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
   • About 3 years
   • No time; it was instant
   • About 5.1 billion 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 1.8 billion years
   • About 600 million years
   • About 3 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
   • Photosynthesis used up oxygen
   • The air already held plenty
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?

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

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

   • About two million years ago
   • It was blue from the start
   • At Earth's start
   • About two billion years ago
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
   • Less carbon dioxide, more oxygen
   • No change
   • More methane from their breathing
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?

   • About 4.1 billion years ago
   • Right at 4.6 billion years ago
   • About 5.1 billion years ago
   • About 0.5 billion years ago
9. 8.

   The young Sun gave about 70% of today's light. A dimmer Sun means a colder Earth unless what, by Ren's reading (Smithsonian)?

   • Unless greenhouse gases trap extra heat
   • Unless volcanoes make oxygen
   • Unless oxygen fills the air
   • Unless the oceans freeze
10. 9.

    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 20 times
    • About 100 times
    • About 5 times
    • About 2 times

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