Evolving Earth and Life Midterm I

Book of Genesis

Biblical account of creation and the Flood. Early naturalists used it as a framework for explaining rock layers and fossils.

The Flood

The Noachian Deluge. In the lecture's "one interpretation," 40 days of rain, then 150 days of flooding, then the waters receded and the earth dried.

Flood rocks

Under a strict literal reading, every rock layer deposited during the Flood.

Pre-Flood rocks

Also called "original creation rocks." They are the only rocks that formed before the Flood and so lie beneath the Flood deposits.

Sacred Theory of the Earth

Thomas Burnet's 1681 book, which explained Earth's geology as the result of the Deluge.

Catastrophism

The view that Earth's features came from sudden, large-scale events rather than slow processes.

Original (diluvial) catastrophism

The earliest form. A single worldwide flood created the rock layers, and the past could not be reconstructed from present-day processes.

Huttonian uniformitarianism

Hutton's view that nature's operations are "equable and steady." Current historical research suggests Hutton did not believe in uniform rates.

Episodic catastrophism

The view that there were multiple floods or catastrophes, with the Biblical Flood being the last. Cuvier is the key figure.

Lyellian uniformitarianism

Processes are unchanged and rates are slow and unchanged. Geologic history is "cumulative slow change at relatively constant rates."

Substantive uniformitarianism

The claim that rates of processes were uniform throughout geologic history, which is false.

Methodological uniformitarianism

The claim that the physical laws governing geologic processes are unchanged. It is an assumption in all sciences.

Actualism

The modern, modified form of Lyellian uniformitarianism. Present-day processes, slow or rapid, explain the past, without assuming constant rates.

Neo-catastrophism

Sudden, high-magnitude events (volcanic eruptions, large regional floods, asteroid impacts, sudden burial) are part of geologic history. It is a concept within modern uniformitarianism, not a return to old catastrophism.

Thomas Burnet

Wrote Sacred Theory of the Earth (1681). He is one of the "original catastrophists," and held that the Earth's surface was ruined by the Flood.

James Hutton (1726-1797)

Argued for deep time and uniformitarianism ("the present is the key to the past"). Siccar Point was a key locality for his thinking.

George Cuvier (1769-1832)

Worked at the Natural History Museum in Paris, where he studied fossil vertebrates. He used evidence from the Paris Basin to argue for episodic catastrophism and extinction.

Charles Lyell (1797-1875)

Published Principles of Geology (1830). He popularized uniformitarianism with slow, unchanging rates.

1830

Publication of Volume 1 of Lyell's Principles of Geology, subtitled "An attempt to explain the former changes of the Earth's surface by reference to causes now in operation."

Siccar Point, Scotland

Hutton's unconformity.

- Tilted Devonian layers sit on top of vertical Silurian beds.

- It was one of the localities that led Hutton to conclude the Earth had a very long history

Paris Basin, France

Rocks that alternate between marine and freshwater deposits, which Cuvier used to argue for cyclic inundations by the sea.

What does the nature of the Flood imply about the time frame and conditions under which Flood rocks formed?

- With the exception of pre-Flood rocks, all rock layers formed within 314 days while underwater (150 days of flooding plus the remaining days to dry, which would add up to 314)

- No slow-forming rocks can exist, meaning rocks that take longer than 314 days to form.

- No terrestrial rocks can exist, because everything formed under floodwaters.

- No freshwater rocks can exist, because everything formed under saline water.

- The rock record shows slow-forming, terrestrial, and freshwater rocks, so this interpretation conflicts with the evidence.

Explain Thomas Burnett's idea about the natural processes used by God to cause the Flood.

- Burnet's Sacred Theory of the Earth (1681) explained the Flood through natural processes. His diagram shows the early Earth as layered: solid interior, water, an oily layer, and a dusty atmosphere that "turns into land."

- During the Deluge, the original surface broke up. Today's oceans and continents are the leftovers of that collapse.

