CHAPTER 7 SECTION 5 EARLY EARTH

HOW DID EARTH FORM? The age of Earth suspected to be > 4 billion yrs old Hypothesis: Moon formed from material knocked loose when a very young Earth collided with another object. Hypothesis: Earth began as a ball of dust, rock, and ice in space. Gravity pulled this mass together. As Earth got bigger, gravity increased, pulling in even more matter. Hypothesis: Energy from collisions raised Earth’s temperature until it became so hot it melted. Denser material sank to center (core).

THE ATMOSPHERE Early Earth may have included light gases like hydrogen and helium. Then the sun released bursts of particles called solar wind. The light gases were blown away. A second atmosphere then formed. Volcanic eruptions and collisions with comets added carbon dioxide, water vapor, nitrogen, and other gases.

THE OCEANS As Earth’s surface cooled, the water vapor began to condense to form rain. The rainwater accumulated and formed oceans. The oceans absorbed much of the carbon dioxide from the atmosphere.

THE CONTINENTS Eventually rock at the surface of Earth formed continents. Continents move very slowly over Earth’s surface because of forces inside Earth.

EARLY ORGANISMS Scientists have found fossils of single-celled organisms in rocks that formed about 3.5 billion years ago. About 2.5 billion years ago, many organisms began using energy from the sun to make food (photosynthesis). The amount of waste product, oxygen, slowly increased creating an ozone layer that blocked the ultraviolet rays of the sun. Now organisms could live on land.