The early Earth

Goal

To understand modern hypotheses and theories about the formation of the Universe, our solar system, and the Earth.

The Doppler effect• Waves emitted from a source moving

towards you are compressed (increased frequency).

• Waves from a source moving away from you are expanded (decreased frequency).

The Doppler effect

In light this is known as a blue shift (shortening of waves) or a red shift (expansion of waves)

The Doppler effect

Think of the change in sound of a race car from high-pitched to low-pitched as it goes past you.

Red shift

In the 1920’s astronomers noticed that EVERY galaxy exhibits a red shift relative to nearby stars in our own galaxy.

Therefore, the Universe must be uniformly expanding—this has been tested many times since then.

The Big Bang

Big Bang theory: All matter and energy in the Universe started at a single point which exploded ~14 b.y. ago—giving rise to our continually expanding Universe.

Expansion of the universe with time

As the Universe expanded and cooled

• After a few moments atomic nuclei began to form

• After a few 100 k.y., nuclei trapped electrons to become atoms of hydrogen and helium

• Further cooling allowed clouds of gas we call nebulae (plural of nebula) to form

• Nebulae swirl together and coalesce under the force of gravity

• Once a central ball of matter becomes large enough, pressure and heat start fusion reactions—it is now a star

Protostar

• Multiple generations of stars must form and die to generate heavier elements

• Our own solar system began to coalesce from a nebular cloud about 4.6 b.y. ago.

Formation of Earth

In our own solar system 99.8% of matter went into the sun. The remaining 0.2% remained swirling around the sun as a disk-shaped cloud of gas and dust.

protostar

rocky volatile

Formation of Earth

Ages of meteorites range from 4.53 to 4.58 b.y. old. These ages are the minimum age for formation of the solar system.

This is likely close to the maximum age as well.

protostarrocky volatile

Formation of Earth• Matter in this disk-shaped cloud rapidly formed

small bodies called planetesimals• Planetesimals continued to collide and grow,

eventually forming the planets, ~4.5 b.y. ago.

Formation of Earth• Proto-Earth was almost entirely molten.• Gravitational stratification of the earth into:

– Iron-nickel core– surrounded by a magnesium-silicate mantle

Formation of Earth

Soon after this differentiation, a large planetesimal collided with the proto-Earth and blasted out material that became our moon.

Formation of EarthDuring first 600–700 m.y., no permanent

crust could form due to continual meteorite bombardment and volcanic activity

Formation of EarthAbout 3.8 b.y. ago, the Earth stabilized and

a semi-permanent crust formed.

Earth’s layers• Earth initially formed a

metalic core surrounded by a magnesium-silicate mantle

• As the it cooled, the inner part of the core became solid

• Lighter elements continually move from the mantle to the surface.

Earth’s layers

• Core made of iron-nickel alloy including:– Solid inner core– Liquid outer core

• Mantle made of magnesium-silicates including:– The inner mantle– The asthenosphere —

weak, partially molten layer

Relative thickness of layers

Earth’s layersThe lithosphere• The rigid upper-

most layer of the mantle

• The crust — thin rind of light elements floating on top of the earth. Also what you’re sitting on.

So, how do we know what the mantle and core are made of?

Actually, we use a number of different lines of evidence

1. Composition of meteorites

Iron-nickel alloy

Magnesium-silicate rocks

2. Inclusions of the mantle in volcanic rocks

The mantle

3. Ophiolites: Fragments of the uppermost mantle trapped in mountain belts

The mantle in Oman

4. Behavior of sound waves from earthquakes & large explosions

Internal structure of mantle modeled from seismic velocities