- The lecture summarizes his view as "today's geology is the result of the ruinous nature of the deluge."

- Burnet believed natural truth and religion could not conflict: "God is not divided against himself."

- This undercuts the common misconception that early naturalists rejected empirical study because of religion.

What philosophical conclusions did the original catastrophists draw from their approach to explaining how rocks formed?

- Other original catastrophists were John Woodward (surface dissolved by Flood, 1695) and William Whiston (comet caused Flood, 1696).

- They envisioned processes that are no longer observable and assumed the historicity of Genesis events, but did not feel the need to take it all literally.

- They concluded that a study of present-day processes would not lead to valid conclusions about Earth history.

- Gohau's quote sums it up: the past is contingent and cannot be reconstructed from natural law and the present state alone.

What did Hutton mean by "the present is the key to the past"?

- Present-day processes can be used to interpret rocks formed long ago.

- His supposition was that nature's operations are "equable and steady," so processes are unchanged. Rates are also traditionally said to be unchanged, which the lecture questions.

- The emerging consensus is that Hutton did NOT believe in uniform rates. He accepted both gradual change and violent events.

How did Siccar Point influence Hutton's thinking about geology?

- The site shows Devonian layers resting at an angle on Silurian layers that stand vertical.

- Hutton reconstructed the sequence of events:

1) Silurian sediments were deposited horizontally at the sea bottom.

2) They were violently folded and tilted to vertical, then uplifted.

3) The raised rocks were eroded.

4) They were submerged again, and Devonian sediments were deposited on the eroded surface.

5) Everything was uplifted and tilted again, then partly eroded, leaving the exposure.

- This sequence requires repeated cycles of deposition, uplift, and erosion. These processes are slow, so he concluded the Earth had a very long history: deep time.

How did episodic catastrophism differ from original catastrophism?

- Number of events: Original catastrophism had a single worldwide Flood. Episodic catastrophism had multiple floods or catastrophes, with the Biblical Flood the last. Naturalists no longer held to a literal reading of Noah by the late 1700s.

- Philosophical approach: Original catastrophism started from Genesis and assumed the story's historicity. Cuvier's catastrophism was inferred from evidence in the rock record, which is an empirical approach.

Explain the nature of the rocks in the Paris Basin and how this led Cuvier to conclude that catastrophism was episodic and that organisms could go extinct.

- The Paris Basin rocks alternate between marine and freshwater deposits, which showed cyclic inundations by the sea.

- Cuvier argued the inundations were sudden because of:

- the disappearance of consecutive faunas, so he accepted extinction

- sudden breaks between freshwater and marine rocks

- thick layers of boulders and cobbles

- frozen mammoths in Siberia

- Repeated inundations meant multiple events, hence episodic. Different faunas in successive layers that vanish and don't return implied extinction.

How were "frozen mammoths" and their popular but incorrect depiction evidence for the catastrophists?

- Cuvier cited Siberian mammoths "encased in ice and preserved with their skin, hair, and flesh down to our times."

- He argued they were caught in the last sudden inundation of the land. For their flesh to be preserved, the event must have been rapid.

- The popular depiction was of animals flash-frozen in a single sudden event. The question's wording says this depiction was incorrect, though the slides don't explain why, so add details from class if you have them.

Based on modern re-readings of Hutton, how was Lyell's uniformitarianism subtly but significantly different?

- Hutton (as reread): Processes are unchanged, but he recognized violent events (strata "violently bent, broken and removed"). He did not necessarily insist on uniform rates.

- Lyell: Processes are unchanged and rates are slow and unchanged, with change "cumulative, slow... at relatively constant rates."

- Lyell's version is what modern "creationists" claim dominated the 19th and 20th centuries. The lecture says it was criticized immediately after publication.

- The related misconception is that naturalists accepted the catastrophic Deluge until Lyell. In fact, by the late 1700s most had moved away from a literal Noah.

