Historical Geology - Hadean and Archean

Question 1

How is the Hadean Eon described?

Answer 1

• named after Hades
• no rocks Earth this old except some meteorites and isolated Zircon crystals.
• provides an estimate of the age of the solar system and the planets within it.
• during the early Hadean the solar system was forming.

Question 2

Explain the Accretion Disk.

Answer 2

• The sun was formed by gasses getting trapped in a self-gravitational collapse until it began to undergo nuclear fusion and give off light and heat.
• Surrounding particles began to coalesce due to gravity into larger lumps or planetisimals, which continued to aggregate into planets

Question 3

Explain stratification of the earth.

Answer 3

• center is called the core - surrounded by the mantle - surrounded by the lithosphere (upper mantle and crust)

Question 4

What is a meteorite and what can it be made of?

Answer 4

A solid piece of debris that survives the fall to earth. It can be made of stone (most common) or can be metallic or a combination of both.
- many meteorites fell early on which scientists believe accounts for most of the heavy metals found on the surface of the earth.

Question 5

Describe Earth during Hadean?

Answer 5

• atmosphere was different from today

- Earth’s crust cooled enough that rocks and continental plates began to form

Question 6

What are possible causes for the formation of the moon?

Answer 6

• asteroid captured by earth’s gravitational field

- portion of earth knocked loose by a large impact

Question 7

What formed the ocean?

Answer 7

• formed mostly from water vapor emitted during volcanism

- salts derived from weathering

Question 8

What formed the Earth’s atmosphere early on?

Answer 8

• composed primarily of volcanic gasses

- O2 was produced in very small quantities

Question 9

Discuss the origins for the continental crust. (What it was made of)

Answer 9

• original crust would have been largely mafic (rich in magnesium and iron) in composition
• Felsic (feldspar and quartz) minerals formed from mafic slabs that were partially melted upon subduction

Question 10

How is the Archean Eon characterized?

Answer 10

• derived from Greek “arche” which means “beginning” or “origin”
• continents were small and thinner than post-Archean continents.

Question 11

What is a protocontinent?

Answer 11

An actual or hypothetical landmass that might later be enlarged into a major continent or broken up into smaller one.

Question 12

What is a craton?

Answer 12

Old stable portion of the continent.

Question 13

What is a greenstone belt?

Answer 13

• a block of lithosphere that has been attached to a craton (stable rock) while preserving its geologic history
• zones of variably mafic rock that occur within Archaean and Proterozoic cratons between granite and gneiss bodies. It is primarily formed of volcanic rocks, dominated by basalt, with minor sedimentary rocks inter-leaving the volcanic formations.
• Protocontinents accrete (form) during Archean to form continental cratons.
• Rock deposits by Archean volcanic arcs in a series of terranes
• Pillow basalt structures and fine-grained sedimentary rocks

Question 14

What are the two most abundant gasses in the Earth’s atmosphere?

Answer 14

N2 and O2

Question 15

How was the majority of the O2 in the earth’s atmosphere produced?

Answer 15

Photosynthesis

Question 16

What were the primary producers of O2 in the Precambrian?

Answer 16

Photosynthetic bacteria (cyanobacteria) in stromatolites!
Giant, reef-forming stromatolites from the Archean of Canada(2.7 Ga) would have contributed O2 to early atmosphere

Question 17

What does an organism need to be alive?

Answer 17

• Grow
• Metabolize (extract energy from its surroundings in some manner)
• Reproduce
• Maintain some sort of homeostasis (regulated internal
  environment)

Question 18

Name the 4 things life requires.

Answer 18

– Protein – strings of comparatively simple organic molecules called amino acids -These are the building materials of organisms
– Nucleic acids – DNA and RNA for heredity and to code
for protein construction
– Organic phosphorous compounds – to extract energy
from light or chemical fuel for cell activities
– A container – some sort of cell membrane to keep the
innards in, and the environment out

Question 19

What are hydrothermal vents?

Answer 19

Chimney like holes with sulfur-rich clouds expelling from the ocean floor.

Question 20

Why might hydrothermal vents be important in understanding the origin of life?

Answer 20

• Warm waters at mid-oceanic ridge would have been anoxic, so chemical compounds necessary for life would not be oxidized and destroyed.
• Mid-oceanic ridges are protected from UV radiation (thought to be intense at Earth’s surface prior to ozone build-up in atmosphere)
• There’s a lot of phosphorus at mid-oceanic ridges, an element that all living organisms need.
• Mid-oceanic ridges contain metals such as nickel and zinc that organisms need in trace amounts.
• Bacteria that live at today’s mid-oceanic ridges harness several naturally occurring chemical reactions to produce energy.

Question 21

Where might life have originated?

Answer 21

• Warm little pond/primordial soup hypothesis
• At hydrothermal vents along mid-oceanic ridges
• In/under the ice
• Elsewhere in the galaxy

Question 22

Where is the Isua formation and what is it know for?

Answer 22

Located in Greenland it preserves the oldest possible evidence of life. (Pillow basalts)

Question 23

Give evidence of life in the Isua formation.

Answer 23

– Indirect evidence:
• Higher than expected percentage of organic 12C
• Interpreted as evidence of photosynthesis

• Two stable isotopes of Carbon
– 12C (98.89% of Earth’s carbon; 6 protons, 6 neutrons) – – 13C (1.11% of Earth’s carbon; 6 protons, 7 neutrons)
• It is more metabolically efficient to use the lighter atom (it weighs less), so livingorganisms tend to be enriched in 12C