Fundamentals of Earth Science: Fossils, Tectonics, and Time

Posted on Oct 7, 2025 in Geography

Fossils and the Geological Record

Fossils are remnants of ancient life formed through fossilization, collectively creating the fossil record. This record, alongside rock and event data, helps reconstruct Earth’s geological history. The oldest fossils, around 3.5 billion years old, are a subject of scientific debate but contribute to our understanding. A distinction exists between “true” fossils, over 11,000 years old, and younger subfossils. However, a comprehensive record of all life forms is unattainable due to the limited number of organisms preserved as fossils throughout Earth’s vast history.

Types of Fossils

Three types of fossils exist: body fossils, trace fossils, and chemical fossils. They are distinguished by what parts of the original organism are preserved:

Body Fossils: The body itself (or parts thereof).
Trace Fossils: Traces of the organism’s activities.
Chemical Fossils: Specific substances combined within the organism’s body and the embedding sediments.

Body Fossils

Body fossils, the most common type, require preservation of either a part or the entirety of the organism to be classified as such. The vast majority include the ancient organism’s hard parts (e.g., shell, carapace, test, etc.), which are more easily fossilized due to their mineral nature. The soft body of an organism occasionally can be preserved if it is buried rapidly after death and is protected from scavengers. Paleontology is concerned with the study of body fossils.

Trace Fossils (Ichnofossils)

Trace fossils capture the activities of organisms (like feeding or movement), while the original organism’s body that created these traces is rarely preserved. Only the trace fossils are preserved, leading to a specialized classification and naming system. Soft-bodied organisms with low chances of becoming body fossils, like worms or insects, often leave trace fossils. Both paleontology and ichnology deal with the study of trace fossils.

Chemical Fossils

Chemical fossils are created from substances produced by an organism or found within its body, bonding with surrounding minerals. These chemical reactions might occur during the organism’s life or after its death. This kind of fossil is crucial when examining remnants of small, soft-bodied organisms like bacteria or algae. Chemical fossils play a vital role in understanding ancient life in sediments where fossilization chances are minimal. Geochemistry is involved in the study of chemical fossils.

Fossilization Processes

Common fossilization processes preserve hard organism parts while soft tissues decay or might leave an impression in the rock. Six processes fall into this category: permineralization, recrystallization, dissolution, replacement, carbonization, and metasomatism.

Permineralization

This happens in fossils with pores or cavities in their hard parts, like the porous structure found in vertebrate bones. After an organism dies and its soft tissues decay, leaving these spaces empty, fluids carrying substances like calcium carbonate flow through them. When the fluid concentration reaches a critical point, new minerals can form in these cavities, partially or completely filling them. This process preserves the original fossil material, such as phosphatic tissue in bones or woody tissue in fossil trees, while the newly added material consists only of what is precipitated in the cavities.

Recrystallization

Recrystallization is a common process leading to a partial or complete change in the mineral composition of shells after an organism’s death. For instance, mollusc shells made of aragonite transform into calcite. This change occurs due to the unstable nature of aragonite, seeking stability in the form of calcite. During recrystallization, only the mineral composition shifts while the chemical composition remains unchanged.

Dissolution

Dissolution typically happens after the sediment holding a fossil turns into rock. When a fossil shell encounters fluids within the rock pores, both the shell and fluids, often high in chemical reactivity, can cause partial or complete dissolution of the fossil debris. This results in the creation of an empty space in the rock, preserving the inner and outer characteristics of the dissolved fossil. Internal features of the shell, valve, or carapace can be preserved on the mold, which occupies the internal position. The cast, which is in the exterior position, preserves the fossil’s external features.

Replacement

Replacement occurs after fossil dissolution, if fluids with high concentrations of a mineral substance continue to flow through the layer that embeds the fossil, especially in the space left by the dissolved fossil. This process triggers the precipitation of a new mineral once a critical concentration is reached. A very spectacular and relatively frequent case of replacement is pyritization, which occurs when pyrite is precipitated.

Structural Geology and Rock Deformation

Structural geology studies rock deformation caused by external forces. Early observations by Steno linked tilted layers to crustal movements, although he proposed other explanations, like cave collapses in the Alps. Further studies highlighted its importance for understanding mountain formation and economic benefits, like predicting coal and mineral veins for mining. This field directly aids mineral and hydrocarbon exploration and helps evaluate rock stability in engineering geology for construction purposes.