How does neo-catastrophism differ from the original forms of catastrophism?

- It is part of modern uniformitarianism (methodological uniformitarianism), not a rejection of it.

- It accepts that sudden, high-magnitude events occurred (volcanic eruptions, large regional floods, asteroid impacts, sudden burial) but assumes the same physical laws and processes as today.

- It is based on evidence, not on a literal reading of Genesis.

- It does not claim a single global event made the rock layers. The evidence still shows that much of the rock record reflects slow, gradual, non-catastrophic events.

-The lecture's summary is that both gradual processes and sudden events are important.

Nebular Theory of Planet Formation

A rotating disk (nebula) of gas and dust surrounds a central ball that becomes dense and hot enough for fusion to begin, forming the Sun. Dust (solid particles) condenses in the surrounding rings and goes on to form the planets.

Planetesimal

A small body that forms as dust condenses in the disk. Planetesimals grow by continuous collisions, and gradually an irregularly shaped proto-Earth develops. Its interior heats up and melts.

Theia

The Mars-sized planet that collided with the early Earth. Material orbiting Earth after the destruction coalesced to form the Moon.

Trans-Neptunian objects

Objects orbiting beyond Neptune. The dwarf planets Pluto, Haumea, Makemake, and Eris are trans-Neptunian, and their orbits are tilted out of the ecliptic plane.

Mantle

The outer layer of the differentiated Earth, surrounding the core.

Core

The center of the differentiated Earth, made of liquid iron (Fe) and nickel (Ni).

Differentiation

Within the first tens of millions of years, the early Earth, a mixed mass of molten material, separated chemically and physically into layers of different densities and compositions. This produced the core and mantle.

Axial tilt

The angle of a planet's rotation axis relative to its orbit. The slides compare Mercury (0.1°), Venus (177°), Earth (23°), Mars (25°), Jupiter (3°), plus Saturn, Uranus, and Neptune. Earth's Moon does not orbit in Earth's equatorial plane.

Accretionary bombardment

The asteroids and meteorites that struck the young Earth, helping build the planet while its surface was molten seas of lava.

Asteroids

Rocky bodies. One of two proposed extraterrestrial sources of Earth's water, and the objects behind the Late Heavy Bombardment.

Comets

Icy bodies. The other proposed extraterrestrial source of Earth's water.

Outgassing

Release of gases, including water vapor, from the interior through volcanism. The slides state that much of Earth's water came from volcanic outgassing, in contrast with the extraterrestrial (comet and asteroid) view.

Late Heavy Bombardment (LHB)

A dramatic increase in asteroid bombardment about 4.0 to 3.8 billion years ago. It is inferred from impact melt rocks on the Moon.

Nice model

A solar system model introduced in 2005 in three Nature articles. The gas giants did not form in their present orbits but migrated there.

Nuvvuagittuq Greenstone Belt

A site on the east side of Hudson Bay (Porpoise Bay) with some of the oldest crustal rocks on Earth.

Age of the Earth

4.57 billion years. This is the age of the oldest meteorites and the start of planet formation.

Oldest Moon rocks

4.51 billion years. The Moon had formed by then.

Oldest mineral grain

4.40 billion years. A zircon-type grain dated at about 4404 Ma, showing some crust was present.

Oldest crustal rocks

4.32 billion years. These are the Nuvvuagittuq rocks (4.321 by), which show Earth had a solid crust and oceans.

Late Heavy Bombardment

4.00 to 3.80 billion years ago.

Which four dwarf planets have orbits that are unusual compared to all other planets in the solar system?

- Pluto, Haumea, Makemake, and Eris, the trans-Neptunian dwarf planets.

- All planets (and Ceres) orbit nearly within a single plane, the ecliptic plane. These four are tilted out of it. Eris is the most extreme, and Pluto the least.

- Something must have happened to move these objects out of the ecliptic plane. The Nice model offers an explanation.

