Principles of Geology
Essential Questions What is Uniformitarianism? What is Catastrophism? What is Steno s s Law? What are the other geologic principles? How is relative age determined using these principles?
What is Uniformitarianism?
Development of the Theory In the mid-seventeenth century, biblical scholar and Archbishop James Ussher determined that the earth had been created in the year 4004 BCE Just over a century later James Hutton, known as the father of geology,, suggested that the earth was much older. The 19th century scholar Sir Charles Lyell whose Principles of Geology (1830) popularized the concept of uniformitarianism.
Uniformitarianism can be summarized by the phrase "the present is the key to the past." based on the slow, natural processes observed on the landscape if given enough time, a stream could carve a valley, ice could erode rock, sediment could accumulate and form new landforms
What is Catastrophism?
Catastrophism is the idea that many of Earth s s crustal features (strata layers, erosion, polystrate fossils, etc) formed as a result of past cataclysmic activity.
What is Steno s s Law?
The 3 principles of Steno s s Law The Principle of Superposition The Principle of Original Horizontality The Principle of Original Lateral Continuity
The Principle of Superposition In any series of layered rocks, the older rocks are on the bottom and they become younger as you move up through the beds.
The Principle of Original Horizontality All sediments are originally deposited in flat layers.
The Principle of Original Lateral Continuity Sedimentary beds form over a large area in a continuous sheet, gradually thinning toward the edges. Often, coarser-grained material can no longer be transported to an area because the transporting medium has insufficient energy to carry it to that location. In its place, the particles that settle from the transporting medium will be finer-grained, and there will be a lateral transition from coarser- to finer-grained material.
What are the other geologic principles?
The Principles of Intrusive Relationships The Principle of Cross-cutting Relationships The Principle of Inclusions The Principle of Faunal Succession Principle of Unconformities
The Principles of Intrusive Relationships The sedimentary rocks are older than the igneous rock which intrudes them. In other words, the sedimentary rocks had to be there first, so that the igneous rocks would have something to intrude. Examples of types of igneous intrusions are: Dikes Sills Laccoliths Stocks Batholiths Discordant - a differing type of rock cutting across a formation Concordant a differing type of rock which lies parallel to a formation
Dikes is a type of sheet intrusion referring to any geologic body that cuts discordantly across Sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava,, or even along the direction of foliation in metamorphic rock
Stocks is a discordant igneous intrusion having a surface exposure of less than 40 square miles may have been feeders for volcanic eruptions Batholiths is a large emplacement of igneous intrusive rock that forms from cooled magma deep in the earth's crust almost always made mostly of felsic or intermediate rock-types very large intrusive bodies, usually so large that there bottoms are rarely exposed
Laccoliths is a sheet intrusion that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base
Diagram (1): Dike B is younger than Sedimentary Rock A. Erosion surface C is younger than Dike B. Sedimentary Rock D is younger than Erosion Surface C. Diagram (2) Sill B is younger than Sedimentary Rock A. Dike C is younger than sill B. Diagram (3) Stock B is younger than Sedimentary Rock A. Dike C is the youngest.
The Principle of Cross-cutting Relationships Originally developed by James Hutton in Theory of the Earth (1795) embellished upon by Charles Lyell in Principles of Geology (1830), the principle states that the fault which cuts a rock is the younger of the two features if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault
Structural Types of Cross cutting A fault or fracture cuts through an older rock Stratigraphic An erosional surface (or unconformity) ) cuts across older rock layers, geological structures, or other geological features. Sedimentological Occur where currents have eroded or scoured older sediment in a local area to produce, for example, a channel filled with sand. Paleontologic Occur where animal activity or plant growth produces truncation. This happens, for example, where animal burrows penetrate into pre-existing existing sedimentary deposits. Geomorphic Occur where a surficial feature, such as a river, flows through a gap in a ridge of rock. In a similar example, an impact crater excavates into a subsurface layer of rock.
The Principle of Inclusions developed by James Hutton with sedimentary rocks, if inclusions (or clasts) ) are found in a formation, then the inclusions must be older than the formation that contains them
The Principle of Faunal Succession developed by the English geologist William Smith is based on the observation that sedimentary rock strata contain fossilized flora and fauna, and that these fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found
Principle of Unconformities Any place where there is missing time is called an unconformity Unconformities normally happen either through erosion or non-deposition An unconformity represents time during which no sediments were preserved in a region
The Great Unconformity Grand Canyon, AZ
Types of Unconformities
Angular Unconformity is where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers. younger sediments rest upon the eroded surface of tilted or folded older rocks
Disconformity is an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition contact between younger and older beds is marked by a visible, irregular or uneven erosional surface
Disconformity continued
Nonconformity exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing existing and eroded metamorphic or igneous rock develops between sedimentary rock and older igneous or metamorphic rock that has been exposed to erosion
Nonconformity continued
How is relative age determined using these principles?
What is relative dating? is the science determining the relative order of past events, without necessarily determining their absolute age can only determine the sequential order in which a series of events occurred, not when they occur