summing-up
1A planet made of layers
The Earth is formed from three
•
Under the oceans the crust has an
•
average thickness of 6km, and 35km
beneath the continents.
• The mantle has a thickness of about
2,900km.
• The core has a thickness of 3,470km.
The asthenosphere is a zone of the
•
mantle located at a depth of between
70 – 250 km; in this zone, the mantle
is formed in small part by molten
material, and for this reason it is less
rigid.
The entirety of the Earth’s crust and
•
the mantle down to the
asthenosphere is called the
lithosphere (rigid).
2The structures of the
oceanic crust: ridges and
trenches
• Oceanographic explorations have
floor of the oceans is not flat and
uniform, but has two types of
structures of dimensional
importance:
–the oceanic ridges that constitute a
system of raised areas of the sea floor
that stretch for more than 60,000
km; and
–the oceanic trenches, very deep
depressions of the ocean floor.
3Ocean floor spreading
The release of magma on the floor of
•
Since the planet’s surface remains
•
constant over time, a mechanism
must exist that “consumes” the
oceanic crust.
• It is thought that the lithosphere is
“recycled” within the oceanic
trenches.
Evidence for the expansion includes:
•
–changes of intensity in the magnetic
field strength in bands parallel to the
ridges;
–the gradually increasing age of the
ocean floor (basalts) away from the
axis of a ridge.
4Plate tectonics
The theory of plate tectonics is a
•
move away from each other. The
continents, embedded in plates,
“drift”, being drawn along by the
movement of the plates.
The plates are separated by
•
boundaries, along which seismic
events and volcanic activity are
concentrated.
There are three types of boundaries
•
between the plates:
–divergent (or constructive)
boundaries, corresponding to
oceanic ridges;
–convergent (or destructive)
boundaries, corresponding to
oceanic trenches; and
–transform-faults (conservative)
boundaries, corresponding to
transform faults.
5New mountains and new
oceans
• A chain of mountains such as the
formed as a result of the subduction
of an oceanic plate beneath a
continental plate and is characterised
by the activity of many explosive
volcanoes.
• An arc of volcanic islands, such as
the Marianas, is formed as a result of
the subduction of an oceanic plate
under another oceanic plate.
• The separation of two lithospheric
plates leads to the formation of a new
ocean.
6Verification of the model
95% of active volcanoes are located
•
occurs together with the formation
of volcanic arcs or arcs of volcanic
islands.
• Volcanism associated with
constructive margins involves the
leakage of basaltic magmas on the
floor of the rift valleys of oceanic
ridges.
Volcanoes within the plates are
•
associated with “plumes” of hot
material rising from the mantle,
which appear at the surface as hot
spots.
concentric shells:
–the crust, divided into continental
and oceanic, which are very different
in nature and origin;
–the mantle; and
–the core, divided into outer and
inner.
found that the crust that forms the
the rift valleys of oceanic ridges bring
about the progressive addition of
new oceanic crust to the existing
one, i.e. spreading of the ocean
floors.
global theory that is able to explain
the main geological phenomena of
the planet.
• According to this theory, the
lithosphere is subdivided into
around 20 rigid plates that, floating
on the asthenosphere, slide past each
other, collide with each other, or
Himalayas are raised following the
collision between two continental
plates.
• A mountain range like the Andes is
along the margins between
lithosphere plates, while the
remaining 5% are located within
plates.
• Volcanism along destructive margins
is related to the process of
subduction of lithosphere plates and
Lupia Palmieri, Parotto Osservare e capire la Terra - edizione azzurra © Zanichelli 2012
unità 5•La struttura della Terra
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summing-up
7The motor of the plates
The temperature within the Earth
•
increases with depth: this internal
heat is fueled by natural
radioactivity.
It is thought that convection currents
•
are active in the mantle, caused by
local differences in temperature.
• Lithosphere plates, in their
movements, respond in some way to
Lupia Palmieri, Parotto Osservare e capire la Terra - edizione azzurra © Zanichelli 2012
flows in the underlying mantle: it is
possible that they are “dragged” or
in some way transported by the
convection currents that involve the
material in the mantle.
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