To log this earthcache, send
me an email with the answer to the following questions:
1. Briefly
describe the decompression process that gives rise
to slabs
2.What is the difference between
chips and slabs? What are the three causes of
fracture?
3.From the front you can see Pena
Amarela. What type of fracture predominates?
(horizontal, vertical, diagonal)
4. Add a photograph of you at the
indicated place, or another in which you can see an
object, or your nick on a piece of paper
If you believe you have successfully completed this
Earth Cache goals and has already sent to me all the
requirements as requested, Please, feel free to log
it as found. Later i will verify the requirements
sent and, if necessary, contact you in order to make
the necessary corrections to your log.
|
|
Granitic geoforms
In
the PNDI it is possible to find extensive granite areas, where
medium and large geoforms occur, which contribute decisively to
the Park's landscape, with emphasis on the extensive areas of
the surface of the Mirandês Plateau itself. There are also
several detailed geoforms in the granite, such as those shaped
into balls, found in geoform fields, namely in Barrocal do
Carrascalinho. There are also sinks, depressions excavated in
solid rock, as in Miradouro do Carrascalinho, Barrocal do Douro
and Gamoal, split blocks, with flat fracture surfaces,
observable in Barrocal do Carrascalinho, polygonal fracture,
visible in Trigueiras and surfaces in existing flames, for
example, in Gamoal and Trigueiras. Tor, rocks and pedunculate
blocks, tafoni, and flutes are also visible in several barrocal
areas in the PNDI
Geological framework
The region under study is
included in the Hesperian or Iberian Massif and is
characterized by Precambrian and Paleozoic formations that
were metamorphosed, deformed and intruded by granitic
plutonites during the Variscan orogeny.
The Fundamental Surface
of the Iberian Plateau is modeled, on average, between 600 and
800 meters in altitude. The Mirandese Plateau is a significant
portion of the North Meseta. It develops essentially in
granitic and metasedimentary rocks, but is also modeled on
Cenozoic sedimentary deposits. The Trás-os-Montes plateau is
only interrupted by the strong flow of the Douro River and its
main tributaries, the Águeda River, the Sabor River and the
Tua River.
Sheet fractures and
structures
Many granite outcrops are subdivided not only by orthogonal or
rhomboidal sets of fractures but also by flat-lying or gently
arcuate partings. The latter are of two types. One set is
surficial partings, and is referred to as pseudobedding,
pseudostratification, or flaggy joints.
They are essentially discontinuous, occur within a few metres of
the surface and affect morphology at a small scale, banks of
shallow clefts being produced by preferential weathering of the
partings
Sets of horizontal or
curvilinear fractures that extend to greater depths than
pseudobedding are known by various names: flat-lying joints,
Lägerklufte, Bankung, structure en gros bancs, estructura en
capas, stretching planes, shells, and exfoliation, or offloading,
relief of load, pressure release, sheeting or sheet joints. For
various reasons, but mainly because they preempt discussion of
origin, several of these terms are unsuitable, and here sheet, or
sheeting fractures, is preferred as genetically neutral but yet
descriptive; for though the word sheet may suggest a thin layer,
whereas some of the forms discussed here are 10 m or more thick,
they are nevertheless thin in the global, continental or regional
contexts. Fracture is preferred to joint because dislocation is
evident along some of the partings, which are, therefore, small
displacement faults. Sheet structure is used to denote the massive
slabs defined by the sheeting fractures
DESCRIPTION AND CHARACTERISTICS
Sheet structure consists of thick arcuate slabs defined by
sheet fractures. The slabs are up to 10 m thick, and are
arbitrarily defined as being more than about 0.2 m thick
(slabslless than that are termed spalls or flakes). Sheet
fractures have been observed at depths of 100 m or more in some
quarries, though elsewhere they fade with depth. Indeed, in some
areas with well developed orthogonal fracture systems, as in some
of the residuals found on northwestern Eyre Peninsula, and in the
silicic volcanics of the Gawler Ranges, in the arid interior of
South Australia, sheet fractures appear to be superficial. On the
other hand, it is evident in some deep mines and other excavations
that sheet fractures extend to great depths. Some sheet fractures
take the form of simple arcuate partings. Others consist of
several separate fractures arranged en echelon and together
forming an arcuate parting. It is frequently claimed that the
thickness of sheet structure increases systematically with depth,
but there are many exceptions.
THEORIES OF
ORIGIN
Two diametrically opposed views of the relationship between the
form of the land surface and the geometry of sheeting joints have
evolved over the past 150 years or so.
