Geology

Back to AgeOfEarth

Rock

Igneous

Feldspar

Sedimentary

Metamorphic

Slate from Shale
Marble from Limestone
Quartzite from Sandstone
Hornfels
Schist

Principles of Sedimentary Geology

Law of superposition: Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top

Principle of lateral continuity: Layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, are assumed to have been originally continuous.  Sediment is confined to a sedimentary basin representing the original lake or sea.  The margins of the basin are typically thinner and finer-grained.  The margins are also usually very complex due to historical changes in the shape of the basin.

Principle of original horizontality: Layers of sediment are originally deposited horizontally. Therefore, a tilted strata has been disturbed from it's original position.  This principle is very useful, but not universal, since sediment can be deposited at angles of up to 15 degrees, especially at the margins of basins.

Principle of cross-cutting relationships: A rock or fault is younger than any rock through which it cuts.  For example, if there is a series of horizontal sedimentary strata, and a band of igneous (from molten) rock cuts diagonally across it, then the igneous rock is younger than the horizontal strata.  It can't have been in place before the sediment was laid down, because it is diagonal.  Instead, it must have intruded upwards through a crack in the sedimentary rock.

Principle of included fragments: Any rock fragment that is included in another rock must be older than the enclosing one.  This applies equally to igneous bodies and to grains in sediment.   Therefore, if a fragment in sediment is absolutely dated using radiometric dating, then it can be said conclusively that the sedimentary rock is younger.

Principle of faunal succession: Sedimentary rock strata contain fossilized flora and fauna.  These fossils appear in a specific vertically-reliable order that can be identified over wide horizontal distances.  This principle says absolutely nothing about how or why the fossils are different.  The word "succession" simply means sequence and does not in any way imply lineal descent.

Correlation

The aim is to demonstrate that a particular layer in one geological section represents the same period of time as another layer at some other section.

Lithostratigraphy: Correlation based on lithology, color, structure, thickness, etc.

Chronostratigraphy: Correlation with absolute times.  Relies heavily on radiometric dating of igneous rocks.  The law of superposition and the principle of cross-cutting relationships are used to correlate igneous rocks with sedimentary rocks.  Also, sedimentary rocks frequently have material such as fragments or such as volcanic ash in distinct layers which can be dated radiometrically.

Magnetostratigraphy: Correlation by using fine-grained magnetic minerals which are oriented with Earth's magnetic field like tiny compasses.  The earth's magnetic field varies substantially in both orientation and intensity through time.  It also completely reverses poles with an average interval of approximately 250,000 years.  Rocks are correlated with the Global Magnetic Polarity Time Scale, which summarizes all known changes in the historical magnetic field.

Biostratigraphy: Correlation by using the fossils contained within them.

Biostratigraphy

Index Fossils: Fossils that are useful for correlation.  To be useful, they must be very common, easy-to-identify at species level, and have a broad distribution.  Most importantly, they must only be found in a narrow age range of sedimentary rock. There are a number of specific species which fit these criteria and are used regularly.

Microscopic Index Fossils: Although they cannot be seen in the field, microscopic fossils are much more common.  They are routinely used by the oil and mineral industries as very reliable indicators of the ages of a rocks.  Dead microscopic creatures, in fact, provide the dominant form of sediment across vast areas of the ocean floor.  They continuously accumulate at rates of about 20-50 mm per thousand years.  Specific species appear and then quickly disappear from the stratigraphic record.  The vertical sequence is preserved uniformly around the world.

Plate Tectonics

We can directly measure the movement of the various continents as about 4 to 5 cm per year.  Some of the continents are moving at different rates than others, so the numbers given here will be averages for simplicity.  There is vast and diverse evidence for the historical rate of movement and for the original positions of the plates. A nice visual reference is scotese.com/earth.htm

Midoceanic Ridges: A continuous chain of mountains in the middle of the oceans that are 2 miles high and 45,000 miles long.  A rift valley runs along the ridge crests.  They were discovered in the 1950's.

Pangea: This was the supercontinent that began to break up about 250 million years ago (Ma).  Its existence does not preclude the existence of previous cycles of continental breakup and formation.  The breakup was a bit chaotic.  Rifting began in what is now the middle of the Atlantic Ocean, creating a southern landmass called Godwana, and a northern landmass called Laurasia.  By 180 Ma, Laurasia and Godwana were completely separated.  Laurasia was eventually home to dogs, cats, and hooved animals.  Godwana, on the other hand, was home to primates, rodents, meridiungulata (extinct hoofed animals), and marsupials.  200 Ma, Africa, South America, and Antarctica broke apart as the South Atlantic opened up.  60 Ma, North America finally split from Eurasia.  35 Ma, India began to collide with Asia forming the Himalayas.  3 Ma, a land bridge was formed between North and South America, leading to the Great American Interchange of animals between the two landmasses.

