Structure of the Earth
Different researchers might recognize various subdivisions of the spheres. Here is a list, running from the most interior (deepest and densest) outwards:

• Core: metal
  • Solid crystalline inner core
  • Liquid outer core
• Mantle: rocky, but denser and more compact rock than any found on the surface
• Lithosphere: also rocky. Contains the uppermost layer of the mantle and the brittle crust.

The deep interior of the Earth interacts with the parts in which climate happens and we live, but generally only very slowly. Our knowledge of the interior comes almost exclusively from various forms of remote sensing: despite movies the contrary, we do not have the means to drill deep into the mantle or to the core.

The innermost part of the earth is the core, comprised largely of the metals iron and nickel. The inner core is solid, despite having temperatures over 5700 K: with pressures of 330-360 gigapascals the metals are compressed into a solid crystalline structure. The radius of the inner core is 1220 km. The inner core is surrounded by the 2260 km thick outer core, which is liquid. Motions of this vast inner sea of molten metal generates the magnetic field of our planet. The core is hot because of heat left over from the initial formation of the planet, but also (far more importantly) from radioactive decay of various isotopes and the heat of crystallization of the growing inner core.

Surrounding the core is the 2890 km thick rocky mantle. The mantle represents 85% of the Earth's volume. It is basically solid, but because it is hot and under pressure it can flow like tremendously dense silly putty. The mantle rock is very dense: much denser than the typical rocks found on the continent or the ocean floor.

Heat from the core-mantle boundary is dissipated by the formation of vast convection cells in the mantle:

This motion (moving at rates comparable to finger nail growth: a few cm per year) drives the action of shallower geology. The mantle plays a role in the long term carbon cycle, but is otherwise mostly isolated from climate actions.

Technically speaking, the lithosphere is a dynamic subdivision of the Earth, whereas the core and mantle are compositional subdivisions. The mantle is covered by the brittle rocky part of the Earth: the crust (which ranges from about 5 km to 50 km deep). Functionally, however, the outermost mantle shell and the crust move as a single unit, collectively the lithosphere. The lithosphere is divided into various plates, which move relative to each other as a result of the mantle convection cells below. Interaction between plates results in nearly all of geological phenomena:

Such action results in the widening of oceans; the motion of continents; the loss (subduction of older oceanic crust back into the mantle; the driving of volcanoes and earthquakes; the uplift of new mountains; and more.

The lithosphere rides along a mobile asthenosphere, a portion of the mantle where temperature-pressure conditions support the presence of many molten droplets within the rock.

Compositionally, the crust is phenomenally diverse. All sorts of rocks are formed and deposited here. The lithosphere is also a region of various types of activity, continuous or episodic, small-scale to catastrophic. Some of the major ones to consider are:

• Tectonics, the building of mountains and rifting of valleys, the spreading of mid-ocean ridges, etc.
• Eruptions of volcanoes on land or under the ocean, releasing new material (solids, liquids, and gasses) from the deeper parts of the lithosphere or the mantle into the surface spheres
• Uplift of mountains and the like, which become subject to
• Erosion, which breaks rocks down into particles or ions, which can then be
• Deposited, forming new layers of rock and removing some material out of the atmosphere, hydrosphere, or biosphere and adding it to the lithosphere. This is a major form of sequestration.

Lithospheric processes thus both add and subtract material from surficial Earth systems, and these might be as slow as the erosion of a mountain range or as rapid as the eruption of a volcano.

• First suggested in 16th Century based on coastlines
• A common alternative model: the continental masses remain in the same spot laterally, but move vertically:
  • Ancient ocean floor uplifted to form land; what is now sea floor may have been once above water

Some data difficult to explain if the continents did not move:

• Shared sets of animal and plant fossils with highly discordant distributions
• Matching mountain ranges, geological formations across modern oceans
• Patterns of ancient glacial movement did not match modern climate zones

It was discovered in late 19th Century that the sea floor is flat, everywhere dense volcanic rock: very different from continents with mixed rock types and much lower average density. Thus, the ocean floor does NOT represent simply submerged versions of today's continents. The submergence/emergence model of past geography was clearly rejected. Additionally, it was discovered that when mapped out, earthquakes and volcanoes tend to follow particular tracks along the margins of some continents, in the middle of oceans, and other additional patterns that called for some explanatory theory.

Continental Drift: Theory proposed by Alfred Wegener, German geophysicist and glaciologist, in 1915. His model: the light continental masses move over dense layer of oceanic crust (by analogy to motion of light glacial ice moving over bedrock below.) Volcanoes, mountain building, earthquakes caused by crumpling of continental masses as they move along. In the distant past the continents were united together, but subsequently some force broke them apart and is moving them ever since.

