3. Spreminjajoče obličje Zemlje / Dynamic Earth
DYNAMIC EARTH
Earth is a dynamic planet! If we could go back in time a few hundred million years, we would find the face of our planet dramatically different from what we see today. Tehre would be no Alps or even Atlantic ocean!
We would find continents having different sizes and shapes and located in different positions than today's landmasses.
THEORY OF PLATE TECTONICS
How is it possible to have such a changing Earth's surface? Well, for very long time people beleived that position of the continents and oceans is fixed. But in the early 20th century the radical proposal of "continental drift" has contradicted the established view. It has been received with skepticism and even ridicule. In the following 50 years enough data were gathered to transform this controversial hypotesis into a sound theory that wove together the basic processes known to operate on Earth.
According to the plate tectonics model, Earth's rigid outer shell (litosphere) is broken into numerous slabs called lithospheric plates, which are in continual motion (see Figure 3.1).
Several large plates include an entire continet plus a large area of seafloor. In general, plates move as coherent units relative to all other plates.
PLATE BOUNDARIES
All major interactions among individual plates occur along their boundaries. Plates are bounded by three distinct types of boundaries, differentiated by the type of relative movement they exhibit:
1. Divergent boundaries (figure 3.2) - where plates move apart, resulting in upwelling of material from the mantle to create new seafloor.
2. Convergent boundaries (figures 3.3 and 3.4) - where plates move together, resulting in the subduction (consumption) of oceanic lithosphere into the mantle. Convergence can also result in the collision of two continental margins to create a major mountain system.
3. Transform fault boundaries (figure 3.5) - where plates grind past each other without production or destruction of lithosphere.
Well, plates movement, grindign to each other, subthrusting, overthrusting... cause unwanted sudden floor movement - earthquakes, volcanoes eruption, and many (from human point of view) catastrophic events linked with them. Out EarthCache EC14 - Potres / Earthquake 1511 describes one of strong earthquakes in Slovenia and its consequences.
/ Figure 3.1: Tectonic plates and "contonental drift"
/ Figure 3.2: Divergent boundary - plates moving apart at mid ocean ridge
/ Figure 3.3: Convergent boundary - subduction of oceanic lithosphere under continental
/ Figure 3.4: Convergent boundary - collision of two continental lithospheric plates
/ Figure 3.5: Transform fault boundary - lithospheric plates grind past each other
DINAMIČNA ZEMLJA
Zemlja je dinamičen planet! Če bi lahko pogledali nekaj sto milijonov let v preteklost, bi videli obličje planeta zelo drugačno od današnjega. Ne bi bilo Alp, niti Atlantskega oceana ne!
Ugotovili bi, da imajo celine drugačno velikost, obliko in ležijo drugje kot današnje kopno.
TEKTONIKA PLOŠČ, OZIROMA TEORIJA O POTOVANJU CELIN
Kako ima lahko Zemlja tako spreminjajoče se površje? No, ljudje so zelo dolgo verjeli, da so celine in dna oceanov nepremični. A v zgodnjem 20. stoletju je stare poglede izzval predlagani, korenito drugačen model "potovanja celin". Model je bil sprejet z velikim nezaupanjem in celo posmehom. V naslednjih 50 letih pa je bilo zbranih dovolj podatkov, da je ta revolucionarna hipoteza postala trdna teorija, ki je pomagala razložiti osnovne procese, ki potekajo na Zemlji.
V skladu z modelom tektonskih plošč ima Zemlja togo zunanjo lupino (litosfero), ki je razlomljena na številne plošče, imenovane litosferske plošče, ki so v neprestanem gibanju. (glej sliko 3.1).
Več velikih plošč vključuje celo celino in hkrati velik del oceanskega dna. V splošnem se vsaka plošča premika kot kompaktna celota glede na vse ostale plošče.
MEJE ALI STIKI PLOŠČ
Skoraj vse medsebojno dogajanje med ploščami je skoncentrirano na meje plošč. Plošče se stikajo na tri glavne načine, ki jih ločimo po načinu medsebojnega gibanja plošč:
1. Divergentni stik - razpirajoči se (slika 3.2) - kjer se plošči razmikata, rezultat pa je dvigovanje gradiva iz zemeljskega plašča, ki ustvarja novo oceansko dno.
2. Konvergentni stik (sliki 3.3 in 3.4) - kjer se plošči približujeta, rezultat pa je podrivanje oceanske litosferske plošče in potapljanje le-te v zemeljski plašč. Konvergentni stik dveh celinskih plošč lahko povzroči trk in ustvarjanje večjih gorskih sistemov.
3. Zmični stik (slika 3.5) - kjer plošči drsita ena ob drugi ter se ne uničuje litosferska plošča niti nastaja nova.
Seveda premikanje plošč, medsebojno drsenje, podrivanje... povzročajo prebivalcem planeta neljube nenadne premike tal - potrese, bruhanje ognjenikov, in mnogo s tem povezanih (za nas) katastrofičnih pojavov. Naš geološki zaklad EC14 - Potres / Earthquake 1511 opisuje enega najmočnejših zabeleženih potresov pri nas in njegove posledice.