The Archean Earth

Question 1

What is the upper and lower ages of the boundaries to the Archean?

Answer 1

4000Ma-2500Ma

Question 2

What defines the start of the Archean? Why are rocks formed before 3.85Ga so rare?

Answer 2

The emergence of the first rocks that still exist today. Heavy bombardment, tectonic activity and rare preservation conditions, such as cratons, are the reasons here are so few rocks from the Early Archeamn

Question 3

What key events took place during the Archean?

Answer 3

Rock record begins, most continental crust formed (60-70%), first continent stabilised, tectonic style of Earth evolved, thick sequences of sediments were deposited in the deep oceans, the first life appeared and enigmatic iron-rich sedimentary rock was formed, which hasn’t been formed since.

Question 4

What proportion of Earth history does the Archean represent?

Answer 4

A third

Question 5

What is the age and name of the oldest rocks?

Answer 5

Name: Acasta gneiss, which is 4.03Ma

Question 6

What is the age and name of the oldest sedimentary rocks?

Answer 6

Name: isua supracrustal belt, 3.8Ga

Question 7

What is the age of the oldest macroscopic fossils?

Answer 7

From the Ediacaran, 575-560Ma, these are earliest multicellular life that can be seen without a microscope

Question 8

What is a craton? Where are the Archean cratons located?

Answer 8

A large, stable block of the Earth’s crust that forms the ancient core of the continent. They are found on every continent.

Question 9

When was the period of most rapid growth of the crust?

Answer 9

3-2.5Ga, during the major crustal accretion (or Archean crustal growth peak)

Question 10

What are the key differences in tectonics from the end of the Hadean to the end of the Archaean?

Answer 10

Hadean (4.54–4.0 Ga): Proto-Tectonics
Dominant Processes:
Magma Ocean- During the early Hadean, the Earth’s surface was dominated by a magma ocean, preventing traditional tectonic processes.
Crust Formation- The crust was thin, unstable, and frequently recycled into the mantle due to extreme heat and intense asteroid bombardment.
Heat Transfer- Heat was primarily lost through vertical tectonics, involving upwelling plumes and gravitational sinking of dense crust, rather than horizontal plate movements.
The Late Heavy Bombardment (~4.1–3.8 Ga)- disrupted early crust formation and erased much of the Hadean geological record.

Late Archaean (3.0–2.5 Ga): Transition to Modern-Like Tectonics

Emergence of Modern Plate Tectonics:
Evidence for true subduction zones and plate collisions becomes more widespread. Tectonic features like greenstone belts and granite-gneiss complexes reflect convergent margin processes.
Many cratons formed during this period and became tectonically stable, providing the foundation for modern continents. Cratonization involved thickening of the crust and isolation from mantle convection processes.
As the Earth’s mantle cooled, magmatic activity and heat flow declined, slowing crustal growth.
Horizontal plate movements became more organized, with evidence of collisional orogenies (mountain-building) and the first continental supercrustal blocks.

Question 11

What was the tectonic style of the early Earth before plate tectonics began? What role did mantle plumes play?

Answer 11

Stagnant-lid tectonics, where the lithosphere acted as a single, rigid shell without large-scale lateral plate motions. Heat-driven mantle convection caused intense upwelling (mantle plumes) and downwelling (delamination), recycling crust back into the mantle. Small, unstable crustal fragments formed (proto-continents) and were frequently destroyed due to high mantle heat flow. Crust recycling occurred via gravitational sinking of dense lithospheric material, not modern-style subduction.

Role of Mantle Plumes:
Mantle plumes supplied magma to the surface, forming the first crust through extensive volcanism and contributing to the growth of early felsic continental material.
Created doming, rifting, and transient crustal fragments, acting as the primary drivers of surface dynamics.
Persistent plume activity, combined with crustal thickening, likely weakened the stagnant lid, eventually initiating horizontal movements and proto-plate tectonics.

Question 12

When did plate tectonics start?

Answer 12

Debated but between 3.2 and 2.5Ga during the late Archean Eon

Question 13

What evidence is used to indicate the presence of plate tectonics?

Answer 13

Mountain Belts: Linear chains of deformed rocks (e.g., Himalayas, Alps) formed by plate collision or subduction.
Faults and Rift Zones: Large-scale faults (e.g., San Andreas Fault) and rift systems (e.g., East African Rift) indicate plate boundaries and motions.
Granite-Gneiss Complexes: Formed in settings involving crustal thickening, subduction, or collision.
Magnetic Anomalies on the Ocean Floor: Symmetrical patterns of magnetic stripes on either side of mid-ocean ridges record seafloor spreading.
Isotope Signatures: Hafnium, oxygen, and neodymium isotopes in ancient zircons and rocks suggest crustal recycling and subduction-like processes.
Growth of Cratons: Stabilization of continental nuclei (cratons) with evidence of subduction zones around their margins.
Presence of TTG Suites (Tonaloite-Trondhjemite-Granodiorite): Derived from partial melting of subducted oceanic crust in early tectonic settings.

Question 14

How did the first protocontinents form in the early Archaean? What was the crust made of?