Resistance to continental drift was strong in the US, Canada, and the UK (although more widely accepted by Southern Hemisphere geologists.) In part, northern resistance because Wegener failed to propose causal mechanism that could be well-verified (not that their own stabilist model had a verified causal mechanism, either!) But there were ad hominem components to the rejection, too: in part, post-war Germanophobia, and in part, cross-disciplinary "snobbery". At a 1926 Meeting of American Association of Petroleum Geologists, the majority of the talks were strongly against Continental Drift. From this point on, continental drift became a fringe subject among northern hemisphere geologists

Sea-Floor Spreading: In the 1940s and 1950s some geologists (notably Arthur Holmes and Harry Hess) had proposed a mechanism to move continents: the coninents did not move OVER the oceanic crust, but carried along with it as the sea-floor itself was recycled. In post-WWII era, additional discoveries concerning depth of earthquakes, age of oceanic crust confirmed sea-floor spreading.

Plate Tectonics: models of continental drift and sea-floor spreading were combined by John Tuzo Wilson and colleagues to form plate tectonics.

• Earth's surface is comprised of numerous rigid lithospheric plates
• Plates themselves carry thick continental and/or thin oceanic crustal rock
• Since plates are mostly rigid (although they can be broken or crushed or pulled), most motion occurs at the boundaries of plates:
  • New material generated at divergent boundaries (mid-ocean ridges in sea, rift valleys on land)
  • Plates slide past each other at transform boundaries
  • Plates come together at convergent boundaries, whose type depends on whether or not one of the plates is oceanic crust:
    • Oceanic crust is lost under other ocean crust or continents at subduction zones (site of deep-sea trenches, earthquakes, volcanoes, etc.)
    • Two continental masses meet at collisional boundaries: after the collision, they will fuse ("suture") together

Major plate boundaries, from online USGS pamphlet "This Dynamic Earth".

Plate velocities predicted by theory confirmed by GPS studies in 1990s

Heat from Earth's core moves plates, forming mountain ranges at subduction and collision boundaries. Weather erodes uplifting mountains, wind and water and ice transports sediment to depositional environments. Over time, material becomes buried.

Plates wander over Earth's surface, so continents move from tropics to poles or back. Also, action of mid-ocean ridges causes sea levels to rise up (flooding continents) or lower (draining continents). (Current situation is very low sea level).

Big change from the 1960s-1970s model: now recognize there are LOTS of little plates (terranes) rather than just a few big plates.

Here is a brief animation of estimates of the position of the continental masses over the last half-billion years or so (thus, the time scale of the course):

It is important to consider, however, that not all environments are environments of deposition. Many locations will be environments of erosion: these places are sources of sediment, but because material is being lost from there rather than accumulating there, they will not wind up in the geologic record. A particular location can shift between deposition and erosion ("D-world" vs. "E-world") as local environmental conditions change.

Here are some aspects of depositional environment to consider:

• Energy of the environment: is it fast-running water, or still water, or wind? The higher the energy, the larger the size of sediment that will be moved around: fast rivers can move cobbles, but slow rivers only silt and mud. If the energy is too high (whitewater rapids, for instance), it will be an erosional rather than depositional setting. For a brief overview of sediment size and the types of energies associated with them:
  • Boulders (basically, anything bigger than a baseball): extremely high energy, but short term, events, such as avalanches, tsunamis, etc.; also, glaciers (which move objects slowly but can transport sediment as fine as flour to the size of buildings)
  • Cobbles and Pebbles: high energy, sometimes short term (flash floods) but also continuous (the faster moving parts of a stream)
  • Sand: moderate energy, such as waves along a beach, the flow of a river channel, or wind in a desert
  • Silt: lower energy versions of the same environments as sand
  • Mud (such as clay): extremely low energy environments, like lake bottoms, swamps, lagoons, the bottom of the sea, etc.
• Whether the primary sediment source is silicate rocks (such as most igneous or metamorphic rocks) or carbonate minerals (sea shells, carbonate mud, etc.). Most continental (land and freshwater) environments are dominated by silicate sediment deposition, as are marine environments where there is a substantial input from land (e.g., the Atlantic seaboard of North America). If the water is warm, salty (typically), and free of much input of silicate material, then carbonate minerals are likely to be formed: some as chemical precipitates, but far more often in the form of biogenic sediment (broken bits of sea shells or algal skeletons or coral or the like) (e.g., white sand beaches in the Bahamas or the Caribbean).
• The nature of the agent of transportation of sediment.
  • Is it wind? If so, the ever-shifting direction of the wind will deposit, uncover, and redeposit the sediment to produce characteristic patterns when you view the sediment (and the rock it becomes) in cross section. These patterns on the side are an example of the sedimentary structure called "cross-beds" (in this case, trough cross-beds).
  • Is it wave action (back-and-forth oscillating flow)? This will leave one form of ripples and cross-beds. In contrast, current action (one-directional flow, as in a stream) leaves a different pattern.
  • Is it from glaciers? As mentioned above, glaciers simultaneous deposit sediment of all different sizes
  • Is it simply settling in quiet water? In that case, only fine particles (clay, silt, etc.) will be present.
• After depositing, was it subjected to:
  • Drying? If so, you may find mudcracks.
  • Rain? If so, you might (if you are lucky) find preserved raindrop marks.
  • Burrowing and mixing by worms, clams, etc. (technically called bioturbation)? You may find the traces of these burrows, but if it goes on enough it might just be a homogenous mixture of sand and silt and clay left over.
  • Anoxic (very low oxygen) conditions below the reach of waves or currents? In this case, with no worms or clams to disturb the sediment, the fine layers of clay and silt and so forth will form nice laminations.
• Was the sediment actually a structure built by organisms (like a reef) or a mass of organic material (like plant material (coal) or broken bits of shell)?