Some interpret the joints and associated sheet structure as a
primary feature of the rock which has closely determined the gross
morphology of the land surface. According to this view the joints
were first developed in the bedrock and the shape of the land
surface is a response to this internal structure.
These are the two major competing interpretations of sheet
fractures and associated sheet structure, but over the years many
explanations and mechanisms have been proposed. Though most fail
as general explanations, some may have local validity. All fall
into one of two major categories exogenetic or endogenetic.
Exogenetic
explanations
Insolation was long ago suggested as a possible cause of sheet
fractures. As rocks are poor conductors of heat it has been argued
that solar radiation heats the outer exposed zones of rock which
expand and become detached from the main mass, forming
more-or-less thick slabs or sheets. But because the effect of the
Sun’s radiation penetrates only a few centimetres at most into the
rock,whereas sheet jointing extends to considerable depths, this
view can be discounted
The suggestion that sheet fractures are an expression of
offloading or pressure release is widely accepted. And all rock
fractures are an expression of erosional offloading in the sense
that at depth other stresses are subordinate to the pressures
exerted by the superincumbent load. It is only through the release
of vertical pressure that the other stresses are manifested as
obvious fractures. But, a basically different interpretation of
sheeting joints, which attributes them solely and wholly to
pressure release without the previous application of stress, has
long found favour
The gist of the pressure release, or erosional offloading,
hypothesis is that rocks which cool and solidify deep in the
Earth’s crust (for example granites, whether of metasomatic or
igneous origin) do so under conditions of high lithostatic
pressure, i.e. loading by overlying and adjacent rock.
Decompression
When large masses of igneous rock, especially granite, are
exposed to erosion, the concentric slabs begin to loosen. The
process that generates these onion-like layers is called
loosening. This is believed to occur, at least in part, due to the
large reduction in pressure that occurs when the rock above is
eroded, a process called decompression. Accompanying this
decompression, the outer layers expand more than the rock below
and, in this way, separate from the rocky body. Continuous
weathering ends up separating and breaking the slabs, creating
exfoliation domes (ex exterior; folium foliar).
Although many fractures are created by expansion, others are
produced by contraction during magma crystallization and others
are due to tectonic forces acting during mountain formation. The
fractures produced by these activities generally form a defined
pattern and are called joints. Joints are important rock
structures that allow water to penetrate deep areas and the
weathering process to begin long before the rock is exposed.
Achieved through removal of the superincumbent load causes the
development of radial stress which is tensional and is relieved by
the development of fractures tangential to the stress and parallel
to the earth's surface; these are, according to the proponents of
this hypothesis, sheet joints.
The fundamental premise of the hypothesis is that the shape of the
land surface in broad terms determines the geometry of the sheet
joint, as it is in relation to it that radial tension develops.
Lamination is a secondary feature formed after the development of
topography.
Although plausible and persuasive, the theory of unloading in the
sense outlined by Gilbert (1904) and adopted by many later
workers, namely, that the release of pressure is the sole cause of
sheet joining, can be questioned on several grounds.
Endogenetic
explanations
Turning to endogenetic explanations, several writers, including
some of the earliest to consider the problems of sheet jointing,
related sheet structure to the stresses imposed on magmas during
injection or emplacement and, hence, to the shape of the original
pluton. The parallelism of sheeting joints and the margins of the
igneous body on Dartmoor were noted early last century. Some
workers attributed sheet structure to a combination of stresses
developed during emplacement of the granite mass and later
cooling. Thus, although it may apply at a few sites, this
suggestion cannot stand as a general hypothesis, for inselbergs
and associated sheet jointing are well-developed in rocks such as
sedimentary and volcanic sequences, which have not been emplaced
and even in granites the magnetic structures contemporaneous with
the emplacement are clearly discordant to the sheet structure
Though any of several possible explanations of sheet jointing may
be valid in particular areas, the hypothesis offering the best
general explanation is that involving lateral compression, induced
by horizontal stresses, either relic or modern, and the
manifestation of stress patterns affecting brittle rock within,
say, a kilometre of the surface when vertical loading is decreased
by erosion. In these terms, sheet fractures and sheet structures
are associated with tectonism. Such an explanation accounts for
many details of the field evidence, is consistent with measured
stress conditions, and offers a comprehensive view of the
preservation of inselbergs and the sheet structure widely
associated with them.
Fuentes:
LANDFORMS AND GEOLOGY OF GRANITE TERRAINS
Património
Geológico no Parque Natural do Douro Internacional:
caracterização, quantificação da relevância e estratégias
de valorização dos geossítios
|
|