The Lithosphere Part 2 A Cinder Cone A cinder cone is a cone-shaped hill surrounding a volcanic vent, formed by the accumulation of small pieces of fragmented lava (pyroclastic cinders and ash) that was spewn out of the vent. Volcanic Features A dyke is a hardened sheet of magma that cuts across layers of surrounding rock. A sill is a hardened magma layer that flowed between two surrounding rock layers. A laccolith is a dome-shaped lava chamber that forces adjacent rock layers apart. A batholith is a massive, deep intrusion of magma. Eroded Batholiths When surface layers over batholiths are eroded away and the batholiths are lifted, massive granitic mountains may be formed. The mountains from Vancouver to Alaska show batholith roots. The Sierra Mountains of California are batholiths. Kinds of Lava Pahoehoe lava is smooth and wrinkled while aa lava is rough and slag- like with broken fragments. Pahoehoe lava runs smoothly because it contains many dissolved gases. Cratons The oldest and most stable parts of continents are called cratons. A craton is a section of a continent that has not had mountain-building activity for hundreds of millions of years. Cratons have less surface relief like valleys and nearby mountains. The Canadian Shield is a North American craton. Accretion Accretion is a process by which bits of islands or continent collide, stick together and accumulate into larger masses. Most of British Columbia was formed by accretion as pieces of continental plates collided with western North America and stuck to the west coast of North America. The formerly separate pieces now stuck together are called terranes. Accretion and Vancouver Island Vancouver Island is thought to be made up of at least three major accretions. Isostasy The state of balance between ocean crusts and continental crusts floating on the asthenosphere is called isostasy. For mountains to stay uplifted on continental crusts, there must be roots below the mountains or they will sink. When continental glaciers covered Canada and the Northern States, the glacier weight pushed the continental crust downward. With the melting of the glaciers in recent Geologic times, the Canadian crust is rebounding upward because of isostasy while the Southwestern US is sinking as the continental crust seeks an equilibrium or balance. Earthquakes Earthquakes can occur anywhere. More earthquakes occur near subduction zones than anywhere else. Unlike most other environmental hazards, earthquakes give no warning. Earthquakes take more lives than any other type of natural disaster. Some effects of earthquakes are fires, floods, landslides and tidal waves. Earthquake Causes Earthquakes are caused by tension built up as rocks are distorted. As rocks under tension suddenly slip and move, they release shock waves that move in all directions from the point making the waves. Focus and Epicentre The focus of an earthquake is the point below the earths surface where the energy of the moving rocks is released. The epicenter is the point on the earths surface directly above the focus. At the epicenter, the shock waves from the earthquake are the strongest. Earthquake Magnitude Earthquake magnitude measures the energy difference between earthquakes and is measured by the Richter Scale. The numbers of the Richter Scale are logarithmic which means that for each higher number, the earthquake is ten times more energetic. An earthquake of Richter 5 is 100 times more energetic than an earthquake of Richter 3. The Mercali Scale measure the intensity of an earthquake on the earths surface in terms of an observers judgements. The Mercali Scale is much less precise than the Richer Scale. Earthquake Effects on Gravels Unconsolidated deposits (not cemented together) like gravel beds often behave like liquids during earthquakes which causes objects resting on these beds to sink in them. This process is referred to as liquefaction of the gravel beds. Relative Position of Earthquakes Most earthquakes happen on the landward side of a subduction trench. The more inland the earthquake epicentres, the deeper the foci are found. This is explained by the deeper a plate is subducted as it moves from a coastal area. When Earthquakes are Devastating The devastation of an earthquake is directly related to the density of the human population in the earthquake area. Most deaths due to an earthquake are related to side effects of the earthquake like fires caused by broken gas and electric systems, disease caused by broken sewer systems, lack of food and water due to broken water mains and road damage preventing food transport. Earthquake Sites Japan has many earthquakes because it is in a subduction zone. California has many earthquakes due to many transverse faults. Oklahoma has more earthquakes than any other place in the United States and these are related to the practice of fracking for natural gas in which drills sent deep underground release water and chemicals which breaks apart deep rocks releasing natural gas but also weakening lower rock layers leading to earthquakes. Measuring Earthquakes A seismograph is an instrument used to measure earth movements. Earthquakes generate three main types of shock waves called primary waves (p waves), secondary waves (s waves), and long waves (l waves). P waves travel the fastest, s waves slower, and l waves the slowest. P and S Waves P waves are compression waves which push and pull back and forth as they move forward. P waves can travel through solids and liquids. S waves are transverse waves which move sideways to the direction that the waves travel in. S waves travel slower than P waves and can not travel in liquids but do travel through solids. Locating the Epicenter of an Earthquake Seismographic stations watch for P and S waves from earthquakes. The stations record P waves first and a bit later they record the slower S waves. The time between the first P and first S waves tells the recording station how far away the epicenter is. A circle is drawn whose radius represents the distance away that the epicenter is. Three Seismic Stations Needed to Fix Epicentre Location When three seismic stations share their circles of epicenter distance, where the circles all intersect is where the epicenter of the earthquake is. L Waves L waves travel only on the earths surface and shake rocks sideways as they move forward. P and S Waves Through the Earth P waves travel through the earth in all directions but S waves can not pass through the earths outer core which is proof that the outer core is liquid because S waves cant travel in liquids. A Past Supercontinent Scientists have evidence that millions of years ago there was a single supercontinent made up of todays separate continents. They have named this supercontinent Pangea. Evidence suggests that this supercontinent trapped heat energy beneath it, creating hot spots which caused cracks and rifts to form. These cracks and rifts eventually split Pangea into a Northern part that has been named Laurasia and a Southern part called Gondwanaland. The Fates of Laurasia and Gondwanaland Laurasia later split into North America and Asia. Gondwanaland split into South America, Africa, India, Antarctica and Australia. Dinosaur fossils have been found on all of the present day continents as well as tropical fossils, indicating a warm climate for the ages of Pangea, Laurasia, and Gondwanaland as well as the ability for dinosaurs to roam over a single land mass. End of Presentation A A