Earth's early air and oceans

Grade 9 Science Worksheet

Circle the best answer.

1. Read this first

   Oxygen in the air over time (Smithsonian, NASA)

   Time	Oxygen in the air
   before about 2.7 billion years ago	almost none
   about 2.4 billion years ago	first fills the air (NASA)
   about 600 million years ago	about one-fifth of today's level
   today	21%

2. 1.

   Using the table, about how long after cyanobacteria began (about 2.7 billion years ago) did oxygen first fill the air (about 2.4), by Bo's math?

   • About 300 million years
   • No time; it was instant
   • About 5.1 billion years
   • About 3 years
3. 2.

   Using the table, about how long did it take from the first oxygen in the air (2.4 billion years ago) to one-fifth of today's level (600 million), by Esme's math?

   • About 600 million years
   • About 3 billion years
   • About 1.8 billion years
   • About 1.8 million years
4. 3.

   Why did oxygen take hundreds of millions of years to build up in the air, by Jun's reading (Smithsonian)?

   • It first reacted with methane and with iron in rocks
   • Volcanoes kept taking oxygen in
   • The air already held plenty
   • Photosynthesis used up oxygen
5. 4.

   Drawing of reddish rock layers. Smithsonian: after about 300 million years of oxygen-making, reddish redbeds formed. What do they show, by Kira's reading?

   • The rocks were heated by the Sun
   • Volcanoes were giving off oxygen
   • There was still no oxygen
   • Oxygen had built up enough to rust iron in rocks
6. 5.

   About when did the methane haze clear and the sky turn blue, by Luz's reading (Smithsonian)?

   • About two billion years ago
   • About two million years ago
   • At Earth's start
   • It was blue from the start
7. 6.

   Carbon dioxide + water + sunlight → sugar + oxygen. Cyanobacteria take in carbon dioxide and give off oxygen. How did they change the early air, by Milo's reading?

   • More carbon dioxide, less oxygen
   • More methane from their breathing
   • Less carbon dioxide, more oxygen
   • No change
8. 7.

   Today's air and a lifeless model. If volcanoes had kept adding gas but no life had made oxygen, what would Earth's air be like today, by Noah's reading?

   • Just like today's, 21% oxygen
   • Pure oxygen
   • Rich in carbon dioxide, with almost no oxygen
   • No gases
9. 8.

   1% of today's level, then one-fifth (20%) of today's level. Oxygen rose from about 1% of today's level to about one-fifth of it. How many times larger is that, by Tala's math?

   • About 2 times
   • About 20 times
   • About 5 times
   • About 100 times
10. 9.

    NASA describes oxygen oases: patches where cyanobacteria made oxygen before it spread worldwide. What does this suggest, by Zee's reading?

    • The whole air filled with oxygen at once, with no patches
    • The patches took in oxygen
    • Oxygen built up in patches first, then spread
    • Volcanoes made the patches

11. Oxygen in the air over time (Smithsonian, NASA)

    Time	Oxygen in the air
    before about 2.7 billion years ago	almost none
    about 2.4 billion years ago	first fills the air (NASA)
    about 600 million years ago	about one-fifth of today's level
    today	21%

12. 10.

    Earth is about 4.6 billion years old. For about what fraction of that time has its air held oxygen (since about 2.4 billion years ago), by Ari's math?

    • About half
    • The whole time
    • About one-tenth
    • About 2.4%