- Background: a planet is defined as an object that orbits the Sun, is large enough to be a spheroid, and has cleared its orbit. A dwarf planet orbits the Sun, is a spheroid, has not cleared its orbit, and is not a moon.

What are three unusual facts about our Moon that any hypothesis of its formation must explain?

- It does not orbit in Earth's equatorial plane. The Moon's orbital plane is tilted relative to Earth's axis.

- It is unusually large. The ratio of the Moon's mass to its planet's mass is far higher than for any other planet and its largest moon.

- Earth's Moon is the fifth largest moon in the solar system and is "planet-like" (planetoidal) rather than small and irregular.

- It is greatly depleted in iron. It may even lack a metallic core altogether, whereas Earth has a large iron-nickel core.

Describe the four hypotheses for the origin of the Moon. How have three been falsified, except the impact hypothesis?

Fission: A piece of the molten early Earth spun off to form the Moon.

- Orbit plane: NO. Size: YES. Iron depletion: YES.

- Falsified because it fails to explain the Moon's orbital plane.

Capture: Earth's gravity captured a body formed elsewhere in the solar system.

- Orbit plane: YES. Size: NO. Iron depletion: NO.

- Falsified because it fails to explain the Moon's large size or its iron depletion.

Co-formation: The Moon formed alongside Earth from the same disk material.

- Orbit plane: NO. Size: YES. Iron depletion: NO.

- Falsified because it fails to explain the orbital plane and iron depletion.

Collisional (giant impact): Earth collided with a Mars-sized planet, Theia. Debris orbiting Earth coalesced into the Moon.

- Orbit plane: YES. Size: YES. Iron depletion: YES.

- This is the only hypothesis that explains all three facts, and it is the current scientific consensus. Computer simulations by Robin Canup (Southwest Research Institute, a Duke physics B.S.) model the collision with iron/nickel cores and mantles for both Earth and Theia.

How does the "Nice model" re-envision the formation of our early solar system?

- It still envisions a nebular disk, but the gas giants did NOT form in the orbits where they are today. They migrated there.

- Published in 2005 in three Nature articles, covering the origin of the Late Heavy Bombardment, the chaotic capture of Jupiter's Trojan asteroids, and the origin of the giant planets' orbital architecture.

- It is becoming widely accepted because it explains:

- the Late Heavy Bombardment

- peculiarities in the orbits of Neptune and Uranus

- the presence of Jupiter's Trojan asteroids

- the scattered nature of the Kuiper Belt objects

- It also fits the question of what moved the trans-Neptunian dwarf planets out of the ecliptic plane.

- The Nature abstracts shown add that planets migrated by exchanging angular momentum with a disk of planetesimals, and that Jupiter and Saturn crossing their 1:2 orbital resonance was a key trigger.

How are the rocks of the Nuvvuagittuq Greenstone Belt evidence for both an early ocean and significantly thick crust early in Earth's history?

Early ocean:

- Their chemical signature implies they formed as seafloor basalts underwater.

- Their underwater origin is further supported by banded iron formations in the same rocks.

- At 4.32 billion years, this means liquid water oceans already existed. This fits the story that the Earth cooled after the Moon formed, and it rained for the first time.

Thick crust:

- The age (4.321 by) is a metamorphic age.

- The grade of metamorphism implies pressures equal to about 16 km of overlying rock. Rock must have been buried that deeply, so substantial crust must have existed already.

Together with the oldest mineral grains (4.40 by) and the oldest Moon rocks (4.51 by), this supports the "developing consensus" timeline of a cool early Earth with a solid crust and oceans well before the end of the LHB.

Lithification

The process by which loose sediment, such as river gravel, sand, and mud, becomes solid sedimentary rock. Sediments are compacted and cemented together by minerals that precipitate between the grains.

