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?

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

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

   • The air already held plenty
   • Photosynthesis used up oxygen
   • It first reacted with methane and with iron in rocks
   • Volcanoes kept taking oxygen in
4. 3.

   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?

   • There was still no oxygen
   • Oxygen had built up enough to rust iron in rocks
   • Volcanoes were giving off oxygen
   • The rocks were heated by the Sun
5. 4.

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

   • About two million years ago
   • About two billion years ago
   • It was blue from the start
   • At Earth's start
6. 5.

   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?

   • Less carbon dioxide, more oxygen
   • More carbon dioxide, less oxygen
   • No change
   • More methane from their breathing
7. 6.

   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?

   • Pure oxygen
   • Just like today's, 21% oxygen
   • No gases
   • Rich in carbon dioxide, with almost no oxygen
8. 7.

   Earth formed about 4.6 billion years ago, and its surface cooled enough for water about half a billion years later (Smithsonian). About when could oceans begin to collect, by Pia's math?

   • Right at 4.6 billion years ago
   • About 0.5 billion years ago
   • About 5.1 billion years ago
   • About 4.1 billion years ago
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 5 times
   • About 2 times
   • About 20 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?

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

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?

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