 4000 Ma
What is the average thickness of oceanic crust?
7 km
What is the maximum thickness of oceanic crust?
10 km
What is the average thickness of continental crust?
35 km
What is the maximum thickness of continental crust?
c. 90 km
What is the composition of the lithospheric upper mantle?
peridotite
What is the composition of the asthenosphere?
peridotite
Define the term kimberlite.
an ultramafic igneous rock with fine crystal size
