PLATE TECTONICS THE DIFFERENT TYPES OF PLATE BOUNDARIES

The Three Boundaries Every tectonic plate is bounded on all sides by a combination of the following three: Divergent Boundaries – where plates pull away from each other Transform Boundaries – where plates slide and grind past each other Convergent Boundaries – where plates collide and smash into each other

DIVERGENT BOUNDARY A weak zone in the lithosphere occurs, and magma from the mantle begins to rise up and force the plate to crack into two pieces Divergent margins are almost always found along the mid-ocean ridges under water We call this process seafloor spreading Seafloor spreading will produce new ocean crust as the two separating plates pull apart and the asthenosphere wells up New crust is formed at divergent margins Seafloor spreading causes the plate to increase in size

DIVERGENT BOUNDARY A close-up of mid ocean ridges reveals a very interesting and unique geology The long mountain chains of the mid-ocean ridges are actually an intricate system of high volcanic valleys that form when sea-floor spreading occurs

DIVERGENT BOUNDARY Divergent boundaries usually begins forming in the middle of a continent The continent begins to rip apart, and down- dropped rift valleys form As the spreading continues, a shallow sea then forms Finally a full expanding ocean is formed

DIVERGENT BOUNDARY The progressive stages of a divergent margin can be found on earth today: A) Rio Grande Rift Valley B) East African Rift Valley C) The Red Sea D) The Atlantic Ocean

Divergent – A Case Study The East African Rift Valley System is where the second stage of divergent boundary formation is taking place Large lakes and river systems fill up the down- dropped rift valleys Volcanoes are found on the valley floors East Africa will one day be its own plate

TRANSFORM BOUNDARY Transform boundaries do not form new lithosphere nor do they destroy it Plates grind past each other like two opposing trains passing each other on parallel tracks Most transform faults are on the ocean bottom, but the San Andreas Fault in CA is the one we most often hear about

TRANSFORM BOUNDARY Transform faults form long fracture zones along the sea floor that stretch away from mid-ocean ridges They connect offset areas where sea floor spreading is occurring The mid-ocean ridges are connected by the laterally trending transform faults These transform faults form long scars on the ocean floor that are called fracture zones

TRANSFORM BOUNDARY The San Andreas Fault is part of the boundary between the North American Plate and the Pacific Plate This boundary is much more extensive than just the San Andreas, and actually includes all three boundary types: convergent, divergent and transform

CONVERGENT BOUNDARY Remember that the crust can be of two varieties: continental (granitic) or oceanic (basaltic) This means that there are three possible ways that a convergent boundary can form 1. Continent/Ocean 2. Continent/Continent 3. Ocean/Ocean

CONVERGENT BOUNDARY Convergent boundaries form when plates smash into one another One of the plates, the one that is more dense, will plunge under the less dense one – this is called subduction As it descends into the mantle, the plate is melted and ultimately absorbed back into the rock cycle Some of the melted magma rises up through the lithosphere to form volcanic mountains

CONVERGENT BOUNDARY Continent/Ocean convergence – the oceanic basalt is more dense and is subducted below the continental crust The ocean crust buckles and forms a deep ocean trench just off shore When the subducting plate reaches the proper depth it begins to melt Some of the magma rises up through the continental plate and forms a volcanic arc This volcanic arc does not form directly on the coast, but rather it forms inland Subduction zones form when ocean crust is forced under the continental crust Ocean trenches are the deepest places on earth and they parallel subduction zones Volcanic arcs form inland from the subduction, where rising magma reaches the surface

CONVERGENT BOUNDARY Continent/Ocean – can be found in several places around the globe When subduction runs along the coastline of a continent, cont/ocean convergence is present Modern day examples of this boundary include such volcanic mountain ranges as the 1.Andes Mountains in South America 2.The Cascades in Pacific Northwest

CONVERGENT BOUNDARY Continent/Continent convergence – both crusts are of low density and will only partially subduct Both plates will buckle, fracture, and crumple when collision occurs When continents collide the largest mountain ranges in the world are formed Subduction is not a main factor

CONVERGENT BOUNDARY Before a cont/cont collision, subduction would have been occurring at a ocean/continent margin Therefore, there would have been volcanic arcs along the mainland As the landmasses approach each other, the sea becomes smaller and smaller and smaller Eventually, the landmasses meet and HUGE mountains are thrust upward The Himalayan Mountains are a current location of this boundary In addition, the Appalachian Mountains & the Ural Mountains in Russia are both ancient examples of this boundary

Convergent – A Case Study The Himalayan Mountains A) India advanced towards the Eurasian continent and the ocean basin in between was reduced Subduction took place underneath Asia and a large volcanic arc was present There would have also been a trench paralleling the Asian coastline C) Once the plates collided around 15 million years ago subduction would cease and the Himalayan mountains were uplifted Most volcanism stopped and India continues to drive the mountains and the Tibetan Plateau further upward

CONVERGENT BOUNDARY Ocean/Ocean convergence – both plates are made of high density ocean crust One of the plates will still be subducted Magma will rise up due to melting and form volcanoes These volcanoes will form on the sea floor though, not on land Eventually, the volcanoes will build up to the ocean surface and form and island chain Ocean/ocean collisions are very similar to continent ocean collisions Instead of volcanic arcs, these margins will form island arcs

CONVERGENT BOUNDARY An ocean/ocean boundary is very similar to continent/ocean collision because subduction occurs There is also a deep ocean trench present Magma will rise up from the subducting plate, but instead of forming a volcanic arc, an island arc is formed An Ocean/ocean collision will form large volcanic islands Modern day examples of this boundary are: Japan Islands Philippine Islands Aleutian Islands (Alaska)

Geology at the Boundaries Divergent (B) – under sea mountains, non-explosive volcanoes, minor earthquakes (< 5 on Richter Scale), new crust formed Transform (B) – small mountains, no volcanoes, large earthquakes (> 7 on Richter Scale) Convergent (A & D) – large mountains, explosive volcanoes, large earthquakes (> 7 on Richter Scale), crust consumed into the mantle A good way to remember the geology/formations is that everything is bigger at convergent boundaries

Know the Current Boundaries Divergent –Rio Grande Rift Valley –East African Rift Valley –Red Sea –Mid Atlantic Ridge –Mid Indian Ridge –East Pacific Rise Transform –San Andreas Fault Convergent CO –Andes Mountains –Cascade Mountains Convergent CC –Urals –Appalachians –Himalayans Convergent OO –Japanese Island Arc –Philippine Island Arc –Aleutian Island Arc

Divergent Boundaries Rio Grande Rift Valley East Pacific Rise Mid Atlantic Ridge East African Rift Valley Red Sea Mid Indian Ridge

Transform Boundaries San Andreas Fault

Convergent Boundaries Andes Mountains Japanese Island Arc Appalachian Mountains Himalayan Mountains Cascade Mountains Ural Mountains Philippine Island Arc Aleutian Island Arc

To Review: Divergent margins will form new lithosphere via the process of seafloor spreading Convergent margins will destroy lithosphere via the process of subduction Transform margins will neither destroy nor create lithosphere All major geologic features will occur along these three plate boundaries Know the modern examples of all five types of boundaries Rift Valleys will form at divergent boundaries Trenches will form along convergent boundaries paralleling subduction zones Volcanic arcs will form at continent/ocean collisions Island arcs will form along ocean/ocean collisions Earthquakes, volcanoes, and mountain ranges will indicate where the current boundaries can be found