Geology Photograph Album

Transcription

1 Geology Photograph Album Josh Gibson Geology 100 July 2, 2007

2 Ridges of Resistant Rock Gravel Beds Feature: Differential Weathering Location: Otay Mesa, CA N W Date: June 19, 2007 Description: The erosion pattern on this cliff face is a type of differential weathering known as badlands style, which refers to intricately streamdissected topography that develops on surfaces with little to no vegetation cover (AGI, 2007). Differential weathering occurs when rock masses do not weather uniformly. Softer, less resistant rock is weathered more rapidly creating the valleys and more resistant rock protrudes as ridges or pinnacles (Tarbuck & Lutgens, 2005). This Formation is in the loosely bound sandstone of the San Diego formation; also notice the thin beds of gravel that were deposited near the Pliocene shoreline (Abbott, 1999)

3 O Horizon A Horizon E Horizon Feature: Soil horizons Location: Otay Mesa, CA N W Date: June 19, 2007 Description: A soil profile is a vertical cross-section from the surface through all of the soil s horizons and into the parent material (Tarbuck & Lutgens, 2005). An ideal soil profile will consist of five horizons or layers. At the top is the O horizon consisting of loose and partly decayed organic matter, next is the A horizon where mineral matter is mixed with decayed organic matter this is also where most biologic activity is found. Further down is the light colored E horizon; this is the zone of leaching, where water has carried away fine soil particles and soluble components. Next we find the B horizon where much of the leached material from above is deposited. The O, A, E and B horizons together constitute the solum or true soil (Tarbuck & Lutgens, 2005). Below the solum we find the C horizon of partially altered parent material which rests upon the unweathered parent material. The O, A and E horizons are visible in this photograph

4 San Diego Formation Pleistocene Rock Relative Movement Feature: Fault Location: Paradise Hills, CA N W Date: June 20, 2007 Description: A fault is a break in a rock mass along which movement has occurred (Tarbuck & Lutgens 2005). Here the La Nacion fault has offset two rock masses. The orangish-white sedimentary rock of the San Diego Formation on the right (north) is side by side with dirt brown Pleistocene rocks on the left (south) (Abbott 1999). Using the law of superposition we can determine the relative motion along this fault. The Pleistocene rocks on the left are younger than the San Diego Formation, which is of the Pliocene epoch. Therefore the Pleistocene sediment was deposited above the Pliocene sediment. From this we know that the San Diego Formation has moved up in relation to the darker rock on the left

5 Horizontal Beds (Deposited after Movement) Up Tilted Beds Feature: Tilted Bedding Location: Paradise Hills, CA N W Date: June 20, 2007 Description: The principle of original horizontality states that layers of sediment are generally deposited in a horizontal position (Tarbuck & Lutgens 2005). This means that if rock layers are found in a horizontal position we can assume they are undisturbed. We may also conclude that if rock layers that are observed in a tilted, inclined or folded position they must have been disturbed by crustal deformation sometime after their deposition. This photo also shows horizontal beds that were deposited after the underlying beds were tilted (law of superposition)

6 Vesicles Feature: Vesicular Texture N W Date: June 22, 2007 Description: Vesicular texture refers to holes, pores or cavities (vesicles) within an igneous rock. Vesicles are the result of gas expansion (bubbles), which often occurs during volcanic eruptions. (CSU Pomona, n.d.) Lava with higher silica content tend to be thicker than those with lower silica content, this higher viscosity contributes to the vesicles being left behind by the escaping gasses as the lava cools and hardens. Pumice and scoria are common vesicular rocks; this example appears to be vesicular basalt or scoria. (Pocket knife is 10cm for scale) - 6 -

7 Point Bar Cut Bank Feature: Ephemeral Stream N W Date: June 22, 2007 Description: Ephemeral streams only carry water in response to specific episodes of rainfall. Most desert streams are of this type. (Tarbuck & Lutgens, 2005) In spite of the fact that these streams only flow following sporadic rainstorms, which may only account for a few days or several hours throughout the year, they can perform impressive amounts of erosion during these short lived events. Although wind erosion is indeed more significant in dry areas than elsewhere, most desert landforms are carved by running water. (Tarbuck & Lutgens, 2005) This photo shows cutbanks and point bars that have been eroded and deposited respectively during the sporadic flow in this stream

8 Concretions Feature: Concretions N W Date: June 23, 2007 Description: Concretions form as minerals within a rock, segregate and begin to precipitate within cracks and cavities, or as sediment builds up in successive layers around a nucleus such as a shell or pebble (P.R.I.). Therefore they have a different composition than the rock they form in. The presence of water in the rock is important to the formation of concretions because it allows dissolved minerals to migrate and collect where they precipitate out in one area. Concretions often form spheres because they build up in layers around a nucleus, they may also take on many other shapes including oval such as the ones seen here. These concretions are visible in a road cut, but more often they are exposed by differential erosion. Because they are harder and more resilient than the material they are formed in they are left behind after the other rock is eroded away

