navigation image map next pagetable of contents previous page



The Six Fundamental Concepts about the Earth's Geology

1. The Earth formed about 4.6 billion years ago, along with the other solar planets and the Sun itself. The planets built up by accretion of rocky and gaseous debris (asteroidal, planetesimal [meteoritic] materials and comets) through collision of orbiting bodies. Aided by gravitational attraction, early on the assembling Earth underwent partial to complete melting, separation of different materials into an inner and outer core (iron-nickel), and extensive interior mantle, (iron/magnesium/calcium-rich silicates), and a thin crust (enriched in silica, sodium/potassium/aluminum), all (except the outer core) solidifying by cooling over the first few hundred million years; escaping gases produced an atmosphere (H, CO2, N, CH4) were held above the solid Earth by gravity owing to its large mass; in time (about 4 billion years ago), the Earth's exterior cooled sufficiently to allow vast volumes of water vapor to condense, forming in lower areas great concentrations of water collection into oceanic basins.

Slice through the Earth showing its layered, concentric shells.

2. The Earth's materials are diverse and variable. Most variation occurs in the outermost 200 kilometers, in the lithosphere. Igneous rocks form directly by crystallizationof hot melts made up of silicates (SimOn) combined with Fe, Mg, Ca, Al, Na, K, Ti, H2O). Minerals formed from these make up nearly all the mantle and crust. Rocks at the surface decompose/disintegrate by reaction with the atmosphere/hydrosphere to produce solid debris and soluble chemicals that are transported/deposited to form sediments, that upon burial are converted to Sedimentary rocks. Previously formed rocks that are heated and pressurized when buried to shallow to moderate depths (5 to 70 km) of the crust recrystallize as solids to form Metamorphic rocks (some may melt). The above processes comprise the Rock Cycle, shown below, and discussed in more detail on this page.

Preview of the Rock Cycle.

3. The Earth's outer shells (crust and upper mantle = lithosphere) about150-200 kilometers thick under the continents (less so under the oceans) are subjected to dynamic forces that cause segments of the shells and materials at the top, to break up into plates and deposits on them that move laterally, bringing about deformation of their constituent rocks (mainly in and on the crust) by bending, folding, flowing, fracturing, and movement of blocks along faults. The dybamic processes, driven mainly by heat and gravity and resultant convection within and below the lithosphere (in the mantle), move plate units either away from each other or against each other (both situations can affect a plate); this general motion is called plate tectonics. Plates diverge from ridges within oceanic basics (lower areas underlain by basaltic crust) and converge against boundaries of other plates (whose outer rocks are either oceanic or continental in nature and composition), causing melting, volcanism, metamorphism, mountain building, rise/fall of crustal blocks, continental growth and splitting.

The Earth's major lithospheric plates.

The principal types of folds and faults are shown in the two diagrams below; the extensional fault is commonly known as a normal fault, the compressional type is called a reverse fault if high angle and a thrust fault if low angle, the transform fault is one type of wrench or rift faults that is associated with oceanic ridges.

Types of folds Types of faults

4. The distortions (lateral and/or with up-down movements) of crustal materials combine with physical and chemical reactions between atmospheric constituents (mainly oxygen and water) that weather (breakdown and/or dissolve) rocks which are then eroded, transported (by running water, ice, wind, gravity) and deposited in low surficial locations on land or in water bodies (oceans). These actions contiually modify the shape of the land and ocean surfaces producing a wide range of continental and oceanic landforms (mountains, valleys, plateaus, plains, volcanic edifices, etc.), developing a wide variety of landscapes.(

Derivation of some typical and general landforms through rock cycle processes.

Schematic scene showing many common landforms, most geologic, some with geographic names.

