Earth’s Structure and Composition

Earth’s Crust---

1.  Earth’s outermost layer is the crust. 

a)      made of relatively light and brittle rocks

b)      Earth’s thinnest layer---ranging from about 5 km to about 100 km thick

c)      makes up only about 1 % of Earth’s mass

d)      main elements---oxygen, silicon, aluminum

e)      average density of the crust ranges from about 2.7 g/cm³ to 3.0 g/cm³

f)        2 kinds of crust---continental crust and oceanic crust

·        Continental crust is the lighter, older, and thicker part of the crust, which makes up the continents.

·        Oceanic crust is the rock that makes up the ocean floor. Oceanic crust has nearly twice as much iron, magnesium, and calcium as the continental crust. These elements make it much denser.

Earth’s Mantle---

1.  The mantle is the layer of Earth beneath the crust.

a)      about 2,900 km thick and makes up nearly 67 % of Earth’s mass

b)      density ranging from 3.3 g/cm³ to 6 g/cm³

c)      The uppermost part of the mantle is solid, but tremendous heat and pressure make the middle of the mantle act more like a very thick liquid.

d)      In the lowest part of the mantle, the pressure is so great that it keeps the rock from melting.

e)      Magma is melted rock that forms below Earth’s surface. In some places, melted mantle rock rises to the surface.

f)        Active volcanoes on the ocean floor spew out lava. Lava is molten rock---magma---that reaches Earth’s surface.

2. Earth’s Core---2 parts---inner and outer core---inner core is denser

Tectonic Plates---

1.  The crust and the rigid and immovable part of the mantle make up the lithosphere. 

a)      resembles a giant jigsaw puzzle---each piece of the puzzle is a tectonic plate

b)      rigid layer

c)      about 100km thick

d)      less dense that lower layers

2. A tectonic plate is a large section of lithosphere that is in slow but constant motion (sometimes referred to as lithospheric plates).

3. Tectonic plates are able to move because they float on a denser, semi-molten layer of Earth known as the asthenosphere (hot and flowing).

a)      The asthenosphere is the soft layer within the mantle that flows like a very thick liquid.

b)      hot, plasticlike rock layer of the mantle below the lithosphere

c)      The matter making up the asthenosphere moves in a circular path (convection current).

d)      Convection currents in the asthenosphere are set in motion by temperature differences which cause the lithosphere to be in constant motion.

e)      Convection current drives the plate motion.

The Theory of Plate Tectonics

In 1912, Alfred Wegener, a German geophysicist and explorer, stated a hypothesis about Earth’s surface. He proposed that the continents had been in very different places in the past. Over a long time, the continents had drifted into their current locations. That is, the continents were moving. Since this motion is so slow, and at the time was undetectable within a person’s lifetime, Wegener’s ideas were not well received.

Drifting Continents---

1. Wegener called his hypothesis that the continents move continental drift.

a)      believed that hundreds of millions of years ago the continents were joined as one large landmass he called Pangaea

b)      evidence to support his idea---discoveries of similar rocks and fossils on separate continents

c)      most scientists rejected Wegener’s hypothesis (skepticism)

2. The theory of plate tectonics was developed in the late 1960s.

a)      Earth’s crust is made of several rigid plates that move on the asthenosphere

b)      Tectonic plates are large pieces of the lithosphere (sometimes called lithospheric plates) that are in constant motion. The plates float on top of the asthenosphere because it is a soft layer of the mantle that flows like a thick liquid.

Evidence of Plate Tectonics---

1.  At the bottom of the oceans, scientists have found mountain chains that are several thousand kilometers long and some are 5 km high. These mountain chains are known as mid-ocean ridges.

a)      located in the deep sea, between continents

b)      great deal of volcanic activity

c)      form as a result of the formation of new ocean floor

d)      the farther away one moves from the ridges, the older the rocks on the ocean bottom are

e)      youngest rocks on ocean floor are located near mid-ocean ridge

f)        creation of mid-ocean ridges take the same amount of time as mountain formation

2.  Scientists have found that magma squeezes up through the boundaries between oceanic plates. The plates move apart, allowing new crust to form on the seabed. This process is called seafloor spreading.

a)      If magma rises between two continental plates, a deep crack or rift valley forms, and new crust appears in the valley.

b)      rift valley--- where two continental plates pull apart (divergent boundary)

c)      The Great Rift Valley causes volcanic eruptions and earthquakes. The future will probably create an inland sea.

d)      Seafloor spreading and rift valley formation are both evidence for plate tectonics.

3.  Evidence for continental drift also helps support the theory of plate tectonics.

a)      the shapes of some continents seem to fit together like a puzzle

b)      identical rocks and fossil species have been found along the matching parts of the coasts of Africa and South America

c)      similarities in the distributions of ancient organisms on continents that appear to have once been joined

d)      zones of similar ancient climates that match up on different landmasses

e)      geologic similarities, such as mountain ranges that match up on their ends, on landmasses that appear to have once been joined

Plate Motion and Geological Events

Plates and Plate Movement---

1. Scientists have determined that the lithosphere is made up of seven main plates and six or seven smaller ones. Continental plates support the continents, and oceanic plates support the oceans.

