Transcript PPT
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. protostar rocky 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 Earth During first 600–700 m.y., no permanent crust could form due to continual meteorite bombardment and volcanic activity Formation of Earth About 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 layers The lithosphere • The rigid uppermost 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 Magnesiumsilicate 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