By observing modern environments and their sediments and sedimentary structures, we can use the clues mentioned above (as well as other aspects) to reconstruct the paleoenvironment. Major environments of deposition represented in the geologic record include:

• Desert sand dunes
• Debris in front of glaciers
• Swamps
• River channels and their floodplains
• The interiors and margins of lakes
• River deltas
• Reefs
• Lagoons
• Sandy or muddy offshore deposits
• Limy mud from shallow carbonate banks
• Deep sea sediments
• And many, many others

NOTE: Present day sea level is much lower than most of Earth History; also, as new mountains are born, once shallow deposits are uplifted. Consequently, even in the middle of continents, it is common to find sedimentary rocks deposited underwater. In fact, rocks deposited in marine environments are extremely common, even in the interiormost parts of continents.

Whatever the environment of deposition, the sediment is laid down in layers (strata). Since every rock is a record of the environment in which it formed, the strata will be of the same general sort while the environment remains the same, and change to a different sort as the environment changes. Packages of similar strata formed over a region are called formations: at any given spot, if we see a section through the bedrock, we can see the transition from one formation to another, representing a transition from one environment to another.

• Compaction: as new layers are deposited over older ones, the lower layers get squished, squeezing out ground water in between the sedimentary particles and causing the stratum to compact. In some minerals (clays, for instance), this process may be enough to strongly bind together the sedimentary grains, thus lithifying it.
• Cementation: minerals in the pore water can precipitate between grains of sediment to bind them together.
• Recrystallization: crystals of some minerals (esp. some carbonates) can grow and join together at the pressure and temperature conditions present in burial.

These factors -- alone or in combination -- can bind the sediment together, transforming it into sedimentary rock.

Lithification is an example of diagenesis: post-depositional alteration of sediment. We will see diagenetic effects in another context shortly, in fossilization.

Plate tectonics ultimately drives the Rock Cycle:

• Volcanoes (and thus igneous rocks) form at divergence and convergent boundaries
• Heat from these volcanoes, and from compression at convergent boundaries, form metamorphic rocks
• Uplift from convergent boundaries provides source rock for sedimentary rocks

Two different aspects of time to consider:

• Relative Time: sequence of events without consideration of amount of time
• Numerical Time: (sometimes called "absolute time"), dates or durations of events in terms of seconds, years, millions of years, etc.
• "The Wright Brothers flight at Kitty Hawk came after the Signing of the Declaration of Independence, but before the Apollo 11 moon landing" is a statement of relative time.
• "The Signing of the Declaration was in 1776, the flight at Kitty Hawk was in 1903, and the Apollo 11 landing was in 1969"

Relative time was determined LONG before absolute time.

• Principle of Original Horizontality: because strata are deposited under gravity, they form horizontal layers. If the strata are no longer horizontal, something has disturbed the sediments AFTER they became rocks.
• Principle of Superposition: unless they have been disturbed, the strata at the bottom of a stack were deposited first, the ones on top of that are next oldest, and so on, with the youngest strata being the ones on top.
• Principle of Lateral Continuity: sediment extends laterally in all directions until it thins and pinches out or terminates against the edge of a depositional basin.

As Steno and others mapped out strata, they found that sometimes there were types of breaks (discontinuities) in the layers. These are called unconformities, and represent gaps in the rock record (periods of erosion and/or non-deposition). James Hutton studied these and recognized that they represented aspects of relative time.