Force, Stress, and Rock Response

There are two ways to evaluate the action applied to a rock: force and stress.

Forces: Represent the push or pull that results in a change in the motion of a physical body of a given mass.
Stress: Represents the amount of force per unit area.

Rocks respond differently to force and stress, depending on temperature, pressure, chemical and mineralogical composition, occurrence of fluids in their pores, etc. The rocks may change their place, position, and shape as a result of applied force or stress:

Change in Place (Displacement): Moving a rock volume from one place to another.
Change in Position (Rotation): Spinning around a center or axis, like tilting layers.
Change in Shape (Strain): Internal deformation in response to force, seen in clast elongation due to stress. For example, clast preferential elongation is a result of strain, in response to a certain stress.
No Change: Happens if the force or stress is too small to produce such an effect.

Types of Subsurface Stress

There are three types of subsurface stress caused by the weight of overlying rocks, movements in the Earth’s crust, and the presence or absence of fluids in rock pores.

Confining Stress: Is equal in all three directions of space for a certain point in the Earth’s crust; it occurs mainly in the case of rock burial.
Differential Stress: Occurs in the case of crustal movements; therefore, the amount of stress is higher in certain direction(s). There are three kinds of differential stress:
- Compression (stress pushes on a rock).
- Tension (stress stretches the rock).
- Shear (stress is applied in two opposite directions).
Fluid Pressure: Is given by the fluids in rock pores and occurs mostly in the case of sedimentary rocks; such pressure is opposite to the general stress applied on a rock in subsurface conditions, and reduces the effects of the total stress.

Rock Deformation Zones

Rock deformation varies with depth, influenced by increasing pressure and temperature gradients downwards. This creates three zones: one with brittle deformation above, another with ductile deformation below, and a transitional zone between them where both types occur.

The Brittle Deformation Zone: Found in the upper crust, occurs where rocks are weak due to low pressure values, causing them to fracture under differential stress. Fractures (joints and faults) are the dominant structures in this zone.
Ductile Deformation Zone: Occurs with depth increase. The rocks can be fractured with more difficulty due to the increased pressure and temperature; they are prone to flowing in a solid state, and deformation is the most frequent process. The transition between the two zones with different deformation characteristics is gradual.

Folds

Folds are structures that form from lateral compression of the Earth’s crust. The elements of a fold are the branches (limbs), crest, trough, axis, and axial plane.

Branches (Limbs): The two convergent or divergent sides of the fold.
Fold Crest: The apical part of the fold; the fold’s lowest point is referred to as the trough.
Fold Axis: A line defined by the points of maximum curvature.
Axial Plane: Defined by the axes that subdivide the fold into two equal parts.

Fold Classification

The most common fold classification is based on limb positions. Folds can be subdivided into: anticlines (branches converge upward) and synclines (branches converge downward). In anticlines, the oldest layers are at the fold’s center (axial part), while synclines have the youngest layers there. Another classification considers the combined positions of limbs and axial planes in relation to the horizontal plane:

Upright: Axial plane is vertical; the two limbs dip in opposite directions.
Inclined: Axial plane is inclined; the two limbs dip in opposite directions.
Overturned: Axial plane is inclined; the two limbs dip in the same direction.
Recumbent: Axial plane is horizontal; the two limbs dip in opposite directions.

Fractures (Joints and Faults)

Fractures occur in the Earth’s crust at various scales, from microscopic to regional. There are two kinds of fractures, which are recognized based on the movement of the two blocks they define:

Joints: Fractures in which the two resulting blocks present no movement with respect to each other.
Faults: Fractures in which the two resulting blocks present a significant displacement. The two blocks are named based on their position relative to the fault plane; the footwall is situated below the fault plane, and the hanging wall above it.

Fault Classification by Movement

The positions of the footwall and hanging wall are determined in relation to the fault plane’s inclination, creating a three-fold classification for faults:

Normal Faults: Hanging wall moves downward compared to the footwall (downward displacement).
Reverse Faults: Hanging wall moves upward compared to the footwall (upward displacement).
Thrust Faults: Reverse faults which dip at a low angle.

