Plate Tectonics Earth’s internal heat creates convection currents. Convection currents in the mantle drive the movements of the Earth’s crustal plates. Motions of the Earth’s plates create a variety of landforms and phenomena.

Some parts of the Earth’s mantle are especially hot Some parts of the Earth’s mantle are especially hot. Hot material is generally less dense, and less dense things rise. This creates a rising “convection current.”

Rising mantle currents split when they reach the top of the mantle Rising mantle currents split when they reach the top of the mantle. As they separate outward, this mantle material cools and eventually sinks back to the bottom of the mantle. In this way, patterns of circulating cells are created.

Tectonic Feature Background Information: Ocean Crust is more dense than Continental Crust. (That’s why it sits lower and gets covered with water.) It is also typically darker in color. Ocean Crust material is very similar to the material in Earth’s mantle. “Convergent” means “moving together” “Divergent” means “moving apart”

Typical Ocean Crust (Basalt) is dense and dark due to iron and magnesium content. Typical Continental Crust (Granite) Light in color and less dense, due to feldspar and silica content.

Sinking Mantle currents (cooler areas) create convergent boundaries Rising Mantle currents (hotter areas) create divergent plate boundaries

Ocean/Continent Convergent Plate Boundary: When they collide, an ocean plate dives beneath a continental plate (because ocean crust is more dense). Notice that some of this diving crust melts and rises back up to create volcanoes on the continent. Chain of volcanoes on continent Trench Magma melts easily and rises easily due to high water content of ocean crust Earthquakes from friction between plates

Ocean/Ocean Convergent Plate Boundary: When two ocean plates collide, one dives beneath the other. Notice that some of this diving crust melts, and rises back up to create a line of volcanoes (“island arc”) on the upper plate. Chain of volcanic islands on upper plate Trench Magma melts easily and rises easily due to high water content of ocean crust Earthquakes from friction between plates

Ocean/Ocean Convergent Plate Boundary: another view

Continent/Continent Convergent Plate Boundary: When two continental plates collide, neither plate is as dense as the mantle below. Therefore, neither plate dives below the other. They smash together, pushing crust up and down, and creating a tall mountain range. The Himalayas were formed in this way. Tall, non-volcanic mountains Earthquakes caused by crumpling and cracking of plates due to collision

Continent/Continent Convergent Plate Boundary: an example

Continent/Continent Divergent Plate Boundary: Magma pushes up from below, creating a spreading convection current. This is occurring in Eastern Africa. Over time, this turns into a…

Mantle Material hardens to create new ocean crust Ocean/Ocean Divergent Plate Boundary: Ocean crust is made of the same material as the mantle. As these pictures show, when a continent splits apart, magma from the mantle rises up and fills the gap. When this magma hardens, it becomes ocean crust. Since ocean crust is dense, it sinks low in the mantle and becomes covered by water. A new ocean is born. Rift Valley Earthquakes Mantle Material hardens to create new ocean crust Mid-Ocean Ridge

Newest Island. Directly over hotspot Ocean Hotspot: A minor mantle current creates a “plume” or “jet” of magma, called a “hotspot.” The plume shoots straight up through a moving plate. As the plate moves over the plume, this creates a chain of volcanic islands, the youngest of which is right over the hotspot. Newest Island. Directly over hotspot

Which way is the plate moving? Hawaii Hotspot: Typically, the newer islands (closer to hotspot) are larger. Volcanoes cool as they become farther from the hotspot. This causes them to shrink and sink below sea level. Which way is the plate moving?

Older islands cool as they move away from hotspot Older islands cool as they move away from hotspot. Cooling causes them to contract and sink. Hawaii Hotspot

Earthquakes from friction due to shearing Transform Plate Boundary: At this type of boundary, one plate shears across another. [The San Andreas fault, in CA, is a famous example] Earthquakes from friction due to shearing

Transform plate boundaries form along jagged convergent or divergent plate boundaries.

Development of Theory of Plate Tectonics 1620 – Sir Francis Bacon noticed that the shores of the Americas fit together with Europe and Africa 1915 – Alfred Wegener postulated “Pangaea” 1928 – Arthur Holmes suggested that convection currents in the mantle could drive plate movements. 1960s – Ocean floor mapping and other evidence caused most scientists to accept the Theory of Plate Tectonics.

1620 – Sir Francis Bacon noticed that the shores of the Americas fit together with Europe and Africa

1915 – Alfred Wegener postulated “Pangaea” 1915 – Alfred Wegener postulated “Pangaea”. He showed that, when the continents are placed together, traces of fossil remains match up.

Further evidence for Wegener’s Pangaea: when the continents are placed together, glacial remains also match up.

Wegener’s theory was widely dismissed because he could not suggest a “driving force” that would be capable of moving entire continents. 1928 – Arthur Holmes suggested that convection currents in the earth’s mantle could provide the driving force for plate tectonics

1960s – Ocean floor mapping showed a “Mid-Ocean Ridge” in the middle of the Atlantic.

The earth’s magnetic poles have reversed many times during the past The earth’s magnetic poles have reversed many times during the past. When lava hardens to form rock, the polarities of magnetic materials in the rocks are locked in position. This banded magnetic pattern suggested that bands of new rock harden from lava at the ridge and then get pushed outward by newer bands of rock (with opposite magnetism)

Dating of ocean bedrock shows that new rock is “born” at the mid-ocean ridge, and then gets pushed outward by newer rock.