Page 42 - (DK) Ocean - The Definitive Visual Guide

40 OCEAN GEOLOGY

The Formation of the Earth

THE EARTH STARTED TO FORM MORE THAN 4,500 million years ago in a disk of gas, dust, and
ice around the early Sun. This protoplanetary disk, as it is known, was held in orbit by the
gravitational field of the young star. Gravitational attraction between dust particles in the disk
produced small rocks, and collisions concentrated the rocks into several rings. The most densely
populated rings went on to form the
planets of the Solar System.

EARLY SOLAR SYSTEM
The early Solar System Birth of the Earth
contained a disk of
dust, ice, and gas, from Initially, the rocks within each ring drifted together,
which the rocky inner due to their mutual gravitational attraction, in a process
planets and gaseous known as cold accretion. The largest bodies in each ring
outer planets formed. attracted the most material and grew to form objects larger than
0.7 mile (1 km) across, called planetesimals. Planetesimals are loose
collections of rock and ice, with a uniform structure. As the mass
of a planetesimal grows larger, it exerts a stronger gravitational pull,
small pieces of planetesimals becoming more tightly held together and attracting nearby rocks with
rock and ice start to form in
pulled together protoplanetary greater force. Collisions between planetesimals broke them apart or
by gravitational disk around Sun grouped them together. In the inner Solar System, the planetesimals in
attraction
each orbiting ring came together to form much larger objects, called
protoplanets, and these later collided to form the rocky planets. The
Earth was born in this way about 4.560 million years ago.
1 COLD ACCRETION
Under gravity, pieces of
3 HEAVY BOMBARDMENT
rock and ice coalesced. Each growing protoplanet
Material sharing the attracted more planetesimals,
same orbit around the which impacted through more
Sun clumped together energetic, high-speed collisions.
to form planetesimals. Finally the protoplanets
themselves underwent a series
of collisions to form the rocky
2 PROTOPLANET
By attracting more clumps of planets, including Earth.
rock and ice, and through many
collisions, planetesimals grew impact of
into protoplanets. As the size rocks accelerate Mars-sized
body leads
of these increased, gravity toward primordial to total
smoothed out their surfaces. Earth melting
impacts generate
surface heat and
local melting
Internal Heat
The early Earth was hot but mostly solid, although with a partially molten surface, and had a
fairly uniform internal composition. Today, it has layers of different compositions, including MOON FORMATION
a dense, partially liquid core of iron and nickel. The transition may have its roots in several It is thought that early
different energy sources. Localized surface melting would have occurred when the kinetic in Earth’s history, it
energy of incoming rocks was converted to heat during impacts. More significant heat sources was struck by a large
would have been the decay of radioactive elements in the interior rocks and the heat released protoplanet, creating
by the Earth’s contraction under the force of its own gravity—a process that led to an event the Moon, tilting the
Earth’s axis of rotation,
called the iron catastrophe (see below). Impact with a sufficiently large body might have and leaving it with a
released enough heat to melt the Earth’s interior, and this may have happened more than once. slightly eccentric orbit.

THE IRON CATASTROPHE

INTRODUCTION 1 strength of its gravitational field 2 was strong enough to cause the Earth 3 the iron contained in the Earth’s 4 released further heat, enough to melt material ejected

during collision
later cooled and
coalesced to
form the Moon

As the Earth grew larger, the
Eventually, the gravitational field
Enough heat was released to melt
The sinking of large amounts of iron
to contract, converting gravitational
rocks, allowing it to flow down to
increased, which in turn attracted
the entire interior of the planet in the
the center of the Earth.
event called the iron catastrophe.
potential energy into heat.
more material.