Plate Tectonics

Earth’s Layers

  • Asthenosphere:

    • The soft layer of rock beneath the lithosphere made of weak rock that is slowly flowing.

  • Lithosphere

    • The outermost physical layer of Earth composed of solid, rigid rock and is divided into plates.

Lithosphere

  • The Lithosphere is Earth's outermost layer & is made of solid rock.

  • The lithosphere is broken into tectonic plates.

2 Types of Plates:

  1. Oceanic Crust

  • Thinner

  • Newer (seafloor spreading)

  • More dense

  1. Continental Crust

  • Thicker

  • Older

  • Less dense


  • Oceanic-oceanic convergent boundaries occur where 2 oceanic plate collide. Since both oceanic plates are made of basaltic rock, usually the older plate is subducted under the younger plate, as the older plates are cooler & thus denser. 

  • Continental-contiental convergent boundaries occur where 2 continental plates collide. Because of its lower density, continental crust doesn’t (or very rarely) subducts. They smash together and create mountain ranges.

  • The cool, rigid, outermost layer of Earth that consists of the crust & the uppermost part of the mantle; broken into pieces or segments called plates.

  • SOLID WITH RIGIDITY

  • 10-200 kilometers deep

  • Least dense

  • Made of solid rock called lithospheric plates, aka tectonic plates.

  • Crust + Uppermost-mantle

Asthenosphere

  • The asthenosphere is also made of rock, however it is weak & actually flows very slowly – creating convection currents.

  • The solid layer with plasticity in the upper mantle that is located just below the lithosphere; lithospheric plates ‘float’ & move on this layer.

  • THINK: Pulling apart a milky way candy bar, the gooey caramel moves slowly and takes pieces of the chocolate shell with it.

  • Heat & pressure within the Earth causes the hot magma to flow in convection currents. These currents cause the movement of the tectonic plates that make up the Earth's crust.

  • Less dense, warmer material rises while more dense, cooler material sinks. It is this movement that creates circulation patterns known as convection currents in the mantle of the Earth.

  • SOLID WITH PLASTICITY

    • Plasticity means to exist in a solid state, yet having the ability to flow.

    • Meaning to exist in a solid state, yet having the ability to flow.

  • 660 kilometers deep

  • The lithospheric plates ‘float’ on top o the asthenosphere

  • Upper mantle

    • A shallow layer

Mesosphere

  • SOLID WITH RIGIDITY

  • 2900 kilometers deep

  • Lower mantle

Outer Core

  • The outer layer of Earth’s core; surrounds the inner core 

  • MADE OF LIQUID NICKEL AND IRON

  • 5100 kilometers deep

  • LIQUID

Inner Core

  • The sphere of solid nickel & iron at the center of Earth; surrounded by the liquid outer core.

  • SOLID

  • 6378 kilometers deep

  • MOST DENSE!

Crust

  • The thin, solid, outermost layer of Earth; is either continental (landmasses) or oceanic (ocean floors)

  • Made primarily of the silicates; oxygen and silicon

Mantle

  • The solid layer of Earth between the cruDst & the core; made of dense silicates.

  • The mantle is a solid state of matter, but wich differences in density & plasticity.

Core

The core is composed of the densest materials, primarily iron and nickel.

  • The Earth’s outer core is LIQUID MAGMA.

  • Earth’s Inner Core is solid and estimated to be approximately 5700 K which is about the temperature of the Sun.

  • The inner core’s intense pressure – the entire rest of the planet & its atmosphere – prevents the iron from melting.

  • The pressure & density are simply too great for iron atoms to move in a liquid state.

*** AS THE DEPTH OF THE LAYERS INCREASES, THE HEAT & PRESSURE INCREASES. ALL OF THE LAYERS OF THE MANTLE ARE MADE UP OF THE SAME THING. IT’S JUST THE DIFFERENCE OF TEMPERATURE AND PRESSURE. BOTH LAYERS OF THE CORE ARE MADE UP OF THE SAME THING. IT’S JUST THE DIFFERENCE OF TEMPERATURE & PRESSURE.

Plate Types

  • The Earth’s surface is cracked into large pieces called tectonic plates.

