Transcript Chapter 8
Chapter 8 Foundations of Geology Structure of the Earth The earth can be divided into three parts: • Crust • Mantle • Core The Earth’s Crust The crust is the part of the earth we are most familiar with… It is an outer layer of rock… The crust on average is 22 miles deep below the continents and 3 miles thick below the oceans… The thickest part is beneath mountain ranges as thick as 40 miles… The Earth’s Crust Continental and oceanic crust are composed differently… Continental crust is made of granite (a rock composed chiefly of silica and alumina. Oceanic crust is made of basalt (a denser rock that contains silica and magnesium). The Earth’s Crust Most of the crust is covered by a thin layer of sediments, deposits of sand and minerals… These are usually laid down by water and were probably laid down during the worldwide flood… In many places, heat and pressure have compacted these sediments into solid rock… Composition of the Earth’s Crust Most abundant element: Oxygen (50%) Second: Silicon (28%) These are usually found together as: silica (SiO2). Third and Fourth: aluminum and iron (13%) Also found in small amounts: calcium, sodium, potassium, magnesium, copper, tin, lead, silver, gold, and uranium. Carbon and hydrogen compose less than 0.2%. The Mantle • Everything we know about the core is from studying seismic waves as they travel through the earth… • The lowest part of the crust is called the Moho named after the scientist who first studied waves traveling through the earth… The Mantle The mantle is divided into two parts… The upper mantle (From Moho to 250 miles down) The lower mantle (From 650 miles to 1800 miles deep) In between is the “transition zone”… The density increases with depth… The Mantle Like the crust, the mantle is not solid rock… Due to high temperatures and great pressure, the rocks in the mantle flow like thick syrup… These rocks are often referred to as “plastic rock”…it has properties of a solid and liquid. The Earth’s Core The bottom of the mantle is called the Gutenberg discontinuity… The core extends to the center of the earth…4000 miles. The core has two layers: Outer core – molten iron and nickel – convection currents give rise to our magnetic field. Inner core – solid iron or nickel Movements of the Plates The earth’s crust is made of several plates that float on the plastic rock of the upper mantle (asthenoshpere). Most of the U.S. is on the North American Plate…except for parts of southwestern California which is on the Pacific Plate. In all there are seven large plates and several smaller ones. Movements of the Plates • This theory of plate movement is called Plate Tectonics… • According to this, convection currents in the mantle cause huge forces on the plates… Movements of the Plates • The plates slowly then move at a rate of a few centimeters per year… • Some of the plates grind against each other producing earthquakes… • Some plates sink beneath others… • This is called subduction. Movements of the Plates • Some spread apart and create trenches in the ocean… • The trenches are a result of sea floor spreading… Movements of the Plates • The mantle is in motion and it causes chages to occur on the earth’s crust! Major Plates of the World Pangaea Some scientists have taken plate tectonics a step further… They say the continents at one time were one huge continent called “Pangaea”… Pangaea There is no way to prove or disprove this… However, Bible believing scientists have proposed that it is possible a breakup occurred during the flood… Pangaea Folds and Faults It is a fact that the earth’s crust does move… When it moves it creates folds and faults in the crust… Types of Faults There are three major types of faults: 1. Normal Fault 2. Strike-slip Fault 3. Thrust Fault Normal Fault Normal fault occurs when rocks on side of the fault slip vertically… Normal Fault Result: Huge cliffs Strike-slip Fault Strike-slip fault occurs when rocks along one side of a fault move horizontally along the fault. Example: San Andreas Fault Strike-slip Fault Example: San Andreas Fault Thrust Fault Thrust fault occurs when rocks on one side of the fault are thrust on top of the rocks on the other side. Thrust Fault Result: Folding Folds A fold is the bending and buckling of rocks under great pressure. It usually takes GREAT amounts of time!!! But, there is evidence that some have occurred quickly…Most likely as a result of the flood… Folds There are two types of folds: 1. Syncline: Folds forming a trough 2. Anticline: Folds buckling upward Syncline Barstow syncline, a beautiful fold in Miocene shales and sandstones, Rainbow Basin, Mojave Desert, California. This fold formed at a bend in a strike-slip fault. Folds near Calico ghost town, northeast of Barstow, Mojave Desert, California. Like the Barstow syncline, these folds formed at a bend in a strike-slip fault. Their axes strike east-west, indicating that the forces that deformed the rocks squeezed from the north and south. Anticline Mountains Most Bible believing scientists speculate that the mountains before the flood were not the tall, steep, rugged mountains of the Rockies, or Himalayas…they may have been gently sloped. Types of Mountains There are four types of mountains today: 1. 