Answer 14

Partial melting of the Earth’s basaltic crust in a hotter mantle environment. This process created small, buoyant crustal fragments composed of felsic rocks. Accretion of felsic material led to thicker, buoyant crustal fragments, which were more stable and resistant to recycling into the mantle.

Protocontinental Crust: Dominated by TTG rocks derived from basaltic crust. These rocks were similar to modern granitoids but lacked potassium-rich components.
Underlying Crust: Predominantly mafic and ultramafic, consisting of basalt and komatiite, remnants of Earth’s early oceanic crust.

Question 15

What process drove continental collision and accretion of land masses in the mid Archean in the absence of subduction?

Answer 15

Vertical tectonics and gravitational processes rather than modern subduction-driven plate tectonics e.g. mantle plume activity, crustal overturns, gravitational accretion and impact events

Question 16

What evidence is used to indicate the onset of subduction?

Answer 16

Rocks like blueschists (rare before 2.0 Ga) and eclogites indicate subduction of cold oceanic crust into the mantle as they are metamorphic and formed in low temperature high pressure environments. Archean Greenstone belts often show rock assemblages resembling modern volcanic arcs, such as metamorphosed basalts and ultramafic rocks. TTG (Tonalite-Trondhjemite-Granodiorite) are felsic rocks, formed by partial melting of subducted basaltic crust, are widespread in Archaean terranes.

Question 17

How did early subduction differ to subduction today?

Answer 17

Early oceanic crust was thinner and less rigid, making it more buoyant and prone to episodic subduction rather than continuous, stable plate descent.
Higher mantle temperatures led to partial melting of the subducting slab at shallower depths, preventing deep penetration into the mantle.
Subduction zones were less stable, frequently collapsing or switching locations due to intense mantle convection and lithospheric weakness.
Subduction was influenced more by mantle plumes and gravitational delamination than by modern-style slab pull. Subducting plates melted more readily, generating abundant felsic magmas (e.g., TTG suites).
High mantle heat flow prevented the preservation of cold, high-pressure rocks like blueschists, which are common in modern subduction zones.
Early subduction zones were likely smaller and more localized, forming in isolated regions rather than long, continuous convergent margins.

Question 18

When did the first supercontinent form in the Archaean and what is it called?

Answer 18

2.7–2.5 billion years ago (Ga). This supercontinent is often referred to as Kenorland.

Question 19

What is an ironstone?

Answer 19

A type of sedimentary rock that is primarily composed of iron minerals, typically iron oxides such as hematite (Fe2O3) or goethite (FeO(OH)). Forming as a result of the precipitation of iron from water.

Question 20

How old are Banded Iron formations?

Answer 20

3.8Ga

Question 21

What conditions does the formation of Banded Iron Formations (BIFs) require?

Answer 21

Oxygenation Events: The periodic introduction of oxygen into the oceans, likely from early photosynthetic organisms, would oxidize dissolved iron to Fe³⁺, leading to the precipitation of iron oxides like hematite and goethite.
Shallow, Stratified Water Bodies: BIFs typically formed in shallow, stagnant, or stratified water bodies, such as shallow marine basins or lakes, where the upper layer became oxygenated and caused the precipitation of iron minerals, while deeper layers remained anoxic.

Question 22

What are two proposed sources of reduced iron in the oceans?

Answer 22

Volcanic eruptions release significant amounts of iron, particularly in the form of iron-rich gases and particles from submarine volcanic activity. These iron-rich compounds could dissolve in seawater as Fe²⁺ (reduced iron), particularly in the absence of oxygen.
The weathering of iron-rich continental rocks, such as basalts and ferromagnesian minerals, could release Fe²⁺ into rivers that carried it to the oceans. In an anoxic ocean, this dissolved iron would remain in its reduced Fe²⁺ state, available for later oxidation to form iron minerals during oxygenation events.

Question 23

What is the relationship between the formation of BIFs, and changes in oxygen levels in the oceans and in the atmosphere? What is the cause of these changes?

Answer 23

Early Earth’s oceans were anaerobic, with little or no free oxygen in the atmosphere. As a result, iron (Fe²⁺) from volcanic and weathering sources could dissolve in the oceans without being oxidized.
As photosynthetic organisms began producing oxygen, it started to accumulate in the oceans. When the oxygen concentration reached a certain threshold, it began to oxidize the dissolved Fe²⁺ to Fe³⁺, causing the iron to precipitate out of the water as iron oxides like hematite and goethite.
The Great Oxidation Event (GOE), occurring around 2.4 billion years ago, marks a significant increase in atmospheric oxygen levels, leading to a decrease in the deposition of new BIFs. By this time, the oceans had become more oxygenated, and dissolved iron was oxidized in the water, preventing the continued formation of BIFs.

Question 24

What is the most common style of tectonics in the solar system?

Answer 24

Extensional, where the lithosphere is stretched and thinned. This style is observed on many planets, moons, and asteroids, often manifesting as rift zones or stretching of the crust. Extensional tectonics is common because it doesn’t require a complex, large-scale planetary mantle convection system like Earth’s plate tectonics. Instead, it can occur on relatively smaller bodies with weaker, thinner lithospheres.

Question 25

What other planet is the Archaean Earth said to be most like in terms of its geodynamics?

Answer 25

Venus