  • Tectonic plates slowly move, creating mountains, islands, and even rearranging continents.

  • Scientists can track the history of the plate movements by analyzing fossils & rock layers.

Convergent boundaries

Where 2 plates come together 🡂🡀

  • Colliding or towards each other

  • Folded mountains, mountain belts

Oceanic-Oceanic

  • The older plate subducts under the new one.

  • Features:

    • Volcanoes

    • Island arcs

    • Trenches

  • Examples:

    • Mariana’s Volcanic Islands

    • Mariana Trench

Oceanic-Continental

  • The more dense ocean plate subducts under the continental plate.

  • Features:

    • Volcanoes

    • Island Area

    • Trenches

  • Examples:

    • Philippine Islands

    • Aleutian Islands of Alaska

Divergent boundaries

Where 2 plates move apart 🡀 🡂

  • Apart, dividing, or away from each other.

  • Continental Divergence:

    • Rift valleys

    • Fault-block mountains

  • Oceanic Divergence:

    • Seafloor spreading

    • Mid-ocean ridges

***NEW OCEAN BASIN FORMATION***

  • When plates spread apart within a continent, the continental crust at the site of rifting thins.

  • Magma made of basalt moves up & fills the space where continental crust used to be.

  • Eventually, if a continental rift spreads apart enough, oceanic crust forms & creates new ocean basins.

Transform boundaries

Where 2 plates slide past each other 🢛🢙

  • Sliding past each other

  • Earthquakes and fault lines


Continental Drift

  • Continental drift is the hypothesis, proposed by Alfred Wegener, that all of today’s continents were once part of a single landmass.

    • Alfred Wegener was an astronomer, meteorologist, and he proposed the Continental Drift Theory in 1912. (great timing dude. Just in time for world war 1)

  • Wegener called this landmass Pangea and proposed that it broke into pieces that slowly moved or ‘drifted’ over time to the continent’s present locations.

  • Adding to Wegener’s ideas, the geologist Alexander du Toit suggested that Pangea first separated into Laurasia and Gondwana 200 million years ago.

Evidence

Wegener found evidence in the shapes of the continents, climatic evidence, location of rock types, fossils, and land features, such as trends in mountain ranges.

  1. Matching Coastlines suggest that continents were once connected — like puzzle pieces!

  2. Fossils of plant and animal species are found on continents that are now separated by vast oceans.

  3. Climatic Evidence from glacial activity (scraping & sediment clues) can be seen across South America, India, Africa, and Australia – places that do not currently have glaciers.

  • Different continents have matching rock layers & land features, like mountain ranges.

  • Over millions of years, COAL forms from plant remains in swampy, tropical areas.

  • Coal deposits found in cooler climates, even Antarctica, suggests that these continents were once closer to the equator.

  1. Rock Evidence

  • The Caledonian mountain range in northern Europe & the Appalachian Mountains in eastern North America are similar in age & structure. They are made up of he same rock types.

  • Suppose you placed North America & Europe next to each other. These mountains would meet and form one long, continuous mountain belt.

Superposition

  • The law of superposition is a principle of geology that scientists use to determine the relative ages of rock layers.

  • This principle states that layers on rock are layered one on top of another.

  • The oldest rock layer will be on the bottom & the youngest at the top.

Plate Tectonics Theory

Despite all his evidence, Wegener didn’t have an explanation for the energy needed to break apart huge chunks of continent and move them through the oceans.

  • Continental drift was not widely accepted by the scientific community, but the hypothesis evolved into a theory called plate tectonics.

    • Plate tectonics states that Earth’s surface is made up of giant, moving slabs called tectonic plates.

    • Marie Tharp was one of the 1st scientists to map the ocean floor. Her maps supported the theories of seafloor spreading & continental drift!!

      • Using a sonar device to detect depth measurements, Heezen & Tharp created a map of the floors of our oceans.

      • The centerpiece of their map was a large mountain system in the middle of the Atlantic Ocean, called the Mid-Atlantic Ridge. It is just one of the several large mid-oceanic ridges that extend across several oceans. Heezen and Tharp showed that ocean floors weren’t just flat, but had mountains, valleys, and deep trenches, which are the deepest feature on the planet.