9 Slide Area Road Cut Feature: Mass Wasting N W Date: June 24, 2007 Description: Mass wasting is the down slope movement of rock regolith and soil under the direct influence of gravity (Tarbuck & Lutgens, 2005). This process differs from erosion because gravity is the only transport medium needed, water wind or glacial ice is not necessary, although water may trigger movement by adding weight or lubricating a surface. Other triggers include oversteepening of slopes, removal of anchoring vegetation and ground vibrations from earthquake activity (Tarbuck & Lutgens, 2005). Types of mass wasting are classified by type of material moved, type of motion and the speed of the motion. This slide was undoubtedly caused by humans oversteepening the slope by making a road cut and probably triggered by saturation of rain water

10 Anchored Dunes Feature: Anchored Dunes N W Date: June 24, 2007 Description: This is part of the Medanos coastal dune field in Baja Mexico. The dune field formed as a result of ocean sands blown onshore by strong winds coming from the open sea (Minch, 1998). Some of the dunes in this area are stabilized or anchored dunes, meaning they do not migrate. The roots of the vegetation that grow on them hold the sand in place. Sometimes the dunes can be blown out by strong wind or high waves, if this occurs the dunes would no longer be anchored and could start to migrate (Minch, 1998)

11 Columnar Joints Crystallized Magma Eroding Cone Feature: Volcanic Plug N W Date: June 24, 2007 Description: Volcanic plugs are masses of rock which crystallized within the vent or conduit of a volcano (Minch, 1998). These masses of igneous rock are exposed as the less resistant rock which makes up the volcanic cone is eroded away. The rock in this plug also exhibits columnar jointing which forms as igneous rocks cool and develop shrinkage fractures that produce elongated pillar like columns (Tarbuck & Lutgens, 2005)

12 Eroded Edges of Beds Cross Bedding Pliocene Sand Stone Feature: Cross Bedding N W Date: June 24, 2007 Description: Stratification known as bedding is a distinctive sedimentary rock feature. In general, each bedding plane marks the termination of one period of deposition and the beginning of another (Minch, 1998). According to the principle of original horizontality sediment is usually laid down horizontally, but cross bedding occurs when sediment is deposited on a slope forming beds at an angle to the main bedding. The cross bedding at this site resulted from erosion of the beds by waves or shifts in ocean currents along a rugged coastline. Subsequent beds were deposited at an angle to the eroded beds (Minch, 1998)

13 Talus Slope Feature: Talus N W Date: June 24, 2007 Description: Talus is an accumulation of rock debris at the base of a cliff (Tarbuck & Lutgens, 2005). Mechanical weathering breaks and loosens the rock into angular fragments which then fall to the base of the cliff. Frost wedging is the most common weathering agent that produces talus slopes, but is obviously not the mechanism that produced the one shown here because the climate is much too warm. Wave action has produced this talus slope

14 Miocene Basalt Bake Zone Feature: Bake zone N W Date: June 24, 2007 Description: The prominent pink layer in this road cut is a bake zone. It formed in the Middle Miocene as molten basalts flowed over the white tuffaceous material and baked them to extremely hot, 2000 degree F, temperatures (Minch, 1998). This heat transformed the white tuff to the pink/red color we see

15 Cobble Pebble Feature: Conglomerate Location: San Ysidro, CA N W Date: June Description: Detrital sedimentary rocks are made up of solid particles derived from mechanical and chemical weathering known as detrital (Tarbuck & Lutgens, 2005). If the detrital particles are 2-4mm or larger they are called gravel. Gravel particles consist of granules (2-4mm), pebbles (4-64mm), cobble (64-256mm) and boulders (> 256mm). If a rock consists of rounded gravel sized particles cemented together the rock is known as conglomerate. If the gravel particles are angular the rock is called breccia. The particles in conglomerates are commonly large enough to be identified as distinctive rock types and; thus, can be valuable in identifying the source of the sediments (Tarbuck & Lutgens, 2005). This large conglomerate is approximately five feet tall and five or six feet across

16 References Abbott, P. (1999). The rise and fall of San Diego: 150 million years of history recorded in sedimentary rock. San Diego: Sunbelt Publications. American Geological Institute. (2007) Earth Science Image Bank. Retrieved June 27, 2007, from images/search/results.html?keyword=physical%20weathering CSU Pomona. (no date). Igneous rock textures. Retrieved June 29, 2007, from Minch, J., Minch, E., Minch, J. (1998). Roadside geology and biology of Baja California. Mission Viejo, John Minch Associates, Incorporated. Paleontological Research Institution. (no date) Concretions. Retrieved June 26, 2007, from Satprints. (no date) Baja California, Mexico: The satellite map. Retreived June 30, 2007, from Action=VIEWPROD&ProdID=228 (cover photo) Tarbuck, E., Lutgens, F., (2005) Earth: An introduction to physical geology (8 th edition). Upper Saddle River, Pearson Prentice Hall. All coordinates listed in hddd mm.mmm WGS