From unconformities, Hutton added additional Principles of Stratigraphy:

• Principle of Cross-cutting Relationships: any structure (fold, fault, weathering surface, igneous rock intrusion, etc.) which cuts across or otherwise deforms strata is necessarily younger than the rocks and structures it cuts across or deforms.
• Principle of Inclusions: any rock fragments included as sediment or xenoliths in a unit are from an older rock unit than the one in which they are included (really, a special case of cross-cutting)

Use these principles to figure out time sequence in any particular section of rock. BUT, how to extrapolate the sequence at one section with the sequence at another? That is, how could one correlate between locations?

Packages of similar strata formed over a region are called formations. Each represents a unit of rock produced by the same conditions (environment) and having the same history (produced over a particular sequence of time). At any given spot, if we see a section through the bedrock, we can see the transition from one formation to another, representing a transition from one environment to another.

Recognizing and defining formations is one of the main tasks of the discipline of lihthostratigraphy. Formations are given formal names (e.g., the Morrison Formation, the Hell Creek Formation, the Solnhofen Limestone, etc.). Sometimes groups of formations which lie directly on top of or next to each other are catalogued together as formal Groups, and sometimes groups which lie directly on top of or next to each other are placed into formal Supergroups. In the other direction, formations may be subdivided into members and beds.

Mapping out formations, groups, supergroups, members, and beds, geologists could connect sequences of rocks across regions. By the principle of lateral continuity, the formation will extend out to the edge of the depositional basin (or at least as far as that set of environmental conditions extend.) This allows for regional correlations, but what about across continents and oceans?

William "Strata" Smith, creating first geologic maps of southern England (and expanded out to include the Continent) observed that the pattern of fossils through the strata was consistent from location to location. Developed this into a new stratigraphic principle:

• Principle of Fossil Succession: there is a unique, non-repeating pattern (history) of fossils through stratigraphic time.
  • All rocks containing fossils of the same species were deposited during the duration of that species on Earth.

In order to be an index (or guide) fossil, the organism used must have certain desirable features:

• Have been VERY common, so chances of individuals being buried is good
• Have hard parts, so chances of fossilization are good
• Have a wide geographic range, so that correlation over wide region is possible
• Lived in (or could be deposited in) different environments, so can be found in different formations
• Have some distinctive features, so it can be recognized from closely related forms
• Have a short geological duration (a few million years at most), so finding a fossil of the species in a rock means it had to be deposited in those few million years

The method of using index fossils to correlate rocks is called biostratigraphy. Here is an excellent summary of biostratigraphic correlation.

In combination, the principles of stratigraphy were useful for determining a global relative time scale, but questions of numerical time were still unresolved.

Geologic Column divided into a series of units: from largest to smallest eons, eras, periods, epochs, ages (or Stages). No initial understanding of the scale of numerical time for these different units.

Animal and plant fossils are mostly restricted to the last (most recent) Phanerozoic Eon ("visible life eon"). The Phanerozoic Eon is comprised of three Eras:

• The Paleozoic Era ("ancient animal life")
• The Mesozoic Era ("middle animal life"): the "Age of Dinosaurs"
• The Cenozoic Era ("recent animal life"): the "Age of Mammals". We are still in the Cenozoic Era.

Expanding out the Phanerozoic, we can see the different Periods within these three Eras:

We are currently in the Quaternary Period of the Cenozoic Era, and the Holocene Epoch of the Quaternary Period.

No one region has a continuous sequence of time. Any given location has likely had periods of non-deposition or erosion, which would leave gaps in the geological and fossil record at any given spot.

An interactive project on geologic time, for those who want to explore in more detail.

Various early scientific attempts to determine the age of the Earth, included such approaches as:

• Compared ocean salinity to known amount of salt in rivers; assuming fresh water proto-ocean, how long to salinate the seas? (tens of million years)
• Calculated current rates of sedimentation, count total thickness of sedimentary rocks, determine total age (a few billion years): standard for Geology at beginning of 20th Century
• Calculate cooling rates of molten iron, determined known surface temperature of Earth & its thermal gradients, assume no additional source of energy (90 million years or so maximum, possibly less): standard for Physics at beginning of 20th Century, most famously argued by William Thomson, Lord Kelvin

Additionally, attempts were made to date particular rocks (i.e., by comparing the sedimentation-rate based time scale or the cooling-based time scale, and estimating what percentage far back a given rock came from), or the duration over which deposits were generated (for environments with annual deposition, like some lakes and glaciers, this was a matter of counting.)

