Mass Wasting/Landslides Lecture Notes PDF

Mass Wasting/ Landslides 1 Mass Wasting: An Introduction MASS WASTING (aka. SLOPE FAILURE): failure and downslope movement of rock or unconsolidated materials due to gravity. UNCONSOLIDATED MATERIALS: Material derived from the disintegration and erosion of rocks on the land's sur...

Mass Wasting/ Landslides 1 Mass Wasting: An Introduction MASS WASTING (aka. SLOPE FAILURE): failure and downslope movement of rock or unconsolidated materials due to gravity. UNCONSOLIDATED MATERIALS: Material derived from the disintegration and erosion of rocks on the land's surface, including clay, silt, sand and gravel. Is MASS WASTING same as a LANDSLIDE? For some yes! For others LANDSLIDE is a specific type of MASS WASTING characterized by a rapid slope failure. 2 Mass Wasting: An Introduction EROSION: geological process in which materials are worn away and potentially transported naturally by wind or water. EROSION and the break down of hills and mountains is driven largely by GRAVITY. EROSION of hills/mountains results in SLOPED surfaces of varying stability. A SLOPED surface’s stability will depend on: ✓ angle of the slope ✓ strength of the underlying materials Apparent EROSION and resulting SLOPED surface. 3 Mass Wasting: An Introduction Let us assume we are observing a chunk of rock on a SLOPED surface. What factors will determine whether or not it will move down the surface? Firstly, must consider interaction between chunk of rock and SLOPED surface. If rock has already broken from the surface: interaction WEAK. If rock is still a part of the surface (connected to surface): interaction STRONG. MASS WASTING in Yosemite National Park. Strength of this interaction is referred to as SHEAR STRENGTH. 4 Mass Wasting: An Introduction Let us assume we are observing a chunk of rock on a SLOPED surface. What factors will determine whether or not it will move down the surface? Next, must consider force of GRAVITY acting on chunk of rock. Force of GRAVITY pulls everything towards Earth’s center (straight down). However, a fraction of GRAVITATIONAL force will want to pull the rock down SLOPE. Highway near Vancouver closed for boulder removal! Let’s take a closer look. 5 Mass Wasting: Shear Force Let us take GRAVITATIONAL FORCE and break it into two components: SHEAR SHEAR FORCE: a FORCE component parallel to FORCE (fs) SLOPE. NORMAL FORCE (fn) NORMAL FORCE: a FORCE component perpendicular to SLOPE. GRAVITATIONAL FORCE (fg) Both act on chuck of rock. However, it is SHEAR FORCE that can cause it to move downhill. GRAVITY as SHEAR FORCE and NORMAL FORCE STEEPER the SLOPE, GREATER the SHEAR FORCE. 6 Mass Wasting: Shear Force Let us take GRAVITATIONAL FORCE and break it into two components: SHEAR FORCE: a FORCE component parallel to SLOPE. NORMAL FORCE: a FORCE component perpendicular to SLOPE. Both act on chuck of rock. However, it is SHEAR FORCE that can cause it to move downhill. GRAVITY as SHEAR FORCE and NORMAL FORCE STEEPER the SLOPE, GREATER the SHEAR FORCE. 7 Mass Wasting: Shear Force Let us take GRAVITATIONAL FORCE and break it into two components: SHEAR FORCE: a FORCE component parallel to SLOPE. NORMAL FORCE: a FORCE component perpendicular to SLOPE. Both act on chuck of rock. However, it is SHEAR FORCE that can cause it to move downhill. GRAVITY as SHEAR FORCE and NORMAL FORCE STEEPER the SLOPE, GREATER the SHEAR FORCE. 8 Mass Wasting: Shear Force Must now compare SHEAR FORCE and SHEAR STRENGTH. Assume SHEAR STRENGTH is FIXED at a specific value. Remember, SHEAR STRENGTH is strength of INTERACTION between ROCK and SURFACE. CASE (a): If SLOPE is small (not steep); SHEAR FORCE is small. In this case, SHEAR FORCE < SHEAR STRENGTH Compare SHEAR FORCE to SHEAR STRENGTH. Chunk of rock does NOT move. 9 Mass Wasting: Shear Force Must now compare SHEAR FORCE and SHEAR STRENGTH. Assume SHEAR STRENGTH is FIXED at a specific value. Remember, SHEAR STRENGTH is strength of INTERACTION between ROCK and SURFACE. CASE (b): As SLOPE gets steeper; SHEAR FORCE increases. In this case, SHEAR FORCE ≈ SHEAR STRENGTH Compare SHEAR FORCE to SHEAR STRENGTH. Chunk of rock MAY or MAY NOT move. Motion will depend on variety of factors (discussed later). 10 Mass Wasting: Shear Force Must now compare SHEAR FORCE and SHEAR STRENGTH. Assume SHEAR STRENGTH is FIXED at a specific value. Remember, SHEAR STRENGTH is strength of INTERACTION between ROCK and SURFACE. CASE (c): As SLOPE gets steeper still; SHEAR FORCE continues to increase. In this case, SHEAR FORCE > SHEAR STRENGTH Chunk of rock will likely move. Compare SHEAR FORCE to SHEAR STRENGTH. Watch out below!!! 