2. 3. 4. Volcanic Domed Folded Fault-block Volcanic Mountains • Formed when molten rock erupts from a hole in the earth’s crust. Volcanic Mountains • Most volcanic mountains are found in the mountain ranges on the west coast of North and South America. Volcanic Mountains • The tallest Mauna Kea in Hawaii – 32,000 feet above the sea floor (14,000 feet above the water’s surface) Domed Mountains • Formed when molten rock moves up toward the surface, but never breaks through. • Often are found in the same regions as volcanic mountains. Folded Mountains • Formed when the edges of two adjacent rock layers were pushed violently together, causing the layers to buckle like a wrinkled rug. Fault-Block Mountains • These mountains are thought to have formed along faults. • Rocks on one side are forced upward while rocks on the other side sink. Fault-Block Mountains Example: Sierra Nevada Mountains here in California Earthquakes An earthquake is a trembling or shaking of the earth’s crust!!! Wow!!! Even the largest quakes typically only last about a minute… Causes of Earthquakes • Most are caused by the movement of crust…called tectonic earthquakes.. • These are the result of sudden movement of rock masses along a fault. • This process is known as faulting… Causes of Earthquakes • Moving rocks bound up against nonmoving rocks forming faults… • Eventually, the forces and strain on the rocks is too great and the rocks break causing an earthquake… • According to elastic rebound, rocks on either side spring back to a position of little or no strain triggering an earthquake… Causes of Earthquakes • Sometime these elastic rebounds are so strong they can be detected around the globe… • The “rebound” rocks form unusual land features… • The rocks on one side may end up higher on one side forming a “fault scarp”… • The rocks may be polished smooth by the motion, producing smooth-faced fault scarps called “slickenslides”… The San Andreas Fault • This fault exists because the western edge of California is moving northwest at about 2 inches per year… • Sometimes the movement gets stuck in places and when the rock finally “breaks” from pressure an earthquake is produced… • When the 1906 San Francisco quake occurred, the rocks slipped as much as 21 feet in some places… Focus and Epicenter • The point at which an earthquake begins is the focus… • The point on the earth’s surface above the focus is the epicenter… • The effects of the earthquake are most severe near the epicenter… • Focuses can be near the surface…(shallow at 5-40 miles) or deep at 435 miles… Earthquake Waves There are two categories of waves: 1. Subsurface waves 2. Surface Waves Subsurface Waves • When rocks strain against each other and then suddenly snap free, they may release as much energy as thousands of atomic bombs.. • This energy spreads out in the form of two kinds of waves: 1. Primary waves (P waves) 2. Secondary waves (S waves) Primary Waves (P waves) • They are the fastest seismic waves… • Consist of a rhythmic push and pull motion… like a spring • P waves are the first waves you feel… Secondary Wave (S wave) • Consist of a rhythmic side to side motion • S waves produce the dramatic rolling motion that causes great damage to buildings… • They travel at about half the speed as P waves… Surface Waves • Are P and S waves that reach the surface… • They shake the ground side to side… • They move more slowly than P and S waves, but are felt at greater distances… Seismograph • Seismologists record and study earthquakes on seismographs… • Consists of a heavy pendulum that remains stationary while the base moves… Determining Distances • P waves outrun S waves… • After about 50 miles, P waves have gained 10 seconds on the S waves… • Scientist are then able to tell how far away the earthquake is… • To know the exact location, they need 3 seismographs to triangulate the location… Earthquake Zones No part of the earth is earthquake free… but… Most earthquakes do happen in narrow belts or zones… One zone is the Circum-Pacific Belt – It borders the Pacific Ocean…About 80 % of all earthquakes are felt along this zone… Earthquake Strength There are two scales by which earthquakes are measured: 1. The Mercalli Scale – it is not an actual measurement of energy – it is based on damage and perceived strength. 2. The Richter Scale – Rates and earthquake based on seismograph readings – rates quakes 1 to 10 – each step is 31 times greater If a quake is 7 or greater, it is considered a major earthquake. • http://www.exploratorium.edu/snacks/snac kintro.html • http://earthquake.usgs.gov/learning/kids/c oloring/waves.gif