Mountains and Volcanoes---

1.  The region where two tectonic plates meet is called a plate boundary. Interactions at plate boundaries change Earth’s surface.

a)      Earthquakes (faults) and volcanic eruptions most often occur near plate boundaries.

b)      Active volcanoes are most likely to form at convergent boundaries (oceanic-continental boundaries).

c)      Mountain ranges form when two continental plates collide.

d)      Folded mountains form where two plates made of continental crust collide (at continental-continental plate boundaries). A folded mountain forms when rock layers are squeezed together and pushed upward to form folds, or ripples in the curst. The highest mountains in the world are folded mountains.

·        Himalayas of Asia; Alps of Europe; Ural Mountains in Russia; Great Smoky Mountains in the U.S.

e)      Plates made of oceanic crust may collide at oceanic-oceanic boundaries. When this happens, one plate is subducted, or pushed under the other. A deep canyon, called a trench, forms in the ocean floor where the two plates meet.  As the subducted plate sinks into the hot mantle, it melts to form magma. The subduction of one plate under the other causes the melting of the lower plate. The region where one plate slides under another plate is called a subduction zone.

f)        At continental-oceanic boundaries, continental plates collide with oceanic plates. The oceanic plate slides under the continental plate. Chains of volcanic mountains can form on the edge of the continental plate. (western coast of South America)

2. A volcano is an opening in Earth’s surface through which magma is released. As newly formed magma rises and erupts as lava on the ocean floor, it cools and hardens. This causes a chain of volcanic mountains to form on the ocean floor.

a)      Some volcanic mountains grow in size and break through the surface of the ocean to form islands.

b)      An island arc is a long, curved chain of islands that form from volcanic activity. (Aleutian Islands of Alaska)

c)      shield volcano---thick layers of lava pour out of a vent and harden on top of previous layers; gradually building a wide, gently sloping mountain; least explosive; example---Hawaiian Islands

d)      cinder cone volcano---ash, cinders, and bombs build up around the vent in a steep, cone-shaped hill or small mountain; example---Paricutin in Mexico

e)      composite volcano—tall, cone-shaped mountains in which layers of lava alternate with layers of ash; example---Mount Fuji in Japan  and Mount St. Helens in Washington State

The Ring of Fire and Hot Spots---

1.  The edge of the Pacific Ocean is called the Ring of Fire.

a)      zone of frequent volcanic eruptions that circles the Pacific Basin

b)      contains more than 75 % of all volcanoes on Earth

c)      an arc that reaches from New Zealand, north along the eastern edge of Asia, across the Aleutian Island and south along the west coasts of North and South America

2.  Both volcanoes and earthquakes occur most often at or near plate boundaries.

3.  Not all volcanoes are at plate boundaries. An area of volcanic activity near the middle of a tectonic plate is called a hot spot.

a)      located beneath a plate

b)      remains in one place as the tectonic plate above it moves

c)      volcanic activity creates a chain of small volcanoes

d)      these volcanoes do not remain active once they are no longer over the hot spot

e)      The Hawaiian Islands are a chain of volcanic islands that formed as the Pacific Plate has traveled over a hot spot. The island chain continues to form to this day.

Earthquakes---

1.  An earthquake is a shaking of Earth’s surface.

a)      A seismic wave is a wave of energy that travels away from the center of an earthquake in all directions.

b)      Seismic waves increase going into Earth because they are moving from less dense material to more dense material.

c)      The break, or crack, in Earth’s surface along which rock has moved is called a fault. 

d)      Earthquakes are also associated with the folding that occurs when rock layers crumple during plate collisions.

e)      Earthquakes can happen at any type of boundary, but they are most common where plates slide by each other.

f)        Seismology is the branch of science concerned with earthquakes. Seismologists use instruments called seismographs to measure the speed and intensity of seismic waves. The recordings from these instruments are called seismograms.

g)      Richter scale—is a rating of an earthquake’s magnitude based on the size of the earthquake’s seismic waves.

Types of Boundaries---

a)      divergent boundary---boundary between two plates that are moving apart; 2 continental plates moving away from each other; forming a rift valley

b)      convergent boundary---two plates move together; mountain range forms

s         similar rock densities at convergent boundaries will form a mountain range

c)      transform boundary---where two plats slide past or grind past one another; move in opposite directions or same direction at different rates; when this happens suddenly---an earthquake occurs

d) The first waves to arrive are primary waves, or P waves. P waves are seismic waves that compress and expand the ground like an accordion. They travel through both solids and liquids.

e) After P waves, the there are secondary waves, or S waves. S waves are seismic waves that vibrate from side to side as well as up and down. S waves cannot move through liquids.

f) When P and S waves reach the surface, some of them become surface waves. Surface waves move more slowly than P waves and S waves, but they can produce severe ground movements.