Pyrite

An iron sulfide mineral with the chemical formula FeS2, commonly called fool's gold because of its metallic, gold-like appearance. Pyrite cannot form in the presence of oxygen and rapidly rusts away under oxygenated conditions. Its preservation in ancient river deposits therefore provides evidence that early Earth had little or no free oxygen.

Fool's gold

The common nickname for pyrite (FeS2). It resembles gold but is chemically different. In this lecture, its importance is that pyrite can survive in oxygen-free environments but is rapidly oxidized in oxygenated environments.

Pyrite cement

Pyrite mineral material that precipitated between sediment grains, binding them together into sedimentary rock. The presence of pyrite cement in ancient river sediments indicates that the cement formed under anoxic conditions, because oxygen would have oxidized the pyrite.

Pyrite conglomerates

Sedimentary rocks made of rounded pebbles and other coarse sedimentary clasts cemented together, with pyrite present as the pebbles and/or cement. These ancient river deposits are evidence that river water and the surrounding environment lacked free oxygen; otherwise, the pyrite would have rusted away.

Anoxic

Describes an environment with no free, dissolved molecular oxygen (O2). The early Earth's atmosphere and oceans were initially anoxic. Anoxic does not mean there were no oxygen atoms in compounds such as water or carbon dioxide.

Oxygenated

Describes an environment containing free molecular oxygen (O2). As photosynthetic microbes released oxygen, it first reacted with dissolved iron in the oceans. After enough iron had been oxidized, oxygen could accumulate in ocean water and eventually enter the atmosphere.

Banded iron formations (BIFs)

Extensive sedimentary deposits made primarily of alternating layers of silica-rich material and iron oxides. The lecture identifies three major mineral constituents: jasper (a form of quartz, (SiO2), hematite (Fe2O3), and magnetite (Fe3O4). They formed when oxygen produced by photosynthesis reacted with abundant dissolved iron in the early oceans, causing iron oxides to precipitate. BIFs are a major record of the changing oxygen conditions in Earth's early oceans.

Red beds

Terrestrial sedimentary rocks and sediments that are red because iron-bearing minerals have been oxidized, or "rusted," in an oxygen-rich environment. The oldest red beds record the first significant accumulation of free oxygen in Earth's atmosphere and the oxidation of the land surface.

Cyanobacteria

Photosynthetic microbes that were among Earth's earliest oxygen-producing organisms. They used photosynthesis to produce oxygen, helping drive the oxidation of dissolved iron in the oceans and eventually contributing to the rise of atmospheric oxygen.

Stromatolites

Layered structures formed as sticky cyanobacterial mats trap and bind sand and mud. Sediment buries the cyanobacteria, which then grow upward toward sunlight. Repetition of this cycle produces layer upon layer of sediment. Stromatolites are evidence of abundant ancient microbial life and photosynthetic activity.

Great Oxygenation Event (GOE)

2.5-2.3 billion years ago (bya). This is the period when free oxygen began accumulating significantly in Earth's atmosphere. Connected to the development of oxygen-producing photosynthesis and the eventual oxidation of Earth's surface.

Shark Bay, Hamelin Pool, Australia

A modern environment where living cyanobacteria build stromatolites. It provides an example of how microbial mats trap and bind sediment to produce layered structures similar to ancient fossil stromatolites.

How do we know what kinds of gases were present in the early atmosphere?

We infer the composition of Earth's early atmosphere from geological evidence preserved in ancient rocks and sediments.

- The earliest atmosphere contained gases released during Earth's formation and volcanic activity, including water vapor, carbon dioxide, nitrogen, carbon monoxide, and hydrogen sulfide.

- Hydrogen and helium are extremely light and readily escaped into space.

- Pyrite deposits indicate an absence of free oxygen because pyrite rapidly oxidizes in oxygenated environments.

- Banded iron formations (BIFs) record oxygen produced by photosynthetic organisms reacting with dissolved iron in the oceans.

