1. Sci. 2-3 The Earth take Shapes
Pages 48-53

2. A. As planetesimals build up, the Earth got bigger until it reached a diameter of 350 km.

3. B. The rocks in the center of Earth were crushed by gravity causing the planet to become spherical.

4. C. As planetesimals fell to Earth, the energy of their motion made Earth warmer.

5. D. Once Earth reached a certain size, the inside rock could not cool off as quickly as the temperature increased so the rocky material inside the Earth began to melt.

6. E. The Earth is divided into 3 distinct layers.

7. 1) crust- the outermost layers- solid- the thinnest layer

8. 2) mantle- below the crust- molten material

9. 3) core- center- mostly nickel and iron because they are the most dense material

10. F. Materials with different densities separated because of melting inside Earth. Heavy elements sank to the center because of Earth’s gravity.

11. G. Earth’s atmosphere today is composed of 21% oxygen, 78% nitrogen & 1% argon with many other gases.

12. H. Earth’s original atmosphere formed from gases brought to Earth by meteorites and comets. Probably a lot of water vapor and carbon dioxide.

13. I. Earth’s second atmosphere arose from comet impacts and volcanic eruptions. The composition was largely water and carbon dioxide but also chlorine, nitrogen, and sulfur.

14. 1) The carbon dioxide kept the planet much warmer than it is today.

15. J. Early Earth had no ozone layer to protect it from UV rays but water protected early algae life forms.

16. K. Scientists believe that our oceans come from early volcanoes erupting throwing dust, lava, and water vapor in the air. As Earth cooled, the water vapor cooled and condensed on dust particles forming clouds. Eventually the clouds became so saturated that it rained filling the low spots on the crust forming our oceans. Comets also brought a little water with them.

17. L. Sometime between 3.7 and 3.4 million years ago, life forms evolved that could produce oxygen through photosynthesis. Over millions of years, oxygen levels increased.

18. M. Continents were formed when lighter materials gathered on the surface and rose above sea level.