DEFOR
In the geological record, deformation plays a key role: on a
large scale, that of whole mountain ranges, but also in tiny portions of minerals and rocks. The principal drivers are the continuous mobility and flow of the hot and viscous material in the
earth's core and mantle, which are conveyed to the 'lithosphere', Le. the rock formations of the earth's outer crust.
The implications of these phenomena are better known since
the elaboration, in recent decades, of the notions of sea-floor
spreading and continental drift, which are unified under the
concept of plate tectonics. According to this now widely
accepted model, once upon a time there was one large, coherent mass of continental crust, sometimes referred to as
'Pangaea'.This single mass of lithosphere started to break apart
into several large chunks some 250 million years ago. These
chunks, now known to us as the individual continents, move
about on the hotter and more viscous mantle-material
beneath, much like a film of grease floating on a plate of soup.
No wonder that this leads to strains, stresses, and deformation
in the rocks that form the crust. Most of this mobility occurs
along the coastal margins of the continents. These are thought
to be the zones where thin, oceanic crust is pushed down
under the thicker, continental crust. But continental plates also
collide with each other, like in some parts of the near East and
southern Asia. Earthquakes and volcanism, the most telling evidence of all this mobility, occur most frequently in these zones,
for example in the Circum-Pacific realm, in Turkey and in
Greece.
'Panta Rhei' (everything moves) said the old Greeks and with
first hand evidence they knew this to be true not only for the
sea around them, but also for the ground beneath their feet.
C OFO S Originally these were thin, sedimentary layers of quartz sand and clay with plant remains, probably
deposited in a riverbed. But in geology nothing is static, everything is transformed and deformed: minerals, rocks, the face of
the earth itself. Slow, but massive, movements dragged these
sediments to the deep subsurface.The sand and clay beds were
altered into carbon-rich schist and quartzite, and intense compressive forces warped the thin bands into steep microfolds.
Eventually, uplift and erosion brought it all back to the surface.
This example is small-scale (thin section, height 20 mm.), but
folding of bedded rocks occurs on all scales, up to that of
whole mountains.
A 10
128
BIF, short for Banded Iron Formation, is a characteristic type of rock composed of thin, alternating beds of
jasper, hematite and tiger-eye, which may, to greater or lesser
degrees, be deformed into folds or fragments. This type of rock,
of which polished surfaces are pictured here and in several of
the following plates, is the main iron-ore in some of the world's
largest mining districts, most notably in West Australia. Many
BIF's are very ancient sediments, some as old as 3 billion years
and particularly interesting in that they represent a crucial period of evolution on our globe. They originally were sediments
deposited in shallow seas, where primitive bacteria brought
about the oxidation and precipitation of the iron dissolved in
the seawater. This led to the recurring deposition of thin beds,
probably under the influence of climatic cycles. This was a new
phenomenon on planet earth and an indication of crucial
changes in the earth's evolution, notably in the atmosphere.
Massive sediments were thus accumulated and, after a long history of deep burial and exposure to tectonic movement, these
rocks were intensely altered and deformed. This polished slab
is a telling example of such deformation on a small scale.
Actual size 180 x 220 mm.
CL V
0
Intense tectonic forces, that imparted a
cleavage type folding (see last picture) to a schistose rock, have
also buckled up a thin stringer of quartzite transecting it. One
might well imagine to be looking at an arctic landscape, with a
river of ice meandering across the dunes of a barren desert.
Thin section.
INKS Kinks are a characteristic type of microstructure, also
called zigzag folds or knee-folds.They are straight-limbed, angular folds with narrow hinge zones that may occur in thinly bedded rocks or in crystals with a laminated structure, like in the
feldspar displayed here. Kinks form in response to intense,
compressive forces, with slip movements occurring along the
laminar plane surfaces. Thin section.
MIGRAB
In geology, a graben denotes a large structural
feature, a chunk of the earth's crust that has sunk downwards
along two sub-parallel faults. The well-known Rift Valley of East
Africa marks the site of a graben, as do parts of the Rhine Valley
and other marked depressions in the globe's surface. In cross
section a graben would look much like what is seen in this picture.ln fact the miniature graben formed in this polished slab of
In the geological record, deformation plays a key role: on a
large scale, that of whole mountain ranges, but also in tiny portions of minerals and rocks. The principal drivers are the continuous mobility and flow of the hot and viscous material in the
earth's core and mantle, which are conveyed to the 'lithosphere', Le. the rock formations of the earth's outer crust.
