Copy-of-W5-Rock-Deformation.pptx

Rock Deformation and Stratified Rocks  After completing this this powerpoint, you are expected to: 1. explain how the movement of plates leads to the formation of folds and faults (S11/12ES-Id-22); 2. describe how layers of rocks (stratified rocks) are formed (S11/12ES- Id-25); and...

Rock Deformation and Stratified Rocks  After completing this this powerpoint, you are expected to: 1. explain how the movement of plates leads to the formation of folds and faults (S11/12ES-Id-22); 2. describe how layers of rocks (stratified rocks) are formed (S11/12ES- Id-25); and 3. describe the different methods (relative and absolute dating) to determine the age of stratified rocks  On a separate sheet of paper, write the letter of the correct answer to complete the sentence. 1. The type of stress found at a transform fault is ________________. A. compressional B. confining C. shear D. tensional 2. _______________ stress is present at a convergent boundary. B. Compressional B. Shear C. Tensional D. Translational  3. A divergent boundary experiences __________ stress. A. compressional B. confining C. shear D. tensional 4. Stress on rocks caused by parallel forces that move past each other in opposite directions is called _________ stress. B. compressional B. confining C. shear D. tensional 5. ____________ stress pull rocks in opposite directions. C. Compressional B. Confining C. Shear D. Tensional FOLDING AND FAULTING Potentially Active Faults According to PHIVOLCS-DOST, a potentially active fault shows insufficient evidence that the fault moved in the last 10,000 years. However, the possibility of movement along these types of faults may not be discounted. The following images are part of the Active Faults Map of Cebu City generated by PHIVOLCS-DOST. The broken lines represent the approximate trace of potentially active faults in Cebu City. This map may be revised STRESS Stress on rocks is the force applied per unit area. The force is mostly related to the movement of tectonic plates and to the weight of overlying rocks. STRAIN Strain is the resulting deformation because of stress. A strain is a change in size, shape, or volume of a material or any kind of movement of the rocks. Rocks under low confining pressures near the earth’s surface generally deform through fracturing and faulting. Rocks deep within the crust under high confining pressures deform by folding. FOLDS Deep within the crust, as plates collide, rocks sbend or crumple into folds. Once they are folded, they do not return to their original shape. Compressive forces are common along convergent plate boundaries resulting in mountain ranges. FAULTS A fracture is a simple break that does not involve significant movement of the rocks on either side. If the rocks on one or both sides of a fracture move, the fracture is called a fault. A fault is a boundary between two bodies of rock along which there has been relative motion. The San Andres fault in California corresponds to the transform boundary between two continental plates. STRATIFIED ROCKS Sedimentary rocks are formed from pre-existing rocks and pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. The layered rocks are referred to as stratified rocks.  Before you learn more about stratified rocks, recall the different types of sedimentary rocks. The composition of these rocks helps us understand the Earth’s history. STRATIFIED ROCKS Rocks at the surface undergo weathering that break rock into smaller pieces called sediments. Sediments are deposited on beaches and deserts, at the bottom of oceans, and in lakes, ponds, rivers, marshes, and swamps through erosion. These particles may bury dead animals and plants. Accumulated sediments harden into rock.  When sediments settle out of calmer water, they form horizontal layers. One layer is deposited first, and another layer is deposited on top of it. When the sediments harden, the layers are preserved. These rock layers are called rock beds or strata.  If conditions on the surface do not change, only thick, homogenous, and undifferentiated sedimentary rocks will form. Bedding or layering in sedimentary rocks reflects the changing conditions during deposition. Each layer represents an interval of time where conditions have remained uniform. 2.2 PRINCIPLES OF STRATIGRAPHY Stratigraphy is the study of strata in the Earth's crust. The principles of stratigraphy help distinguish younger and older sedimentary layers. The works of Nicholas Steno, William Smith, and James Hutton contributed to the principles of stratigraphy used by geologists today.  