Fault Classification by Displacement Direction

Another fault classification is based on the displacement along the fault:

Strike-slip Faults: The displacement is horizontal, along the strike.
Dip-slip Faults: There is only vertical displacement, along the dip.
Oblique-slip Faults: There are both horizontal and vertical displacement.

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Stratigraphy and Earth’s Chronology

Stratigraphy is the study of the successions of strata and bodies of rocks in the Earth’s interior. It includes the study of igneous, sedimentary, and metamorphic rocks. Stratigraphy is one of the most dynamic branches of geology due to its paramount role in the economy; stratigraphical studies are extensively used in the discovery of new mineral resources. This branch is important as it unlocks Earth’s geological history.

Historical Foundations of Stratigraphy

The origins of stratigraphy are unclear, but evidence from ancient structures like the Egyptian pyramids implies that the ancient Egyptians had advanced knowledge of the strata architecture and continuity in the subsurface. The pyramids are made of a single kind of sandstone or limestone, which means that the builders knew with precision how much rock they needed, and how much could be found in the quarries from where they extracted it. Unfortunately, most of this knowledge remains unknown to us, as it was kept for restricted use by the guild members.

Nicolaus Steno’s Principles (1638–1687)

Nicolaus Steno pioneered the understanding of Earth’s subsurface strata. His fieldwork in Tuscany revealed two distinct rock sequences in the Earth’s crust, separated by a surface of discontinuity, which he linked to the Biblical Great Flood, a common explanation at the time. Religious explanations of natural phenomena were frequent among seventeenth-century scholars. Steno extensively studied geological topics, especially sedimentary layer formation, concluding that these layers originate from suspended particles in fluids like rivers and seas. He proposed four principles of layer formation, largely accepted in modern stratigraphy:

Principle of Layer Superposition: States that the older layer is situated at the bottom of the succession and the younger layers occupy a progressively higher position according to the age of formation, in a sequence of undisturbed layers. The order from the oldest layer at the bottom of a normal stratigraphical succession to the youngest layer situated at the top is known in modern stratigraphy as the stratigraphical order; it corresponds to the order of layer formation.
Principle of Successive Layer Formation: States that at the time of formation of a layer, only fluid was above it and none of the existing layers in the succession existed. Therefore, it takes a long time for a layer to form, and this principle is consistent with the fossil record, which shows that the vast majority of fossils lack any trace of soft tissue, which had time to vanish as the result of the organic matter decay process.
Principle of Original Layer Horizontality: States that the sedimentary layers were horizontal at the time of deposition; tilted layers occur frequently in the Earth’s crust, and Steno regarded those as the result of crustal movements that affected their original position. Later, Steno refined this principle by admitting that at the time of deposition, layers could reflect the irregularities at the basin floor.
Principle of Original Layer Continuity: States that at the time of formation, layers were formed over the entire basin surface, and they terminate by thinning at the basin margins. Steno also took into consideration the existence of possible sedimentation barriers (e.g., islands) that could affect the layer continuity, as well as the lateral transition into a different kind of sediment.

Geologists used layer formation principles to track valuable layers like coal during mining, gathering substantial data on layer ends and crustal structures. Back then, sedimentary layer correlation relied solely on rock characteristics. While it was feasible to determine relative ages in exposed layers, correlating distant sections remained a challenge.

Determining Stratigraphic Order in Deformed Rocks

Geological studies in mountainous regions posed a new challenge for stratigraphy due to tilted and overturned sedimentary layers from crustal movements. Recognizing the difference between normal (in order of formation) and inverted sequences was essential. This challenge found a solution as more studies revealed gradual surface changes. For instance, Giovanni Targioni-Tozzetti (1712–1784) demonstrated that stream erosion results in valley formation in mountain areas. James Hutton (1726–1797), who is considered the founding father of modern geology, demonstrated that the gradual changes at the Earth’s surface are ubiquitous, and their geological implications are profound. Moreover, Hutton advanced a new idea: that we can understand our planet’s geological past if we study the processes and phenomena happening today on Earth.