Direct Measurement: The current movements can be directly measured.  Most impressive is the 5 to 6 cm per year movement of India northward that is causing the rise of the Himalayas.  Likewise, is it clear from direct measurements where the boundaries are of each of the tectonic plates.

Magnetostratigraphy: See the definition further up on this page.  Due to the frequent reversals of the Earth's magnetic field, the rock on each side of the mid-ocean ridges has formed striking magnetic banding.  The banding is very symmetrical on both sides of the ridges.  The exact direction that each of the tiny magnetic particles are pointing shows quite conclusively that the rock originated in the mid-ocean ridges and has since been spread apart.  The movement in latitude can even be deduced from the direction vectors.  Magnetostratigraphy is used with both sedimentary and igneous rock. Both kinds of rocks can be accurately dated as explained above.  There is vast correlative evidence using all four kinds of correlation that gives an accurate history of the breakup of Pangea and the timescale over which it happened.

Mountain Formation: Mountains result from the application of tectonic forces to rocks.  Mountain belts form parallel to continental coastlines because this is where the subduction zones lie.  A subduction zone is where the sea floor is being forced under the continental crust.   The intense tectonic compressional forces fold, fault, and metamorphose the rocks.  The most vivid examples of this cause and effect relationship are the chains of mountains that extend along the entire western edge of both North and South America. Before a mountain range begins forming, the rock in that area starts out as flat sedimentary rock.  Once folded and compressed, it takes up less than half the original width.  The principle of original horizontality is used extensively to correlate the rocks.    Mountains also form where two continents are colliding, such as in the Himalayas and the Alps.  Large mountains and other extreme geologic formations visible today are geologically young, because erosion tends to quickly flatten out extreme vertical features.  Older mountain ranges such as the Appalachians and Ozarks, which were probably once as impressive as the Alps, have now been weathered down to rounded hills.

Erosion and Groundwater

Mass wasting: The movement of rock and soil downhill due to gravity.  It is largely dependent upon the water saturation level and the steepness.  Landslides are common in wet areas with sand or clay soils such as Oregon.

Stream: Any flow of water within a channel regardless of size.  A gradient is the downhill slope of a stream.  A stream's velocity is determined by a number of factors.  A stream's velocity will be the greatest in a steep,narrow, deep, or smooth channel.  Velocity will be the lowest in a flat, wide, shallow, or rocky channel.  The cutting power of a stream is determined primarily by its velocity rather than by its size. Nearly all erosion in a stream channel happens during flood conditions.

Valley: Forms as the result of direct downward cutting by a stream. Erosion and mass wasting then go to work on the vertical exposed sides of the channel.  The valley walls will then move outward away from the stream. Some valley walls will become less steep as they erode, but it is also common for slopes to maintain their original steepness as they erode back from the river channel.  Low levels of mass wasting will result in steep valley walls.  Geologic features with sheer vertical walls are common in the western United States due to low levels of mass wasting in the arid climate.

Base level: The level of the most horizontal flow and lowest velocity. Water in a channel will attempt to erode to a base level, which is ideally at sea level.   Initial valley erosion is downward, deepening the valley in a V shape.  As the erosion approaches the base level, it slows and calms.  The erosion then becomes primarily lateral, with a flood plain surrounding the river.  When tectonic uplifting forces increase the gradient of a stream, the stream responds by downcutting faster.

Graded stream: A young ungraded stream has many sharp drops and irregularities.  It is still actively downcutting and smoothing out its irregular gradient through erosion.  Over time, it becomes a graded stream with a smooth and regular profile.

Water table: Below this level, the ground is saturated with water. Above this level is the unsaturated zone which contains mostly air in the pore spaces.  The saturated zone below the water table extends down about 3 miles into the crust.  Below this level, the rock has no porosity due to compressive forces of gravity.  The level of the water table changes with the season.

Gaining or losing: A gaining stream is one which is below the level of the water table.  It gains water from the saturated zone.  A losing stream is above the water table and loses water to the unsaturated zone.