- Red beds indicate that free oxygen eventually accumulated sufficiently in the atmosphere to oxidize iron-bearing terrestrial rocks and sediments.

Rocks preserve chemical evidence of the environmental conditions under which they formed.

Why is the most abundant sediment and clasts inside rivers made mostly of quartz?

Quartz is highly resistant to chemical weathering compared with most other common rock-forming minerals.

The six major primary rock-forming minerals include quartz, feldspar, mica, amphibole, pyroxene, and calcium carbonate. As rocks weather, their minerals do not all survive equally well:

1) Feldspar, mica, amphibole, and pyroxene can break down chemically during weathering.

2) Many of their constituent elements are released into solution or incorporated into new minerals.

3) Quartz is much more resistant to chemical weathering and can survive transport by rivers.

4) Over time, the repeated weathering, transport, and sorting of sediment enrich river deposits in quartz grains and quartz-rich rocks called quartzites. Modern river gravels are dominated by durable quartz-rich material, whereas metal-rich minerals such as pyrite are generally not preserved in oxygenated rivers. Ancient river conglomerates containing pyrite therefore reveal that the environmental conditions were very different in the Precambrian.

What are the six most common rock-forming minerals?

1. Quartz (SiO4)

2. Feldspar (K,Ca,Na) AlxSiyO8

3. Micas (K(Mg,Fe)3AlSi3O10(F,OH)

4. Amphiboles (K,Na)x(Ca,Na,Fe,Mg)2(Mg,Fe,Al)5(Al,Si)8O22}(OH)2

5. Pyroxenes (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6

6. Calcium carbonate CaCO3

How are pyrite cements and pyrite conglomerates evidence that the early atmosphere was anoxic?

Pyrite (FeS2) cannot form in the presence of oxygen, and it rapidly rusts away in oxygenated environments.

There are two important pieces of evidence:

- Pyrite pebbles in river conglomerates: Rounded pyrite pebbles demonstrate that pyrite survived erosion and transport by flowing river water. If the rivers had contained free oxygen, the pyrite would have oxidized.

- Pyrite cement between sediment grains: Pyrite cement formed between river sediments and bound the grains together. Its preservation indicates that the surrounding environment lacked free oxygen when the cement formed.

Pyrite is evidence of anoxia because oxygen destroys the very mineral being preserved.

What are the three most common cements found in sedimentary rocks today?

1) Quartz: silica-rich material that precipitates between grains.

2) Calcite: calcium carbonate, (CaCO3).

3) Iron oxides: iron-bearing oxide minerals, represented generally as FexOy

Cement is mineral material that precipitates in the spaces between sediment grains, effectively gluing them together during lithification. These cements differ from the sediment grains themselves: the grains are deposited first, and cement later binds them into solid rock.

What are the three major mineral constituents of an average banded iron formation?

1. Jasper (SiO2): a silica-rich material and a form of quartz. It forms the silica-rich layers in many banded iron formations.

2. Hematite (Fe2O3): an iron oxide mineral formed when iron is oxidized.

3. Magnetite (Fe3O4): another iron oxide mineral that makes up part of the iron-rich layers.

BIFs consist of alternating silica-rich and iron oxide-rich layers. They are widespread, can be enormous deposits, and formed throughout much of early Earth history before their formation stopped around 1.9 billion years ago.

Explain how both pyrite cements/conglomerates AND banded iron formations could form at the same time, even though the former requires anoxic conditions, whereas the latter required oxygen to form.

The key is that the oxygen was initially produced and consumed locally in the oceans, while the atmosphere and many other environments remained anoxic.

The process occurred in stages:

Stage 1: Anoxic early Earth

The early atmosphere and oceans lacked free oxygen. Underwater volcanoes supplied large quantities of dissolved iron to the oceans.

Stage 2: Photosynthesis produces oxygen

Cyanobacteria and other early photosynthetic microbes began producing oxygen. However, this oxygen reacted rapidly with dissolved iron in the surrounding seawater.

