Features of Plate Tectonics Scientists believe that Earth began as a molten ball over 4.5 billion years ago! as it cooled, denser materials sank toward Earth’s core, less dense materials (ie. Si, O) floated to top  formed Earth’s crust ¾ crust made of silicates crust + uppermost mantle = lithosphere (65 – 100 km thick) tectonic plates (≈ 12 major + many smaller) form the lithosphere  2 types of tectonic plates 1.continental (mostly granite) 2.oceanic (mostly basalt)

Earth has four distinct layers: 1.Crust - outer solid rock layer 2.Mantle (upper & lower) - thickest layer, mostly solid except for upper mantle, which is able to flow like “thick toothpaste” 3.Outer core - composed of liquid iron and nickel 4.Inner core - mostly solid iron, at tremendous temperature and pressure

The Asthenosphere the lithosphere floats on the asthenosphere  the molten layer of the upper mantle  heat to keep asthenosphere molten comes from radioactive elements (ie. U)  temp. in asthenosphere varies throughout due to large amounts of radioactive elements in some areas  a convection current forms as hot, low density rock rises

Push & Pull magma cools when it reaches Earth’s surface at spreading centres  spreading centres in oceans = spreading / oceanic ridge  spreading centres on land = rift valley magma cools, solidifies, and is pushed aside as new magma pushes from below  ridge push

Plate Motion Tectonic plates are all moving at the same time!  Where continental and oceanic plates meet, subduction occurs (1 plate pushes below another).  Denser oceanic plate subducts under the less dense continental plate.  by “slab pull”, the rest of the plate follows  large EQs and volcanic eruptions are found in subduction zones

Plate Interactions A plate boundary is where 2 plates are in contact. The way the plates interact is based on: 1.the type of plate, and 2.the direction the plates are moving relative to each other. Types of Plate Boundaries:  divergent - spreading apart  convergent - moving together  transform - where plates slide past each other

Plate Interactions

1.Divergent plate boundaries are where plates are spreading apart. Examples: ocean ridges and continental rifts The Mid-Atlantic Ridge is the longest mountain range on Earth (≈ km long) and has the greatest amount of volcanic activity!

Plate Interactions 2. Convergent plate boundaries are where plates collide. A.Oceanic-Continental Plate Convergence The oceanic plate subducts under the continental plate, forming a trench. Cone-shaped volcanoes can form from magma seeping to the surface. Mountain ranges like the Coast Mountain range also form from the collision  continental crust crumples up EQs can occur when pressure build-up between plates is released

Plate Interactions

B.Oceanic-Oceanic Plate Convergence the cooler, denser plate will subduct under the other convergence may produce a volcanic island arc, such as those found in Japan & Indonesia

Plate Interactions (continued) C. Continental-Continental Plate Convergence since both are continental plates, densities are =, so no subduction As they collide, plates fold and crumple, forming mountain ranges.  The Himalayas are the world’s youngest (and tallest) mountain range.

Plate Interactions 3.Transform plate boundaries are where plates move past each other. usually are found near ocean ridges Since rock slides past rock, no mountains or volcanoes form. EQs and faults (breaks in rock layers due to movement on either side) are very transform boundaries

Earthquakes EQs often form when plates can no longer resist the build-up of pressure. 95% of all EQs occur at tectonic plate boundaries all 3 types of plate boundaries are found in / around BC Large EQs hit BC every years.

Describing Earthquakes Scientists understand why EQs happen, but it is very difficult to predict their timing, exact location and strength.  Their build-up happens underground, over very long periods of time.  What we *do* understand has helped scientists understand where & how often EQs occur. We can also prepare structures to survive them. The focus of the EQ is the location inside Earth where the EQ starts. The epicenter is the point on Earth’s surface directly above the focus.

Focus & Epicentre

Describing Earthquakes EQs occur at various depths (depending on the plates involved) & are classified according to depth of foci.  energy travelling from a deep focus may result in an EQ that does not cause much surface  EQs with a shallow focus tend to cause more damage

Seismic Waves are vibrations produced by an EQ when it releases energy reveal the source and strength of an EQ help us learn about the composition and distances of the Earth’s interior, since they behave differently in different Earth layers.  S waves cannot travel through liquid outer core Seismology is the study of these waves.

Types of EQ Waves

Measuring Earthquakes Seismometers / seismographs are used to produce records of ground motion.  These records are called seismograms, which show when an EQ started, how long it lasted, and the magnitude. Magnitude = a # that rates the strength of an EQ  an increase of 1 in magnitude = an EQ that is 10 X stronger  A magnitude 6 EQ is 100 X more powerful than one of magnitude 4.

Measuring Earthquakes Since seismic waves travel at different speeds, a distance- time graph can show the distance from an EQ monitoring station to the epicentre of the EQ.

Volcanoes The movement of tectonic plates causes volcano formation. 1.Composite Volcanoes / Stratovolcanoes large and cone-shaped belches smoke, ash, lava found along plate boundaries made of alternating layers of ash / cinder and lava usually form near subduction zones

Volcanoes 2. Shield Volcanoes these are not found at plate boundaries, but instead form over hot spots (generally in ocean basins) made of gently sloping layers composed entirely of cooled lava magma that forms shield volcanoes is thinner and traps less gas than the magma that makes composite volcanoes largest (in area) of the 3 main types of volcanoes; a low, wide cone The Hawaiian Islands are an example of a chain of shield volcanoes.

Volcanoes 3.Rift Eruptions – occur through long cracks in the lithosphere fountains of lava erupt at spreading ocean ridges / continental crust usually not very explosive, but release massive amounts of lava