The implications of these phenomena are better known since
the elaboration, in recent decades, of the notions of sea-floor
spreading and continental drift, which are unified under the
concept of plate tectonics. According to this now widely
accepted model, once upon a time there was one large, coherent mass of continental crust, sometimes referred to as
'Pangaea'.This single mass of lithosphere started to break apart
into several large chunks some 250 million years ago. These
chunks, now known to us as the individual continents, move
about on the hotter and more viscous mantle-material
beneath, much like a film of grease floating on a plate of soup.
No wonder that this leads to strains, stresses, and deformation
in the rocks that form the crust. Most of this mobility occurs
along the coastal margins of the continents. These are thought
to be the zones where thin, oceanic crust is pushed down
under the thicker, continental crust. But continental plates also
collide with each other, like in some parts of the near East and
southern Asia. Earthquakes and volcanism, the most telling evidence of all this mobility, occur most frequently in these zones,
for example in the Circum-Pacific realm, in Turkey and in
Greece.
'Panta Rhei' (everything moves) said the old Greeks and with
first hand evidence they knew this to be true not only for the
sea around them, but also for the ground beneath their feet.
C OFO S Originally these were thin, sedimentary layers of quartz sand and clay with plant remains, probably
deposited in a riverbed. But in geology nothing is static, everything is transformed and deformed: minerals, rocks, the face of
the earth itself. Slow, but massive, movements dragged these
sediments to the deep subsurface.The sand and clay beds were
altered into carbon-rich schist and quartzite, and intense compressive forces warped the thin bands into steep microfolds.
Eventually, uplift and erosion brought it all back to the surface.
This example is small-scale (thin section, height 20 mm.), but
folding of bedded rocks occurs on all scales, up to that of
whole mountains.
A 10
128
BIF, short for Banded Iron Formation, is a characteristic type of rock composed of thin, alternating beds of
jasper, hematite and tiger-eye, which may, to greater or lesser
degrees, be deformed into folds or fragments. This type of rock,
of which polished surfaces are pictured here and in several of
the following plates, is the main iron-ore in some of the world's
largest mining districts, most notably in West Australia. Many
BIF's are very ancient sediments, some as old as 3 billion years
and particularly interesting in that they represent a crucial period of evolution on our globe. They originally were sediments
deposited in shallow seas, where primitive bacteria brought
about the oxidation and precipitation of the iron dissolved in
the seawater. This led to the recurring deposition of thin beds,
probably under the influence of climatic cycles. This was a new
phenomenon on planet earth and an indication of crucial
changes in the earth's evolution, notably in the atmosphere.
Massive sediments were thus accumulated and, after a long history of deep burial and exposure to tectonic movement, these
rocks were intensely altered and deformed. This polished slab
is a telling example of such deformation on a small scale.
Actual size 180 x 220 mm.
CL V
0
Intense tectonic forces, that imparted a
cleavage type folding (see last picture) to a schistose rock, have
also buckled up a thin stringer of quartzite transecting it. One
might well imagine to be looking at an arctic landscape, with a
river of ice meandering across the dunes of a barren desert.
Thin section.
INKS Kinks are a characteristic type of microstructure, also
called zigzag folds or knee-folds.They are straight-limbed, angular folds with narrow hinge zones that may occur in thinly bedded rocks or in crystals with a laminated structure, like in the
feldspar displayed here. Kinks form in response to intense,
compressive forces, with slip movements occurring along the
laminar plane surfaces. Thin section.
MIGRAB
In geology, a graben denotes a large structural
feature, a chunk of the earth's crust that has sunk downwards
along two sub-parallel faults. The well-known Rift Valley of East
Africa marks the site of a graben, as do parts of the Rhine Valley
and other marked depressions in the globe's surface. In cross
section a graben would look much like what is seen in this picture.ln fact the miniature graben formed in this polished slab of