In 1666, a young doctor named Nicholas Steno (1638-1686) concluded that fossils were once parts of living creatures and sought to explain how fossil seashells could be found in rocks and mountains far from any ocean. He studied layers of sedimentary rocks and proposed a series of conjectures that are now known as Steno’s Laws. 1. Principle of Superposition In a sequence of layers that have not been overturned, the oldest layer will be on the bottom and the youngest layer on top. 2. Principle of Original Horizontality Sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earth's surface. Rocks that we now see inclined or folded have been disturbed. Principle of Cross-cutting Relations Younger features cut across older features. Faults, dikes, erosion, etc., must be younger than the material that is faulted, intruded, or eroded. In Figure 12, 4 is younger than 1, 2, and 3 because it cuts through all the three layers. The layers from oldest to youngest would be 1, 2, 3, 4, 5.  2.3 RELATIVE DATING OF STRATIFIED ROCKS The relative age of a rock is its age in comparison with other rocks. For example, a volcano is younger than the rocks beneath it. Relative dating Tells whether one layer of rock is older than another. It does not tell how old something is. All we know is the sequence of events.  The principles of stratigraphy are essential for determining the relative ages of rocks and rock layers. In the process of relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to try to decipher the times that an event occurred relative to the other events represented in that sequence. Figure 13 is a geologic cross section that shows three (3) layers of sedimentary rocks (A – C), intrusion made of igneous rocks (D), and a fault (E).  By the principle of cross- cutting relationships, fault (E) must be the youngest feature because it cuts through A, B, C, and D. When fault (E) formed, the three sedimentary layers and the intrusion were already present.  Using the principle of cross- cutting relationships, the igneous intrusion (D) is younger than layers A, B, and C because it cuts through these three sedimentary rock layers. By the principle of superposition, C is the oldest sedimentary rock, B is younger, and A is still younger.  The full sequence of events is: 1. Layer C is formed first. 2. It is followed by the formation of Layer B. 3.Layer A forms after the formation of Layer B. 4.After layers A, B, and C were formed, intrusion D cut across all three layers. 5.Then fault E formed, shifting rocks A through C and intrusion D. 6.Finally, weathering and erosion created a layer of soil on top of layer A. 2.4 RELATIVE DATING USING INDEX FOSSILS Once geologists had worked to determine the relative ages of rocks throughout the world, it became clear that fossils that were contained in the rock could also be used to determine relative age. This realization led geologist William Smith (1769-1839) to formulate the principle of faunal succession, which recognizes that:  Some fossil types are never found with certain other fossil types (e.g. human ancestors are never found with dinosaurs) meaning that fossils in a rock layer represent what lived during the period the rock was deposited.  Older features are replaced by more modern features in fossil organisms as species change through time, e.g. feathered dinosaurs precede birds in the fossil record.  Fossil species with features that change distinctly and quickly can be used to determine the age of rock layers quite precisely. They were so characteristic of relative age that they were termed index fossils.  To become an index fossil (also known as marker fossil) the organism must have been widespread so that it is useful for identifying rock layers over large areas and existed for a relatively brief period so that the approximate age of the rock layer is immediately known. If two separated rock units contain the same index fossil, then the rocks are of very similar age. 2.5 ABSOLUTE DATING OF STRATIFIED ROCKS Detailed studies of rocks throughout the world using the principles of stratigraphy allowed geologists to break geologic time into units of known relative age. The breaks in relative geologic time were established and well known even before geologists had the means of determining absolute ages.  Absolute age or numeric age means that we can assign a number (in years, minutes, seconds, or some other units of time) to the amount of time that has passed. Thus, we can say how old something is. For example, a piece of metamorphic rock is 3.96 billion years old. With the discovery of radioactivity in the late 1800s, scientists were able to use absolute dating to measure the exact age of some rocks in years.  Radiometric dating is an absolute dating technique that relies on the decay rate or half-life of radioactive isotopes to estimate the ages of materials. Using more than one radioactive isotope helps scientists to check the accuracy of the ages that they calculate. Table 2 below shows several methods used in radiometric dating.  DIRECTIONS: Number the following events from 1 to 4 according to how it happens. Write your answers on a separate sheet. a) Accumulated sediments harden into rock and the layers are preserved. b) Rocks at the surface undergo weathering. c) Sediments form layers. d) Sediments settle on calmer bodies of water.