The Changing Face of the Planet

Earth’s Interior Information gathered about the Earth’s interior comes from direct evidence from rock samples and indirect evidence from seismic waves Rocks from inside the Earth, as well as those ejected during volcanic eruptions, give geologists clues about Earth’s interior Earthquakes produce shock waves that travel through the Earth Changes occur in the movement of seismic waves due to differences in the structure and makeup of the Earth’s interior Scientist have determined what the interior looks like by monitoring the path and speed of seismic waves

The Earth’s Interior The four layers of the Earth are the crust, mantle, inner core and outer core These layers vary greatly in size, composition, temperature, and pressure

The Earth’s Crust The crust is the Earth’s outermost layer Made up of three types of solid rock Thickness varies with type Oceanic crust (makes up the ocean floor) Less than 10 Km thick Consists mostly of basalt Continental crust (makes up the landmasses) Average thickness of about 32 Km Made mostly of granite

The Earth’s Mantle Earth’s mantle is made up of rock that is very hot, but solid Divided into layers based on the physical characteristics of those layers Extends to a depth of about 3000 Km below the surface Contains about 80% of the volume of the Earth and 68% of its mass Made mostly of the elements silicon, oxygen, iron, and magnesium Density increases with depth

Divisions of the Earth’s Mantle Mohorovicic discontinuity (Moho)- Boundary separating the crust from the mantle Lithosphere- Topmost solid part of the Earth Composed of the crust and part of the upper mantle Broken into large sections called plates Asthenosphere- Located directly beneath the lithosphere A hot weak zone, capable of gradual flow Rock in this portion of the mantle can flow like a thick liquid (Has plasticity) Lower mantle- Zone of solid rock, located directly beneath the asthenosphere, and extends to the core

The Earth’s Core The Earth’s core is subdivided into two layers, an inner and outer core Both layers are composed mostly of the metals iron and nickel

The Outer and Inner Cores Outer Core- Layer of the molten metal surrounding the inner core High temperatures keep the iron and nickel in the outer core molten Inner Core- Dense ball of solid metal found at the Earth’s center Intense pressure causes the particles of iron and nickel to remain solid The inner core rotates within the outer core

The Core & Earth’s Magnetic Field Scientists think that convection currents in the liquid outer core create Earth’s magnetic field The magnetic field helps protect our planet from the Sun’s solar winds

The Earth’s Changing Interior It is believed that the Earth was not originally layered, the divisions we see today formed slowly over time Shortly after the Earth was formed, the decay of radioactive elements, along with heat released by colliding particles, produced melting in the planet’s interior Melting allowed the heavier elements (iron & nickel) to sink toward the center, while lighter, rocky components floated upward Still occurs today on a smaller scale

Plate Tectonics Theory which links the concepts of Continental Drift and Sea-floor Spreading to explain how the Earth has evolved over time Helps to explain the formation, movements, collisions, and destruction of Earth’s outer layers Helps people understand the geologic past and predict its future

Evidence for Plate Tectonics Continental Drift Location of volcanoes, earthquake belts and mountains Sea-floor Spreading Paleomagnetism

Continental Drift Proposed in 1910 by Alfred Wegener States that the continents were once joined together as a super-continent called Pangaea and have since drifted apart Since Wegener could not explain why the continents would move, his theory was originally rejected

Evidence for Continental Drift Shape of the continents Similar fossil deposits on continents thought to have been joined Rock formations that end at the edges of continents Glacial deposits (evidence of past climates) Distinctive rock types

Location of the world’s volcano, earthquake belts, and mountain ranges Most volcanoes, earthquakes, and mountain ranges are found along plate boundaries (places where one plate moves relative to another) Produced by stresses that build up along the boundaries As stresses become too great, fractures form and earthquakes occur Fractures allow magma from the asthenosphere to reach the surface, forming volcanoes Bending and folding of the Earth’s crust can create mountain ranges

Sea-floor Spreading Sea-floor spreading- Process in which old ocean floor is pushed away from a mid-ocean ridge by the formation of new ocean floor As the ocean floor spreads, landmasses on either side move apart Occurs at divergent boundaries (also called spreading centers) Younger rocks are found closer to the spreading center The further you go from the spreading center, the older the rocks become The same pattern of rocks are found on both sides of the center

Paleomagnetism Paleomagnetism- Study of the alignment of magnetic particles in ancient rocks Provides proof for sea-floor spreading and a means of determining how the continents have moved When magma cools, grains of iron line up with the magnetic pole (like little compasses) Polarity reversals occur in parallel bands on opposite sides of the mid-ocean ridges During the past 4 million years, the magnetic poles have reversed themselves 9 times

Theory of Plate Tectonics States that the topmost solid part of the Earth is divided into rigid plates that move resulting in earthquakes, volcanoes, mountains, and the redistribution of landmasses Lithospheric plates are made of a thin layer of crust above a thick layer of rigid mantle rock Usually contain both oceanic and continental crust Seven major plates, each named after its surface features Plates move at different speeds and in different directions

Earth’s Tectonic Plates

Plate Boundaries There are three basic types of plate boundaries Divergent- moving apart Convergent- moving together Transform fault- sliding past each other

Divergent Boundaries Plates move apart (diverge) Also called spreading centers or constructive boundaries New rocks are formed as older rocks are pushed aside (Lithosphere is created) Examples: Mid-Atlantic Ridge, East Pacific Rise, and the Great Rift Valley in Africa

Convergent Boundaries Occur where two plates move towards each other Also called destructive boundaries Lithosphere is destroyed There are three types

Types of Convergent Boundaries Convergent boundary where two continental plates collide Forms a single larger continent (India & Asia) Causes the lithosphere at the boundary to be pushed up, forming a mountain range Ex.: Himalayas, Urals, & Appalachian Mtns.

Types of Convergent Boundaries Convergent boundary where two oceanic plates collide One plate subducts (goes under) the other plate Also called a subduction zone Forms a chain of volcanic islands on the overriding plate and a deep sea trench where the plates meet Ex.: The Mariana Islands and the Mariana Trench are formed where the Pacific Plate subducts under the Philippine Plate

Types of Convergent Boundaries Convergent boundary where an oceanic and a continental plate collides The oceanic plate subducts under the continental plate Forms a chain of volcanic mountains on the continental plate and a deep sea trench along the edge of the continent Ex.: Along the west coast of South America, the Nazca Plate subducts under the South American Plate, forming the Andes Mtns. And the Peru-Chile Trench

Transform Fault Boundaries Also known as strike-slip or sliding boundaries Plates grind together as they try to slip past each other horizontally causing stress to build up Earthquakes occur when the stress is released Examples: The San Andreas Fault in California, is a result of the North American Plate and the Pacific Plate trying to slide past each other Transform fault boundaries connect portions of the mid-ocean ridge system that are moving at different rates

Why the Plates Move Convection currents within the asthenosphere are thought to be the driving force behind plate movement Convection current- the movement of material caused by differences in temperature Hot magma rises to the surface, pushing the older rocks aside and driving the plates apart (occurs at divergent boundaries) Cooler, denser currents sink back into the mantle, pushing the plates together (occurs at convergent boundaries)