Way-Up Indicators

Sedimentary structures can indicate whether a layer is in its normal or inverted position:

Mud-cracks: Form frequently at the surface of the Earth, in fine sediments exposed to atmospheric conditions during arid periods. The mud-crack opening is always upwards-oriented, and it narrows downwards. Layer tops can be readily recognized in the outcrops by the cusp opening upward. If the cusp narrows downward, the layer is in normal position; if it narrows upward, the layer is in inverse position.
Ripple-marks: Form at the top of the layer, where unconsolidated particles are rearranged as a result of current action. Such structures are upwards narrowing when seen in transverse sections in outcrops. As a result, the layer top is indicated by the narrow portion of the ripple-mark.
Burrowings: Produced by organisms that develop their life cycles buried in sediments, as adaptations to avoid predators or for feeding purposes. Many of these organisms dig more or less complex galleries, which, as a rule, present the opening towards the layer top.

Unconformities (Breaks in Sedimentation)

Unconformities are breaks in sedimentation and are features that can be relatively easy to recognize in a stratigraphical succession; hardgrounds, layers of iron oxides, and karst structures are often associated with them. Three kinds of unconformities can be recognized according to the rock types and layering below and above the unconformity:

Disconformity: An unconformity between sedimentary rocks that are parallel to each other. Formed through a period of uplifting and erosion, followed by sedimentation above the unconformity; no significant crustal movements are involved.
Angular Unconformity: Occurs between sedimentary rocks where the layering below and above the unconformity form a distinct angle. This involves significant crustal movements during the succession below uplift and erosion.
Nonconformities: Occur between igneous or metamorphic rocks below, and sedimentary rocks above the unconformity. Such an unconformity commonly encompasses a long time period before sedimentation restarts.

The time span within an unconformity is called a hiatus, defined as the sum of the time of erosion and the time of non-deposition. However, different kinds of unconformities can be generated by a single geological process; therefore, they cannot be used for correlation at regional or intercontinental scale.

Fossils and Relative Time

Fossils’ role in stratigraphy emerged in the late 18th and early 19th centuries due to significant geological paradigm shifts.

James Hutton’s (father of geology) book Theory of the Earth revolutionized geological thinking, introducing gradual surface changes and uniformitarianism, later coined by Charles Lyell.
Léopold Cuvier, advocating catastrophism, believed Earth’s shaping involved catastrophic events like volcanic eruptions and marine transgressions.
Cuvier’s observations on vertebrate fossils in the Paris Basin revealed distinct patterns: varying species in different stratigraphic levels and continental vertebrate fossils between layers with marine fossils.
Sir William Smith, through detailed geological mapping, noted specific fossil species occurring in certain layers, defining their stratigraphic range based on appearance and extinction, providing a correlation method for distant strata and enabling ordering of sedimentary layers chronologically based on the unique fossil record.

Principles for Igneous Rock Sequencing

Three principles from the 19th century were initially used to reconstruct the stratigraphical order of igneous rocks. Subsequently, it was discovered that some of these principles could also be applied to sedimentary or metamorphic rocks.

Principle of Inclusion: States that the inclusions within an igneous rock are older than the rock that includes them. This principle is necessary as the principle of superposition doesn’t always work for igneous rocks as it does for sedimentary ones.
Principle of Cross-Cutting Relationships: States that a cross-cutting rock is younger than the cross-cut one. This helps identify the sequence of igneous rock veins and their associated mineral assemblages.
Principle of Rock-Cooking: An igneous rock that heats or alters other rocks (igneous, sedimentary, or metamorphic) is younger than those it affects. This applies to sills, which impact both the rocks above and below, making them younger than either.

The Relative and Absolute Geological Time Scale

These methods and principles aid geologists in sequencing the stratigraphical record based on formation order, determining the relative ages of layers, rocks, events, and fossils. Over a century, a relative geological time scale was developed, primarily relying on layers, rocks, and fossils to reconstruct Earth’s history. This scale has been continuously refined with ongoing discoveries. The geological time scale comprises four eons—Hadean, Archean, Proterozoic, and Phanerozoic—in chronological order. However, scientific advancements have led to significant changes in defining these eons since their initial establishment.