Landforms development is often a complex process requiring long time periods during which specific landform types take shape, evolve, and disappear. Factors involved, besides time, are the actions of shaping forces such as running water, etc., the type(s) of climate a region experiences (can change from humid to arid or reverse), the nature and resistance to erosion of the various rock type present and their structural configuration, the history of deformation over time, and rises and falls of the regional elevations (through isostasy - a tendency for the crust to assume altitudes that maintain balance [equilibrium] within the Earth's gravitational field). Modern theories of landform development are diverse but most trace their ideas back to 19th Century specialists (Geomorphologists) such as William Davis. While the details have changed, his notions of landform cycles remain largely valid. This illustration generalizes the changing landscape in a humid environment:

The Cycle of Landforms development under wet (humid) climatic conditions

The starting point is the emergence of flat-lying sedimentary rocks from the sea as a coastal plain made up of flat-lying sediments. Streams that develop during Youth follow the gradient (slope) from the highest land to the ocean shoreline start and cut down narrow, steeper-walled valley slopes. The progression then is towards valley widening that leaves uplands as hills and mountains. By a stage called Maturity, the uplands have been carved by enlarging valleys so as to leave only the original uplands at narrow ridges. Thereafter, as Old Age is approached, streams and erosion on the mountains reduce them to local hills with the landscape, having been generally lowered, becomes one of low relief (small differences in elevation). Davis called this end product a "peneplain", a term not now used except as an idealization of what a final stage would be like; uplift (block diagram G) is likely to occur before then, which causes a repeat of the overall process (rejuvenation). If the rocks had been inclined (folded) rather than flat, the cycle would have been modified, with hard, less easily erodied rocks maintaining the upland mountains. Rejuvenation has now acted on the Appalachian Mountains such that the ridges (see page 6-3) represent hard rocks and the valleys occur in softer, more easily removed rocks.

A cycle can be specified for erosion under arid climatic conditions. The end result depends on the structural nature of an eroding region. One case, shown below, relates to mountains uplifted along high angle faults (producing "block-fault mountains") such as in the Basin and Range of the western U.S. (see page 6-8). Here the sequence of change seems simple: from the starting point of high mountains and low valleys, the mountains wear down and their eroded debris fill the valleys, so that the final outcome is a subdued topography with low remnant uplands (pediplains) and deeply filled, raised valleys.

The arid erosion cycle in a mountain-valley initial topography.

5. Since its beginning, the Earth has been an active, dynamic planet that experiences continual changes in its interior and especially its ouer lithosphere and surface. Its continents have grown relative to oceanic crust and have shifted in position on a standard global surface (continental drift). Most of Earth's history (expressed sequentially as the Geologic Time Scale) is best deciphered in its rocks, particularly sedimentary ones,that record sequences of modifying events (deduced in part through patterns of lifeforms [usually as fossils] changes (by evolutionary processes) and from rock age measurements (based on fixed rate radioactive decay).

The geologic time scale, based on stratigraphic methods, fossil evolution, and radioactive measurements; the Precambrian as shown is incomplete; other diagrams in this mini-tutorial will elucidate Precambrian time.

6. The Earth surficial environments operate as a complex, interrelated system of units and features best categorized in terms of the physical/chemical components of the Geosphere. Atmosphere, Hydrosphere, and Biosphere powered by solar and internal heat that interact at, just below, and above the global surface to produce a series of conditions that aid, inhibit, and otherwise affect Humans and all living creatures. The study of how these "Spheres" interact, exchange energy, and produce positive and/or negative feedback is called Earth Systems Science. This version of the definitive Bretherton diagram suggests some of these inputs and effects.

The Bretherton Earth System(s) diagram.

In the remainder of this page, we will explore in more detail three primary topics: The Rock Cycle; Geologic Time; Plate Tectonics. The subject of Landscapes is more fully treated in Section 17, entirely devoted to Landforms from both ground and space prespectives.

The Rock Cycle

To discuss this subject, we will use these two diagrams; the first shows what is known as the Rock Cycle (possible changes from one rock type or mode of origin into others) and the second indicates the names of the major rock types in each of the three main groups: Igneous; Sedimentary; Metamorphic:

The Rock Cycle

Classification of rocks in terms of the common types as named.

