1. Zgodnji razvoj Zemlje / Early Evolution of the Earth
ORIGIN OF PLANET EARTH
According to the (most widely accepted) nebular theory (see Figure 1.1):
A. The birth of our solar system began as dust and gases (nebula) started to gravitationally collapse.
B. The nebula contracted into a rotating disk that was heated by the conversion of gravitational energy into thermal energy.
C. Cooling of the nebular cloud caused rocky and metalic material to condense into tiny particles.
D. Repeated collisions caused the dust-size particles to gradually coalesce into asteroid-size bodies. Within a few million years these bodies accreted into the planets.
FORMATION OF EARTH'S LAYERED STRUCTURE
As material accumulated to form Earth, the high-velocity impact of nebular debris and the decay of radioactive elements caused the temperatuer of the planet to steadily increase. Earth became hot enough that iron and nickel bega to melt. Mleting produced liquid blobs of heavy metal that sank towards the center of the planet. This process occured rapidly on the scale of geologic time and produced Earth's dense iron-rich core.
The early period of heating resulted in another proces of chemical differentiation, whereby melting buoyant masses of molten rock that rose toward the surface, where they solidified to produce a primitive crust. These rocky materials were enriched in oxygene, silicon and aluminium.
This early period of chemical segregation established the three basic divisions of Earth's interior - the iron-rich core, the thin primitive crust, and Earth's largest layer called the mantle - between the core and crust.
/ Figure 1.1: Formation of the solar system
/ Figure 1.2: Sun and planets size comparison
KAKO SO NASTALI PLANETI?
V skladu z (najširše sprejeto) teorijo meglice (glej sliko 1.1):
A. Rojstvo našega sončnega sistema se je pričelo, ko so se prah in plini (meglica) pričeli gravitacijsko sesedati.
B. Meglica se je skrčila v vrteči se protoplanetarni disk, ki se je segrel zaradi pretvorbe gravitacijske energije v toplotno.
C. Ohlajevanje meglice je povzročilo kondenzacijo kamnitih in kovinskih snovi v majcene delce.
D. Ponavljajoči trki povzročijo počasno združevanje kot prah majhnih delcev v večja telesa asteroidnih velikosti. V nekaj milijonih let so nekateral teh teles prerasla v planete.
NASTANEK ZEMLJINE LUPINASTE ZGRADBE
Ko se je kopičilo gradivo za gradnjo Zemlje, so nanjo z visoko hitrostjo udarjali kosi preostalega prostega gradiva protoplanetarnega diska ter skupaj z razpadom radioaktivnih elementov povzročili nenehno segrevanje planeta. Zemlja je postala dovolj vroča, da sta se pričela železo in niklelj topiti. Topljenje je povzročalo nastanek tekočih skupkov težkih kovin, ki so potonile proti središču planeta. Ta proces se je, geološko gledano, odvil hitro in nastalo je gosto Zemljino jedro.
Zgodnja faza segrevanja je povzročila tudi proces kemične diferenciacije in topljenje manj gostih kamninskih plasti, ki so se dvignile proti površju planeta in se strdile v prvotno skorjo. Ti kamniti materiali so bili bogati s kisikom, kremenon in aluminijem.
Zgodnja faza kemičnega ločevanje je Zemljino notranjost razdelila na tri glavne pasove - z železom bogato jedro, nad njim ležeči, najdebelejši sloj imenovan plašč in tanko prvotno skorjo.