The Four Eons

Hadean: The oldest eon in Earth’s history; it began with the formation of our planet and has no rock record. The Earth consisted only of molten matter during Hadean times, and even if the earliest crust was formed, it is not preserved in the rock record.
Archean: Includes the oldest rocks on Earth; the Hadean/Archean boundary is given by the age of the oldest rocks in the stratigraphical record. Originally, it was believed that the Archean rocks were devoid of fossils; fossils, the oldest in the fossil record, were found later in the Archean rock successions.
Proterozoic: Was defined originally as the stratigraphical interval with the oldest fossils on Earth, which were believed to be entirely small-sized (microscopic). Macroscopic fossils, which are visible with the unaided eye, were subsequently found in the rocks from the uppermost part of the Proterozoic. The two eons that contain the oldest rocks and fossils in Earth’s history, Archean and Proterozoic, often are grouped as the Precambrian supereon.
Phanerozoic: The youngest eon in Earth’s geological history, where visible, large-sized fossils can be found in many of the sedimentary rocks accumulated during this stratigraphic interval.

Phanerozoic Eras

The Archean, Proterozoic, and Phanerozoic eons have subdivisions based on rock and fossil successions. These divisions are determined by the similarities between fossils and present-day life forms, and each era contains multiple periods.

The Paleozoic Era: The oldest within the Phanerozoic Eon, has small resemblances between fossils found in these layers and today’s floras and faunas. In North America, it is divided into seven periods: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian. Outside North America, the Mississippian and Pennsylvanian periods are combined as the Carboniferous.
The Mesozoic Era: Showcases fossils resembling modern life forms and is divided into three periods: Triassic, Jurassic, and Cretaceous. The Mesozoic Era’s lower and upper boundaries are defined by two major crises in the history of life. The Permian/Triassic crisis was the most dramatic crisis that affected mostly the species in the seas and oceans; 90% of them became extinct during this crisis. The upper boundary is marked by the Cretaceous/Tertiary crisis, which is the meteorite impact that led to the extinction of several major fossil groups in both continental and marine realms, dinosaurs among them.

Absolute Age Dating

Early attempts to calculate Earth’s age varied widely:

Bishop James Ussher (1581–1656) calculated Earth’s creation at 4004 B.C. based on Biblical chronology in 1655.
Georges Louis Leclerc, Compte de Buffon (1797–1788), using a heated metal rod, estimated Earth’s age to be around 75,000 years, significantly older than the Biblical calculation.
With advances from James Hutton and Sir Charles Lyell, various methods attempted to calculate Earth’s age but indicated a significantly longer duration, likely in the hundreds of millions of years.

The discovery of natural radioactivity by Marie and Pierre Curie in 1903 led to the principles of radiometric age dating proposed by Arthur Holmes in 1911, providing numerical data for minerals of the Paleozoic and Precambrian.

Radiometric age dating involves isotopes transforming through radioactive decay, measuring parent and daughter atoms in rocks to estimate mineral ages.
The time necessary for the decay of one half of the parent atoms is referred to as the half-life.
Radiometric age dating isn’t precise, often offering the age of the youngest mineral transformation, like metamorphism in recrystallized minerals, especially prevalent in igneous rocks.
Besides radiometric dating, other methods like fission track dating, thermoluminescence, and electron spin resonance contribute to understanding the Earth’s geological history.
The oldest rocks, at 3.96 billion years, are in the Northwest Territories (Canada), while the oldest minerals (zircon) found in Australia are 4.4 billion years old.

Evolution of Life and the Fossil Record

The fossil record provides vital insights into Earth’s life evolution, elucidating the rise of life, shifts from sea to land, and major extinctions. It reveals distinct spatial and temporal patterns crucial for understanding life’s history. However, it is incomplete due to non-fossilization of many organisms, weathering of fossil layers, and geological processes eroding fossil-bearing strata. These challenges must be considered when interpreting life’s history. Early life on Earth started soon after the planet formed. The oldest known organisms were tiny and simple, dominating Earth for a long time. As we move up the fossil layers, more complex organisms appear. The theory of evolution helps explain the changes in fossils over time, like how creatures look, where they lived, and when they existed. Younger sediment layers often show clear transitions between species. But in the older layers, we mostly see a gradual rise in complexity without clear transitions. Despite gaps in the fossil record, the study of fossils still gives us a good idea of how life developed on Earth.