Stage 3: BIFs form in the ocean

Oxygen oxidized dissolved iron, causing iron oxides to precipitate and accumulate alongside silica. This produced banded iron formations even while oxygen was being consumed and the wider environment remained anoxic.

Stage 4: Pyrite survives in oxygen-poor environments

Because oxygen was consumed by iron in the ocean, it did not immediately accumulate throughout the atmosphere or in all rivers. Anoxic rivers could still preserve pyrite pebbles and pyrite cement while BIFs formed elsewhere.

Eventually, as the available dissolved iron was depleted, oxygen could remain unconsumed and accumulate in the oceans and atmosphere.

What significant event do the oldest red beds record in our planet's history?

- The oldest red beds record the emergence of an oxygen-rich, oxidizing atmosphere and the beginning of widespread oxidation of Earth's land surface.

- Before sufficient free oxygen accumulated, iron-bearing terrestrial rocks and sediments generally did not undergo the same oxidative "rusting" seen today. As oxygen began building up in the atmosphere, iron in exposed rocks and sediments reacted with oxygen, producing iron oxides that give red beds their characteristic color.

- This event is associated with the Great Oxygenation Event, approximately 2.5-2.3 billion years ago, as listed in the review sheet.

- Timeline connects the development of atmospheric oxygen to the earlier production of oxygen by photosynthetic microbes and its initial consumption by dissolved iron in the oceans.

How is it possible that banded iron formation continued to form in the ocean for nearly 400 million years longer than pyrite cements/conglomerates in fluvial environments?

Banded iron formations continued to form in deeper ocean water even after oxygen began accumulating in the atmosphere and surface ocean. The transition to an oxygenated Earth did not happen uniformly:

Surface ocean: Photosynthetic organisms produced oxygen, and eventually oxygen began accumulating in surface waters.

Atmosphere: Once enough oxygen was no longer being consumed by dissolved iron, it could diffuse out of the oceans and accumulate in the atmosphere.

Deep ocean: Deeper water could remain anoxic even while the surface ocean and atmosphere became oxygenated. Iron could therefore continue to accumulate in deep water and form BIFs.

This explains the roughly 400-million-year overlap between the development of oxygen-rich surface environments and continued BIF formation in deeper waters. BIF formation stopped around 1.9 billion years ago, while atmospheric oxidation and red-bed formation had already begun.

How do stromatolites form? Why are they found only in restricted environments today?

Stromatolites form through repeated cycles of microbial growth, sediment trapping, burial, and regrowth.

Step 1: Trap and bind sediment. Cyanobacteria form sticky microbial mats that capture and bind sand and mud.

Step 2: Become buried. Additional sand and mud settle over the microbial mats.

Step 3: Grow back toward sunlight. The cyanobacteria grow upward through the sediment to reach sunlight again.

Step 4: Repeat the cycle. Over time, successive layers of trapped sediment build up into a laminated stromatolite.

Why are they restricted to certain environments today?

- Shark Bay, Hamelin Pool, Australia, is a modern location where living cyanobacteria build stromatolites. - Cyanobacteria were extremely abundant between 2.5 and 0.7 billion years ago, when stromatolites were widespread.

A key explanation for their restricted distribution today is that many modern environments contain grazing animals and competing organisms that disturb or consume microbial mats. In unusual environments such as the hypersaline waters of Hamelin Pool, these grazers are limited, allowing microbial mats to grow and build layered structures.

Iron and nickel

The two principal metallic elements identified in the lecture as making up Earth's core. The core lies beneath the mantle and is divided into an outer core and inner core.

Ultramafic

Igneous rock or magma extremely rich in magnesium and iron and relatively poor in silica. The early Earth's mantle was much hotter than today's mantle, allowing ultramafic magma to reach the surface and form komatiite.

