Mountains are then built by deforming crust: Deformation & Mountain Building. Mountains form where stresses are high!

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1 Deformation & Mountain Building Where are mountains located? Deformation and Folding Mountain building Mountains form where stresses are high! Mountains form at all three types of plate boundaries where deformation (strain) occurs in response to stresses Mountains are then built by deforming crust: The interactions of tectonic plates create stresses that deforms the crust. Deformed crust thickens (or thins) the lithosphere resulting in uplift of mountains 1

2 Which of the following are caused by tectonic stress, and thus occur at all plate boundaries? A. Volcanoes, Earthquakes, mountains B. Earthquakes and Mountains C. Earthquakes D. Mountains Deformation of rocks: Stresses build at plate margins. When rocks are stressed, they deform as Bends = folds, and Breaks = faults Elastic and plastic deformation or the Spring vs. Silly Putty 2

3 Horizontal layers or bedding Notice that older layers are below younger layers Inclined bedding results from deformation Strike = bearing of line defined by intersection of bedding with horizontal Dip = angle of inclination Elastic Deformation and Faults Rocks flex like diving boards Stress the rock and it Strains (deforms) Elastic deformation: when stress is released, the rock bounces back into relaxed shape Think about a diving board returning to shape after you ve jumped off. If the rocks is stressed beyond its strength ( ) while still in the elastic regime, it breaks producing A Fault An Earthquake 3

4 Types of Faults Normal: Overhanging block (headwall) slides down fault plane Reverse: Overhanging block (headwall) slides up fault plane Strike-Slip: Apposed blocks slide horizontally past each other. Plastic Deformation and Folds Rocks flex like diving boards Stress the rock and it Strains (deforms) Plastic deformation occurs when stress exceeds the yield strength: Stress produces progressively more strain. when stress is released the rock remains deformed (like a bent wire) Rocks stressed beyond their yield strength, result in folded rocks Anticline: limbs of fold point down Oldest rocks in center of fold Types of Folds Syncline: limbs of fold point up Oldest rocks at margins of fold Plunging folds: fun games you can play in three dimensions. 4

5 Monocline Anticline Syncline 5

6 Things can get complicated: Plunging anticlines Things can get really complicated: Cartoon How are mountains uplifted (i.e. why are they elevated above sea level)? A. Deformation thickens the crust, thus uplifting the area by isostacy B. Deformation thins the lithosphere, and buoyant asthenosphere uplifts the area C. Intrusion of magma adds to the crust, making it thicker and thus isostacy uplifts the area D. All of the above 6

7 Mountain Building: Mountains are uplifted sections of crust and mantle lithosphere Caused by stresses at plate boundaries Faults and folds = deformation from uplifting How are mountains uplifted? Isostacy explains why most mountains are high (uplifted). In this example, the mountains are uplifted because the crust and lithosphere were thickened. Thicker lithosphere rides high. How might we do this geologically? Subduction Zones thicken the lithosphere: Volcanic Arc = addition of igneous crust Fold-thrust belt = shortened and thickened curst by compressive stress and reverse faulting 7

8 Rocky Mountain Fold and Thrust Belt Continental collision causes major thickening of the crust Himalayan uplift 8

9 Rifting causes uplift too! Thinning of lithosphere allows asthenosphere to rise and heat the lithosphere The shallow asthenosphere and hot lithosphere expand and lift the region Today s Appalachians are the remnants of Rift- Margin mountains! Basin and Range and East African Rift are modern examples Steens Mt, OR; Basin & Range Wasatch Fault, UT; Basin & Range East African Rift Uplift also occurs at transform margins 9

10 What causes the mountains that form the Mid-Atlantic Ridge? A. Collision between Europe, Africa and Americas. B. Subduction related thickening of lithosphere C. Shallow, hot asthenosphere lifting the ridge. D. Volcanic eruptions build a thick crust, forming mountains The Basin and Range of NA is rifting mountainous region. Is it mountainous due to isostacy? A. No, rifting causes thin lithosphere B. Yes, how else could it be elevated How are mountains uplifted (i.e. why are they elevated above sea level)? A. Deformation thickens the crust, thus uplifting the area by isostacy B. Deformation thins the lithosphere, and buoyant asthenosphere uplifts the area C. Intrusion of magma adds to the crust, making it thicker and thus isostacy uplifts the area D. All of the above 10

11 Review Questions Contrast elastic with plastic deformation. What factors influence deformation of a material and how? Explain how both thickening and thinning of lithosphere forms mountains. What is elastic deformation and what is plastic deformation? When do faults form and when do folds form in the crust? Contrast an anticline with a syncline Compare and contrast the three basic fault types. In which tectonic settings do the types of faults occur? 11