Challenges in Studying Early Life

Oldest Earth layers have scarce, tiny, simple fossils, contrasting sharply with younger layers that feature more complex multicellular life, showcasing an evolution from simple to intricate organisms.
Researching early life is challenging due to sparse and often damaged fossil records caused by the limited preservation of ancient crust and metamorphic alterations.
Molecules crucial to understanding pre-organic life phases don’t fossilize, requiring reliance on models simulating early conditions and data from celestial bodies lacking plate tectonics.
Analysis of ancient rocks’ chemistry and minerals sheds light on early atmosphere changes, primitive organisms’ metabolism, and their interactions with the environment.
Stable isotopes and chemical fossils help date and confirm ancient organic materials in highly transformed layers.
Establishing direct ancestor-descendant links in the Archean and Proterozoic periods is challenging due to incomplete fossil records, but scientists observe a gradual increase in complexity, correlating it with geological events like rock formation and atmospheric shifts.

Origin of Organic Molecules

Life on Earth is largely based on a few key chemical elements—carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur—comprising the building blocks for living organisms. These elements, primarily carbon and hydrogen, come together to form simple compounds known as monomers, like methane, ammonia, and water, which are fundamental to more complex organic molecules.
The chemical composition of living organisms points to a unity in life’s basic structure, suggesting that all life forms on our planet have evolved from a common origin—a primitive single-cell life form.
Early attempts to explain the formation of larger organic molecules faced a challenge: an atmosphere rich in free oxygen would have hindered their formation through oxidation. This discrepancy led Alekxandr Oparin to propose an alternative model, suggesting an early Earth atmosphere that fostered the accumulation of organic molecules, a concept popularly referred to as the “primordial soup.”
Miller and Urey’s 1953 experiment demonstrated that organic molecules could form in an early-Earth-like environment, producing amino acids like glycine and alanine. Similar experiments also generated simple organic molecules but didn’t create life.
Simple organic molecules can join to form polymers, such as cellulose and proteins, revealing that complex molecule formation doesn’t require extraordinary conditions.
The first life forms likely emerged from the primordial soup’s substances, possibly as simple genes capable of self-reproduction outside a cell. Catastrophic meteorite bombardment on Earth might have repeated the process of complex molecule formation multiple times until viable life forms evolved.

The Rise of Cells: Prokaryotes and Eukaryotes

Early life evolution documented in the fossil record showcases a progression of increasing complexity in cell types and groupings over time.

Prokaryotes: The earliest cells, lacked a well-defined nucleus and reproduced asexually, initiating substantial changes on Earth for later life forms.
Oldest fossil cells, found in the Apex Chert (Pilbara Craton, West Australia), date back 3.465 billion years, revealing cyanobacteria capable of photosynthesis.
Cyanobacteria aggregated to form complex stromatolites around 3.2–3.1 billion years ago, dominating Earth’s surface for over two billion years.
Stromatolites were diverse in shape, primarily with photosynthetic organisms concentrated in the upper layers, while anaerobic bacteria resided beneath.
The Gunflint Formation in Ontario (Canada) around 2.1 billion years ago exhibited diverse cyanobacteria, signifying their evolution alongside environmental changes.
Banded Iron Formations (BIF): Around 2.2–1.9 billion years ago indicated a significant rise in atmospheric oxygen generated by stromatolites’ photosynthetic activity. The surge in oxygen levels, estimated to increase over fifteen times, altered Earth’s atmospheric composition, laying the groundwork for the evolution of new organisms and earning BIF the name “Earth rusting.”

Prokaryote diversity and increased molecular oxygen led to the evolution of eukaryotes, cells with defined nuclei and specialized organelles.

Eukaryotes showcased larger sizes compared to prokaryotes, reaching centimeters in some advanced single-celled forms.
Eukaryotes introduced sexual reproduction, boosting morphological variability and evolutionary rates.
Oldest fossil evidence of eukaryotic cells appears in the Bitter Springs Formation (Australia), approximately 850 million years ago.
Specimens resembling modern seaweed (Acetabularia) found in sediments dating back to 2.1 billion years suggest the antiquity of this group.
Size variations in eukaryotes peaked around 1.1 billion years ago, reaching millimeter dimensions but declined near the Precambrian/Cambrian boundary.
Late Proterozoic saw the emergence of multicellular organisms, initially soft-bodied, preserved as impressions in fine sediments.
Eukaryotic evolution showcased two distinct periods: slow evolution in the prokaryotic phase and an exponential increase in diversity after the evolution of sexual reproduction in the late Proterozoic.
Stromatolites dominated the environment in the late Archean, while sexual reproduction in eukaryotes significantly accelerated evolutionary diversity.

