1. Oceans & Continents
Unit 9.4

2. Evolution of Oceans
When Earth cooled sufficiently to allow water vapor to condense, rainwater fell and collected in low-lying areas. By 4 billion years ago, it is estimated that as much as 90% of the current volume of seawater was contained in the developing ocean basins.

3. Evolution of Oceans
Because volcanic eruptions released into the atmosphere large quantities of sulfur dioxide, which readily combines with water to form hydrochloric acid, the earliest rainwater was highly acidic. Consequently, Earth’s rocky surface weathered at an accelerated rate.

4. Evolution of Oceans
The products released by chemical weathering - sodium, calcium, potassium, & silica - were carried into the newly formed oceans. Some of these dissolved substances precipitated to become chemical sediment that covered the ocean floor.

5. Evolution of Oceans
Other substances formed soluble salts, which increased the salinity of seawater. Research suggests that the salinity of the ocean increased rapidly at first but has remained constant over the past 2 billion years.

6. Evolution of Oceans
Earth’s oceans also serve as a repository for tremendous volumes of carbon dioxide, a major constituent in the primitive atmosphere. This is significant because carbon dioxide is a greenhouse gas that strongly influences the heating of the atmosphere.

7. Evolution of Oceans
Venus, once thought to be very similar to Earth, has an atmosphere composed 97% carbon dioxide that produced a “runaway” greenhouse effect. As a result, its surface temperature is 475°C (900°F).

9. Evolution of Oceans
Carbon dioxide is readily soluble in seawater, where it often joins other atoms or molecules to produce various chemical precipitates. The most common compound generated by this process is calcium carbonate(CaCO3), which makes up limestone, the most abundant chemical sedimentary rock.

10. Evolution of Oceans
About 542 million years ago, marine organisms began to extract calcium carbonate from seawater to make their shells and other hard parts. Included were trillions of tiny marine organisms whose shells were deposited on the seafloor at the end of their life cycle.

11. Evolution of Oceans
Today, some of these deposits can be observed in the chalk beds exposed along the White Cliffs of Dover England. By “locking up” carbon dioxide, these limestone deposits store this greenhouse gas so it cannot easily re-enter the atmosphere.

12. White Cliffs of Dover

13. Continents
Geologists have discovered tiny crystals of the mineral zircon that formed 4.4 billion years ago (Precambrian time), evidence that crustal rocks began to form early in Earth’s history. However, geologic records from this time are rather limited because the rock cycle operating has destroyed much of its evidence.

14. Continents
Continental crust was produced over time through the recycling of basaltic crust in an early version of plate tectonics. Small crustal fragments formed and accreted to one another, producing North America and the other continents.

15. Continents
About 1.1 billion years ago, the supercontinent Rodinia formed and then rifted apart, opening new ocean basins.

16. Continents
In time, these also closed and formed a new supercontinent called Pangaea around 300 million years ago. Like Rodinia before it, Pangaea broke up as part of the ongoing supercontinent cycle.

17. Continents
The formation of elevated oceanic ridges upon the breakup of a supercontinent displaced enough water that the sea level rose, and shallow seas flooded low-lying portions of the continents. The break up of continents also influenced the direction of ocean currents, affecting climate.