How to reconcile the sedimentation-based scale (needing 100s of millions of years) with the cooling-based scale (limited to <90 million years?. The discovery of radioactive decay at the dawn of the 20th Century gave the key:

• A way around the problem of Lord Kelvin's short physical estimate of Earth's age, because a new natural heat source for keeping Earth's interior molten was now known
• Also, radioactive decay itself forms a "clock" usable for determining age of rocks.

Radiometric Dating: the single most important method of determining numerical rock ages:

• Radioactive materials decay at predictable, experimentally determined rate, known as the half-life
  • Atoms decay from one form (radioactive parent) to another (daughter product), releasing energy and particles
  • After one half-life has passed, half the original parents in the material will have decayed into the daughter product; after two half-lives, only one-quarter of the parent material remains, with three quarters daughter product; after three half-lives, 1/8 to 7/8; after four half-lives, 1/16 to 15/16; and so on
• Can thus date rocks:
  • Compare the ratio of parent product to daughter product
  • Radiometric dates will only be effective for igneous rocks, since those are the ones that form by cooling and locking atoms into place
    • In sedimentary rocks, can date the individual grains of sediment: tells you age of source rock, but not deposition
    • In metamorphic rock, recrystallization redistributes atoms and obscures signal
  • Since only igneous can best be dated radiometrically, use the principles of superposition, cross-cutting relationships, etc., to determine ages of sedimentary rocks (and their fossils) relative to numerical dates, and tie dates into Geologic Column by correlating with index fossils.

Traditional radiometric dating needs some special conditions, however:

• Daughter product should only be produced naturally by decay from the parent
• Neither parent nor daughter should be able to leave the sample naturally
• Only useful for determining ages of formation of mineral grains (and thus really best for igneous rocks)

When possible, radiometric dates of different isotopes with different decay rates are calculated for same sample. If these converge, good support for that age.

In order to get around the issue the requirement of zero initial daughter product, mid-20th Century geochemists developed the actual version of radiometric dating that is currently used by geologists: the isochron method. We won't go into great detail for this in this particular course, but in brief it requires sampling not just the parent (P) and daughter (D) products of a radioactive decay series, but also a stable (non-radioactively-generated) isotope of the same element as D (D_i). By comparing the ratios D/D_i vs. P/D_i, different minerals in the same rock will plot along a particular line (the isochron), the slope of which is scaled to the number of half-lives the rock has gone through. Scatter of plots around the line is a measure of how much contamination or loss there has been of materials, and thus the degree of confidence we have in the measure.

Using the radiometric methods of dating, geologists have estimated the ages of the various boundaries of the Geologic Timescale.

Living things take in ¹⁴C during life, but not when they die: from that point on, it will simply decay away.

Of special note with radiocarbon: studies showed that the amount of ¹⁴C in the atmosphere (and thus the amount that organisms take up) has varied over time, requiring a calibration curve to convert from the amount of radiocarbon to the number of calendar years.

• Eustatic (global) sea level changes: Matching up the rise and fall of sea level worldwide based on the patterns of distribution of coastal deposits (and erosion). This most accurate in environments (shorelines, mainly) where sea-level changes are recorded.
• Marker beds: One-time events (volcanic eruptions, asteroid impacts, etc.) may send particular types of material over wide region (even globally). These record EXTREMELY short periods of time: essentially instantaneous!
• Stable Isotope Stratigraphy: Some stable isotopes of various elements (carbon, oxygen, strontium) vary over time relative to each other due to a number of factors (productivity, glaciers, temperature, erosion, etc.). When examined against a time scale, these form irregular curves. Individual samples or series of samples can be compared to the known-curve of these isotopes to see where they fall.
• Astrochronology: a relatively new field. Looks at the varying thickness of strata within sedimentary packages to calibrate their changing thicknesses to some astronomically-controlled cycles (such as tidal, daily, monthly, annual, or longer term cyclicity.) Not good for calibrating against a time scale as such, nor for correlation, but very useful for determining the duration in numerical time of packages of sedimentary rock.
• Magnetostratigraphy
  • The magnetic (but NOT the geographic) poles have "flip-flopped" throughout geologic time, so that sometimes a magnet's north pole points towards geographic North, and sometimes toward geographic South.
  • Magnetic polarity can be recovered by some iron-bearing rocks (sedimentary and igneous).
  • Because based on the Earth's magnetic field, the changes occur everywhere on the planet at the same time.
  • Can use the particular "bar code"-like pattern of flip-flops to match any section to known global pattern (based on continuous record of lava on ocean floor)
  • Only really good back to Jurassic: before that, lack the continuous magnetic record of the seafloor
  • Here is the Phanerozoic magnetochronological chart as of February 2010.

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Last modified: 26 January 2024