Plate Tectonics and Continental Movement

Earth’s lithosphere consists of seven major plates, comprising oceanic and continental components. Most major plates hold continents, except the Pacific Plate, which is entirely made of oceanic lithosphere. Smaller plates often result from the fragmentation of larger ones, evolving differently or similarly to their parent plate.

Evidence for Continental Drift

Alfred Wegener’s 1915 book highlighted common geological and paleontological features among continents, especially in the southern hemisphere (South America, Africa, Australia, Antarctica, and India).

Geological Fit: There is a geological fit of the rock complexes situated on the eastern coast of South America and the western coast of Africa. This observation parallels the almost perfect match between the present-day shorelines of the two continents, suggesting that South America and Africa were united in the geological past.
Fossil Distribution: The distribution in the southern hemisphere continents of the late Paleozoic Glossopteris fern was another piece of evidence that those continents were united. Eduard Suess (1831–1914) used the occurrences of Glossopteris in South America, Africa, and India to postulate the existence of a former supercontinent he named Gondwanaland. Reptile distributions present patterns that resemble that of the Glossopteris flora. The late Paleozoic Mesosaurus is a crocodile-like reptile readapted to aquatic life that occurs only in South America and Africa; another well-known reptile distributed only in the two continents is the coeval Cynognathus. In addition, Lystrosaurus is a Triassic mammal-like reptile known in Africa, India, and Antarctica.
Glacial Traces: The distribution of the Carboniferous polar cap can be inferred from the traces left by glaciers; such sedimentary structures can be found in South America, Africa, India, Australia, and Antarctica.

Wegener used this evidence to propose Gondwana, a supercontinent comprising South America, Africa, Australia, Antarctica, and India in the late Paleozoic. He suggested these continents broke apart and moved to their current locations due to seafloor spreading. Despite this, Wegener faced skepticism due to the lack of a convincing mechanism for his continental drift hypothesis.

Deep Sea Drilling and Seafloor Spreading

Drilling in deep oceanic waters during the late 1960s through the Deep Sea Drilling Project (DSDP) significantly advanced our understanding of lithospheric plate movement and formation. Over 1,500 boreholes were drilled across the planet’s oceans, providing extensive sedimentary and crustal data analyzed for various factors. These studies revealed two critical patterns crucial for recognizing the evolution of oceanic basins: the ages of sediment layers and oceanic crust, and the distribution of rocks based on their magnetic properties.

Oceanic Crust Age and Magnetism

Sediment and Oceanic Crust Ages: Studied using both biostratigraphy and chronostratigraphy. Microfossils like foraminifera and stable isotopes helped generate this data. The results showed that the youngest sediments are near mid-ocean ridges, and their age increases as you move towards the ocean margins. This pattern is symmetrical along the ocean axis at the mid-oceanic ridge.
Magnetic Zone Distribution: Minerals align with magnetic field lines during crystallization (igneous rocks) and deposition (sedimentary rocks). This results in two settings:
- Normal Polarity (N): The north magnetic pole is in the same hemisphere as the north geographic pole (similar to today).
- Reversed Polarity (R): The north magnetic pole is situated in the southern hemisphere.

These polarity intervals alternate at varying rates, with transitions (magnetic reversals) happening rapidly, sometimes in just a few thousand years. These reversals stem from molten iron movements at the core/mantle boundary, driven by convection currents in the liquid outer core. Polarity zones along oceanic basins run parallel to mid-oceanic ridges, where the polarity is normal.

Volcanoes and Earthquakes

The distribution of geological phenomena related to volcanoes and earthquakes shows that they do not happen randomly, and are often concentrated in certain regions.

Volcanoes: Are frequent at the plate boundaries, which can be either in the mid-oceanic ridge zone (e.g., Atlantic Ocean), or at the boundary between an ocean and a continent (e.g., western coast of North America, Central America, and western coast of South America). Other volcanoes occur in continental areas, such as eastern Africa and China. Volcanism is frequently associated with seamounts (e.g., Pacific Ocean), and clusters of volcanoes can occur in deep oceanic conditions (e.g., proximity of Iceland in the North Atlantic Ocean). Volcanoes also are frequently found in the island arcs, such as Indonesia.
Earthquakes: Are concentrated along the plate boundaries and in certain continental regions (e.g., eastern Africa, Central Asia, Himalayas, etc.). They are rare events in the interior portion of some continents, such as North America and South America.