To assure you are certain of what a rock is, we define it simply as an assemblage of one or (most commonly) two or more minerals (specific chemical compounds) that form a part of the Earth's solid body. A "rock" normally connotes an individual "specimen" - one that can be held in one's hand or is larger but detached from its outcrop (exposure of the rock's source) so that it has visible boundaries. The dependence on mineral composition and texture in naming rock types is best illustrated as applied to igneous rocks, as in this diagram:

Igneous rock classification in terms of mineralogy and texture (Extrusive refers to magmas that reach the surface; Intrusive to those that cool to rock below the surface.); d refers to rock density.

Key parts of the Rock Cycle (RC) involve magmatic/volcanic/metamorphic processes that produce rocks, the actions of water and air in disintegrating rock materials and transporting/depositing them, the dynamic forces that deform rocks, and specialized actions like wind, waves, and ice in modifying rocks at the surface. The subsurface crust is altered by heat and pressure; the surface by physical/chemical weathering and erosion (W/E). Much of the RC is most active where tectonic plates meet: new rocks form; old rocks are changed.

The general pattern followed in the RC is shown in the first of the two diagrams at the beginning of this subsection. It can be summarized verbally in this sequence: Molten rock ---> Igneous Rock ---> W/E of igneous rock ---> Sediments ---> Sedimentary Rock ---> Sedimentary and Igneous Rocks, on burial, experience heat and pressure ---> Metamorphic Rock ---> further heating/pressure ---> Molten Rock. The process can then repeat. One added feature: any igneous, sedimentary, or metamorphic rock at/near the surface can undergo W/E --- Sediment. The energy driving the RC comes from three principal sources: 1) Solar - the Sun's radiation provides kinetic energy to move air and water/ice; 2) Gravitational - rock and water movements downslope; 3) Thermal - trapped heat emanating mainly from the Earth's interior.

Molten rock is called magma if it remains below the surface and lava if it reaches the surface. Most magma is generated in the crust (mainly upper lithosphere); some may derive from the uppermost mantle (asthenosphere) by upwelling and partial melting. Magma reaches the surface as volcanoes or volcanic outpourings primarily where 1) ocean plates spread; 2) oceanic plate dives under (subducts) another plate, inducing melting that rises upward, often to the surface, 3) under continents that experience basal heating (may remain below surface as batholiths), and 4) where moving plate passes over a mantle thermal plume, causing a hot spot, and melting. The two main igneous rocks are Granite - form within continents, intrude upwards but remain under the surface; light-colored and large feldspar and quartz (minerals) crystals, and Basalt - extrude as lavas on both ocean floor and continental surface, rapidly cool, dark because of amphiboles/pyroxenes occur with gray feldspars in small crystals.

Atmospheric weathering (H2O, O2, and CO2and near surface weathering mainly by water affect all rocks. Physical weathering: roots, still and moving water, wind, human activities fractures, grinds, and flakes rocks into particles. Chemical weathering produces acid conditions that dissolves rock (example: sinkholes in limestone). Erosion loosens and moves particles (solids = clastics) and breaks rock down by solution. These become sediments that are transported by rivers, ocean waves, wind, sliding ice, gravity to collect in settling basins by deposition. Sediments tend to accumulate in layers and convert to sedimentary rocks by lithification - pressure squeezing (compaction) and cementing. Main types are: Sandstone (visible grains of quartz, feldpsar), Shales (fine particles including clay minerals), the most common, Limestones (chemical precipitates and biochemical animal/plant carbonates, and Coal (decayed plant matter).

Metamorphic ("changed form") rocks form from action of heat and pressure on pre-existing rocks. Shales (themselves finely layered) follow this progression with increasing T and P: Shale ---> Slate (fine-grained, brittle) ---> Schist (mica flakes) ---> Gneiss (light and dark crystal banding) ---> melting to a magma. Limestone ---> Marble; Sandstone ---> Quartzite; Basalt ---> Amphibolites or Serpentine.


navigation image map next page previous page

Primary Author: Nicholas M. Short, Sr. email: [email protected]

Collaborators: Code 935 NASA GSFC, GST, USAF Academy
Contributor Information