Themes of the Day:

• Origin of the Earth & Solar System
• Early Geologic History of the Earth
• Structure of the Earth

Origin of the Earth & Solar System

• Big Bang ~12-15 Ga (billion years ago) - chemical elements created in star systems prior to our own
• Nebular Hypothesis - Sun and planets of our solar system condensed from a cosmic dust cloud ~4.6 to 4.5 Ga
• Early Earth was undifferentiated (homogeneous) material similar to average solar system composition
• Early Earth was extremely hot (gravitational potential energy of nebular dust becomes kinetic energy and finally thermal energy as Earth condenses)
• Core formation as molten iron separates from molten silicates and sinks to the center of the planet due to its high density (vinegar and oil analogy)
• Soon after core formation a Mars-sized body collides with the molten Earth spraying material that will become the Moon into Earth orbit
• Meteor bombardment intense on early Earth - crust, oceans, and atmosphere develop slowly
• Most of the evidence for this stage of Earth's history comes from meteorites and lunar samples

Early Geologic History of the Earth

• Age of the Earth ~4.6 to 4.5 Ga (billion years old) - equivalent to formation of our solar system
• Oldest terrestrial mineral ~4.4 Ga (zircon, western Australia); Oldest surviving rocks on Earth ~4.0 Ga (Acasta gneiss - northern Canada)
• Earliest fossil evidence for life (single celled organisms) ~3.5 Ga
• Cambrian explosion (rapid diversification of multicelled organisms) ~570 Ma (million years ago)
• Geologic time scale - many subdivisions since ~500 Ma reflect a preservational bias in the geologic record

Structure of the Earth

• Core - composition: primarily Iron (Fe) - inner core is solid, outer core is liquid
• Mantle - silicate composition, primarily the mineral olivine - solid, but near melting point in places
• Crust - silicate composition, primarily feldspars - thin scum riding atop the mantle, solid
• Oceanic vs. Continental Crust
• Lithosphere vs. Asthenosphere