Tectonic Plate Boundaries

Tectonic plates are huge masses of lithosphere that present distinct movement, behavior, and geological history. There are three distinct geological situations, recognized based on the lithospheric plate movement with respect to each other: divergent, convergent, and transform settings. They are produced by different causes and result in the development of distinct kinds of associated sedimentary basins.

Divergent Settings

Divergent settings occur where the lithospheric plates move away from each other, leading to frequent volcanic activity and earthquakes as ascending currents bring molten matter from the Earth’s interior. This process forms olivine-rich rocks while generating oceanic crust, explaining the arrangement of older sediments, magnetic zone alignment, and the gradual creation of oceanic basins through four stages:

Continental Rift Initiation: Produced by ascending currents in the asthenosphere, beneath the continental crust, resulting in continental uplift. Asthenosphere current pressure stretches the continental crust, initiating the divergent regime and causing the formation of normal faults. This stage is recognized by elevated altitudes and increased temperature due to continental crust thinning.
Continental Rift Formation: As the continental crust stretches and thinning continues, an elongate depression zone, known as a continental rift, forms just above the zone where the asthenosphere currents intersect the continental crust. The rift is formed through continental crust block drop-down along the normal faults. Volcanism is common. Free water occurs as lakes, rivers, and aquifers. Coarse sediments accumulate, forming mixtures of continental and volcanic rocks. The East African Rift is an example.
Early Oceanic Basin Formation: Occurs when the continental crust is split into two distinct parts that move away from each other due to the onset of seafloor spreading. Seafloor spreading is generated by the continuous influx of molten matter from the asthenosphere that crystallizes at the surface, forming new oceanic crust. Most of the sediments accumulated in the new ocean are marine, deeper towards the center and shallower towards the basin margin. The classic modern example is the Red Sea.
Mature Oceanic Basin Formation: Happens with the continuation of seafloor spreading; new oceanic crust is added to form vast abyssal plains between the mid-oceanic ridges and the two continental areas that are continuously pulled apart. Volcanism and earthquakes are frequent phenomena in the mid-oceanic ridge region. The Atlantic Ocean is a typical example.

Divergent settings don’t always result in new oceanic basins, as seen in cases where the rifting stage doesn’t progress to seafloor spreading, referred to as aborted oceans in the geological record.

Convergent Settings

Convergent settings arise when plates collide, whether it involves oceanic-oceanic, continent-ocean, or continent-continent interactions, leading to scenarios where one crustal slab overrides the other or where the two crustal types form a discrete contact without subduction phenomena.

Ocean-Ocean Convergence: One oceanic crust slab descends below the other along a subduction plane, marked by earthquakes. This collision zone features an oceanic trench where the sinking begins, with sediments forming an accretionary prism near the trench. The interaction generates magmas enriched with volatiles, leading to explosive volcanic activity and the formation of island arcs upon reaching the surface.
Ocean-Continent Convergence: The collision of oceanic and continental crusts leads to the subduction of the oceanic slab beneath the continental one, forming an oceanic trench parallel to the nearby coastline. This collision generates an accretionary prism. The pressure exerted by the descending oceanic crust causes the thickening of the continental crust, forming a mountain chain parallel to the collision zone. This process involves frequent earthquakes and magma generation.
Continent-Continent Convergence: The collision of two continental slabs results in the closure of an ocean that separated them, forming a parallel mountain chain, such as the Himalayas formed by the collision of the Indian Subcontinent and Eurasia. This process involves frequent earthquakes and active volcanism in the affected region.

The Wilson Cycle

The Wilson Cycle, named after geologist John Tuzo Wilson, encompasses the stages of ocean initiation, expansion, and closure. It starts with divergent settings in the continental crust, forming rifts. If successful, rifts lead to seafloor spreading, gradually widening the ocean. Eventually, convergent settings occur, causing the ocean’s narrowing as continental slabs converge. Continual subduction and formation of new convergent settings consume oceanic crust until closure. These cycles, lasting hundreds of millions of years, explain continent movements and the current continental layout, following the breakup of the supercontinent Pangea in the Triassic period, which included most large landmasses and was surrounded by the vast ocean Panthalassa.