Ch 22 The Precambrian Earth Mrs Johnson Earth
Ch. 22: The Precambrian Earth Mrs. Johnson Earth Science
Essential Question • What’s the difference between a mold and a cast?
Early Earth Precambrian-includes Hadean, Archean, and Proterozoic Eons-is an informal time unit that spans nearly 90% of Earth’s history Rocks from Hadean Eon are rare because it’s the earliest Eon. Earliest signs of life are from the Archean Eon
Crustal Rock Evidence • Absolute-age dating revealed the oldest known rocks are 4. 28 billion years old • Zircon (Zr. Si. O 4) in metamorphic rock from Australia reveals evidence of the Earth’s 4. 28 billion year age – Zircon is stable and common mineral which can survive erosion and metamorphism; therefore, it’s used to age-date ancient rocks.
Solar System Evidence • Evidence from meteorites suggest Earth is older than 4. 4 billion years old • Meteorites-small fragments of orbiting bodies that have fallen on Earth’s surface • The oldest rock samples from the moon were collected during the Apollo missions in the 1970 s; these samples have been dated to 4. 4 -4. 5 billion years old
Earth’s early heat sources 3 likely sources for Earth being so hot when it formed: – Earth’s Gravitational concentration – Radioactivity – Bombardment of Earth from asteroids, meteorites, etc.
Gravitational Concentration • Earth is thought to have formed by the gradual accumulation of small, rocky bodies in orbit around the sun • As it grew in size, the Earth added mass • With increased mass, gravity increased, causing Earth’s center to squeeze together with so much force, the pressure created raised Earth’s internal temperature
Radioactivity • Radioactive decay is the release of energy in the form of HEAT • There were more radioactive isotopes in existence when the Earth formed than there are now • Why? – Because isotopes DECAY and some have disappeared – The more decay happens, the more heat is generated
Asteroid and Meteorite Bombardment • Asteroids-carbon or mineral-rich objects between 1 m and 950 km in diameter • Most asteroids orbit the sun today between the orbits of Mars and Jupiter • Only about 60 objects with diameters greater than 5 km have hit the Earth during the last 600 million years • The rest have been meteorites-fragments of asteroids
Cooling • Earth started out as EXTREMELY HOT due to asteroid bombardment • The Earth started to cool and eventually got cool enough for land oceans to form over a span of many years • The Earth’s core is still very hot and is undergoing slow cooling • As much as half of Earth’s internal heat remains from its formation!
Formation of Crust and Continents Differentiation-the process by which a planet becomes internally zoned when heavy minerals sink toward its center and lighter materials accumulate near the surface • Iron is in the core of the Earth • Oxygen and Silicon are near the surface • Reference pg. 623 figure 3
Earliest Crust Early crust began to form as soon as Earth’s upper layer began to cool This crust would be similar to basaltic crust in the oceans Recall oceanic floor/crust gets recycled at subduction zones Earth’s early crust was also recycled, though we are unsure of how it happened
Continental Crust • As early crustal pieces returned to the mantle, they carried water. • Introduction of water into the mantle was important because it helped form the first continental crust • The water reacted with the mantle material to produce new, less-dense mantle which could rise to the surface
Growth of Continents • Recall supercontinent Pangaea • Plate tectonic forces that formed Pangaea have been active since Archean Eon • When landmasses collide, they can form mountains
Formation of Atmosphere • Scientists think asteroids and meteorites contained water which was vaporized when the asteroids and meteorites hit the Earth • This vaporization of water (liquid to gas) caused a haze around the Earth • Outgassing-when volcanoes erupt, they release gases; these gases contributed to the atmosphere creation since LOTS of volcanoes erupted during the Precambrian
Oxygen • When Earth began, there was not much free oxygen • Free oxygen didn’t begin to accumulate until the Proterozoic Era • How? • Cyanobacteria-tiny, thread-like organisms • Cyanobacteria use photosynthesis to grow – (light energy + CO 2 + H 2 O sugar + O 2)
Oxygen, con’t. • Scientists look for rust when trying to decide if oxygen is present • WHY? • Rust forms when Iron interacts with Oxygen. There is iron in the early Earth. If it mixes with Oxygen, there would be rust!
Red Beds • Lots of sedimentary rocks are red in color, dating from the Proterozoic Era=red beds • Contain: Iron Oxides • Presence of Red Beds means there was oxygen in the atmosphere
Why is oxygen important? • We need it to breathe (DUH!) • But, wait, there’s more! • Oxygen provides protection from UV rays HOW? ?
Ozone layer! It helps protect the earth and gives organisms a chance to thrive!
Ocean Formation • Oceans got their water from 2 major sources we’ve mentioned before: – volcanic eruptions (water vapor) and meteorite and asteroid collisions with Earth. • As Earth cooled, this vapor changes state to liquid-what phase is that? ? ? ? ? • Condensation!
Rain • As liquid water formed, it rained tremendously! • This rain filled the low-lying basins to create oceans • Rain dissolved minerals and the minerals were transported by rivers, runoff, and groundwater • As minerals dissolved, the oceans became salty
Water and Life • Precambrian started with very little water and ended with LOTS of water • Oceans were created • Life thrives in water • Other planets have/possibly have water which means there is possibly life on other planets!
Questions?
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