Mafic

Igneous rock or magma rich in magnesium and iron, but generally less enriched in these elements than ultramafic rocks. Basalt is the key example in the lecture. As Earth cooled, high-melting-point magnesium- and iron-rich minerals crystallized and remained behind in the mantle, allowing the magma that reached the surface to become mafic.

Intermediate

Igneous rock or magma with a composition between mafic and silicic. Intermediate magma can develop as magma evolves in subduction-zone volcanic arcs. Andesite is a representative intermediate volcanic rock.

Silicic

Igneous rock or magma relatively rich in silica and poorer in magnesium and iron than mafic magma. Silicic compositions can develop as magma evolves through processes such as fractional crystallization. Dacite is an example shown in the lecture's arc-evolution discussion.

Core

Earth's innermost major structural layer, composed primarily of iron and nickel. It consists of a liquid outer core and a solid inner core.

Mantle

The thick rocky layer between the core and crust. The lecture emphasizes that the early mantle was much hotter than the modern mantle and produced ultramafic melts. The mantle includes lithospheric and asthenospheric portions.

Crust

Earth's outermost solid rocky layer. Early crust was predominantly mafic and continental crust developed through subduction, magma evolution, and the assembly of smaller crustal blocks.

Partial melting

The process in which only some minerals or components of a source rock melt. Because different minerals melt at different temperatures and under different conditions, the resulting magma has a different composition from the original rock. Different degrees of partial melting can produce different magma compositions.

Fractional recrystallization / fractional crystallization

The process in which minerals crystallize from cooling magma at different temperatures. If some crystals are separated from the remaining melt, the melt's composition changes. In the magma-chamber example, dense, magnesium- and iron-rich minerals crystallize first and sink, leaving the upper magma relatively depleted in those elements and more felsic.

Magma assimilation

The incorporation and melting of surrounding rock into magma. This changes the magma's original chemical composition. Together with partial melting and fractional crystallization, assimilation helps explain the wide range of igneous rock compositions.

Komatiite

An ultramafic volcanic rock formed when very hot ultramafic lava reached Earth's surface and cooled. The lecture uses komatiite as evidence that the early mantle was much hotter than today's mantle.

Flux melting

Melting caused when volatile substances, especially water, are added to hot mantle rock and lower its melting temperature. At subduction zones, water released from the descending plate enters the overlying mantle and promotes melting, generating magma that can feed volcanic arcs.

Proto-plates

Small, early pieces of lithosphere carrying mafic crust. The lecture proposes that the early lithosphere was broken into many small proto-plates rather than consisting of the large modern plates we see today.

Drip-tectonics (vertical tectonics)

A proposed early-Earth process in which dense portions of crust or lithosphere become unstable and sink or drip downward into the mantle. This differs from horizontal movement and collision of large plates in modern-style plate tectonics. It's an alternative model for early crustal evolution.

Island arcs

Chains of volcanoes formed above subduction zones, typically where one oceanic plate descends beneath another. Arc magmas evolves from initially mafic compositions toward intermediate and silicic compositions, helping build continental crust.

Microcontinents

Small blocks of continental crust that can collide with and become attached to larger continental masses. During the Proterozoic, microcontinents and island arcs were accreted to the margins of older Archean crustal blocks.

Subduction

The process in which one lithospheric plate descends beneath another into the mantle. Water released from the descending plate promotes flux melting in the mantle above it, producing magma and volcanic activity.

Microcontinental accretion

The process by which small continental blocks are added to the edges of older continental crust. Repeated accretion of microcontinents and island arcs enlarged early crustal blocks into larger continental masses.

Archean

A major interval of Precambrian time preceding the Proterozoic. The Archean is associated with a much hotter mantle, predominantly mafic crust, early proto-plates, and the development of some of Earth's oldest preserved crust.

Proterozoic

The interval following the Archean and preceding the Phanerozoic. During this interval, larger continental cratons developed as microcontinents and island arcs were added to older Archean crust.