11 Mass Wasting: Shear Force Younger mountains (more recently formed by tectonic Young mountains collisions) tend to be steeper as they have not present steeper ERODED as much as older mountain chains. SLOPES than old mountains do. Older mountains less steep as have experienced hundreds of millions of years of EROSION. That being said, they can still experience some MASS WASTING. ↑ SLOPE, ↑ SHEAR FORCE 12 Mass Wasting: Rock Strength Now, let’s not fix SHEAR STRENGTH. In reality, SHEAR STRENGTH can vary! Aside from fact that chunk of rock can already be broken from the surface or still a part of the surface, strength of the rock/slope material can vary as well! As rock strength increases, so does SHEAR STRENGTH. Remember, SHEAR STRENGTH is strength of INTERACTION between ROCK and SURFACE. ↑ ROCK STRENGTH, ↑ SHEAR STRENGTH Let’s look at different rock types and their relation to SHEAR STRENGTH! 13 Classification of Rocks: Igneous, Sedimentary, Metamorphic In broadest sense, rocks can be classified as: IGNEOUS SEDIMENTARY METAMORPHIC IGNEOUS ROCKS: rocks formed when molten material/magma or lava cools and solidifies. Visually, these rocks appear to have crystals and are NEVER LAYERED. IGNEOUS ROCKS are further subdivided into INTRUSIVE IGNEOUS ROCKS and EXTRUSIVE IGNEOUS ROCKS (this will be discussed in detail later in the course). Examples of IGNEOUS rocks. 14 Classification of Rocks: Igneous, Sedimentary, Metamorphic In broadest sense, rocks can be classified as: IGNEOUS SEDIMENTARY METAMORPHIC SEDIMENTARY ROCKS: rocks formed Examples of from sediments, bits of preexisting rocks SEDIMENTARY and pieces of once-living organisms rocks. (e.g., mollusks and other invertebrates) Furthermore, SEDIMENTARY ROCKS: have layers feel gritty (i.e., rough texture/feels like it is coated in sand) often break easily 15 Classification of Rocks: Igneous, Sedimentary, Metamorphic In broadest sense, rocks can be classified as: IGNEOUS SEDIMENTARY METAMORPHIC METAMORPHIC ROCKS: IGNEOUS or SEDIMENTARY ROCKS that have been transformed by high pressure, high heat, and/or contact with hot magma. Find such conditions: deep in the Earth where tectonic plates meet NOTE: Transformation/METAMORPHISM process does NOT melt rocks, it just turns them into DENSER and more compact rocks Examples of METAMORPHIC rocks. (remember density is mass/volume). 16 Mass Wasting: Rock Strength As rock strength increases, so does SHEAR STRENGTH. Rocks made of CRYSTALLIZED molten materials (i.e., minerals) are very strong. GRANITE and GRANITE cliff These include (but are not limited to): ✓ GRANITE (igneous rock) ✓ BASALT (igneous rock) ✓ GNEISS (metamorphic rock) BASALT and BASALT cliff CRYSTAL: A solid whose atoms are arranged in a “highly ordered” repeating pattern/solid with a “highly ordered” microscopic arrangement of atoms. Rocks above consist of various CRYSTALLIZED (highly ordered) materials/minerals. GNEISS and GNEISS cliff 17 Mass Wasting: Rock Strength As rock strength increases, so does SHEAR STRENGTH. SEDIMENTARY rocks demonstrate a large range of strengths. Potentially strong SEDIMENTARY ROCKS: DOLOSTONE and DOLOSTONE cliff DOLOSTONE made primarily of mineral DOLOMITE DOLOMITE = CaMg(CO3)2 LIMESTONE made primarily of mineral CALCITE (i.e., calcium carbonate) CALCITE = CaCO3 LIMESTONE and LIMESTONE cliff Both contain carbonate (CO32-) ions. 18 Mass Wasting: Rock Strength As rock strength increases, so does SHEAR STRENGTH. SEDIMENTARY rocks demonstrate a large range of strengths. Moderately strong SEDIMENTARY ROCKS: CONGLOMERATE: A CLASTIC SEDIMENTARY rock largely made up of gravel-sized chunks CEMENTED together. CONGLOMERATE and CONGLOMERATE cliff CLASTIC rocks: rocks composed of fragments derived from preexisting rocks that are bound together. 19 Mass Wasting: Rock Strength As rock strength increases, so does SHEAR STRENGTH. SEDIMENTARY rocks demonstrate a large range of strengths. MUDSTONE and MUDSTONE cliff Weak SEDIMENTARY ROCK: MUDSTONE (i.e., SHALE) Soft and brittle. Made of very fine particles of clay, silt or mud deposited in aquatic environments. Compacted, CEMENTED and transformed FOSSILS in SHALE into rock over time. 20 Mass Wasting: Rock Strength As rock strength increases, so does SHEAR STRENGTH. SEDIMENTARY rocks demonstrate a large range of strengths. SANDSTONE and SANDSTONE cliff Can also have variations within a SEDIMENTARY rock type. SANDSTONE can be moderately strong or weak. SANDSTONE made up of SAND grains held together by a MINERAL CEMENT Cement can include silica (SiO2), calcite (CaCO3) and iron oxides (materials containing iron, Fe, and oxygen, O). SANDSTONE in Prince Edward Island Degree of CEMENTATION (how well sand grains held is red due to high iron oxide content. together) varies, affecting strength. 21 Next lecture will examine: Different types of MASS WASTING encountered. Here is a preview! https://www.youtube.com/watch?v=cA0ay24v5kc Famous PEI landmark comes down! 22