  • At first, continental drift was not widely accepted by the scientific community, but the hypothesis evolved into a theory of plate tectonics.

    • Despite all his evidence, Wegener didn’t have an explanation for the energy needed to break apart huge chunks of continents & move them through the oceans.

    • Harry Hess, another pioneer of plate tectonics, mapped the ocean floor using sonar technology during World War 2. Hess proposed the theory of seafloor spreading, the formation of new areas in oceanic crust/seafloor on either side of the ocean ridges as magma flows out & pushes away the old seafloor.

      • Harry hess was an American geologist and Navy officer in WW2 who is considered one of the founding fathers of plate tectonics.

      • A geologist, Hess proposed the theory of seafloor spreading in 1960.

    • Scientists have discovered that the magnetic poles of the Earth have reversed many times throughout its history. Magnetic Reversals are recorded in the rock making up the ocean crust, and the pattern they create is seafloor spreading.

  • Plate tectonics suggest that Earth’s surface – it’s crust and upper mantle – is made up of giant, slow moving pieces called tectonic plates.

  • There are 7-8 major tectonic plates & many minor plates in between where these plates meet. (These numbers are prone to change 🙄)

    • This explains how the continents were able to drift and move over time.

Evidence For Plate Tectonics

  1. Seafloor Spreading

In the ocean, we see:

  • Plates move away from each other

  • Magma rises up & creates new seafloor, pushing existing rock in opposite directions.

    • Magma is molten rock that is underground & LAVA is the molten rock that breaks through Earth’s surface.

  • This results in a sort of layering of rock, where we can actually see the stripes of new seafloor being made over time.

  1. The Locations of Earthquakes & Volcanoes

Most active volcanoes are found near plate boundaries & most earthquakes take place at or near these boundaries as well.

Magnetic Reversals

***The magnetic poles of the Earth have reversed many times throughout its history. Magnetic Reversals are recorded in the rock making up the oceanic crust, and the pattern they create is evidence of seafloor spreading.

  • These changes result in magnetic striping along the ocean floor.

  • Each red & yellow stripe indicates a change in the Earth’s magnetic field.

  • This is called a magnetic reversal pattern.

  • So, Earth’s magnetic field has recorded the formation of new oceanic crust as the seafloor has spread apart at the mid-ocean ridge. This supports the theories of plate tectonics & seafloor spreading.

  • Dating of seafloor rock also helped confirm that the seafloor is spreading.

  • Rock farther from mid-ocean ridges is older than rock closer to the ridges. This makes sense because new rock forms at mid-ocean ridges & pushes older rock away.

  • When lava is extruded at any mid-ocean ridge, the rock it forms becomes magnetized & acquires the magnetic polarity that exists at the time the lava cools. As the crust moves away from mid-ocean ridges, it contains a continuous record of Earth’s changing magnetic polarity.

Convection Currents

  • The heat from the Earth’s core drives convection currents throughout the mantle because magma moves as a thick fluid, rising & turning over again & again as it is heated from below.

    • Does hot air rise or fall? 

      • Heat decreases density. Less dense materials float. Material that is more dense will sink.

  • Hot magma will RISE & as it moves away from the hot core, it will COOL & FALL BACK DOWN.

  • Hot particles move faster, are more spread out, & are less dense. Cool particles move more slowly & pack in together, making them more dense. This also explains why cool, dense particles sink & hot free flowing particles rise.

Tectonic Plate Interactions

  • Tectonic plate movement puts stress on rock!!

    • Stress is the force applied to a rock.

  • The stress causes deformations, which is the bending, tilting, and breaking of rock.

  • 3 TYPES OF STRESS:

  1. Compression

  • At convergent plate boundaries

  1. Tension

  • At divergent plate boundaries

  1. Shear

  • At transform plate boundaries.


  • Deformations usually occur at boundaries where tectonic plates meet. These plate boundaries may be on the ocean floor, around the edges of continents, or even within continents.

  • Compressive stress happens at convergent plate boundaries where 2 plates move toward each other.

  • Tensional stress happens at divergent plate boundaries where 2 plates are moving away from each other. Shear stress is experienced at transform boundaries where 2 plates are sliding past each other.