Plate Tectonics Quiz

Earth's Layers

• The Earth is made of distinct layers with deeper layers being composed of heavier materials, hotter, denser, and under greater pressure
• Natural forces affect the Earth's crust, creating landforms

Earth's Crust

• The crust is the Earth's rigid outer surface, rocky and made of two main rock types:
  • Continental crust (mostly granite)
  • Oceanic crust (basalt)
• Basalt is denser than granite, and this is why less dense continents ride on denser oceanic plates
• The crust is 3-5 miles (8 km) thick under the oceans and about 25 miles (32 km) thick under the continents

Lithospheric Plates

• The Earth's crust is broken into many pieces called plates
• These plates "float" on the soft, semi-rigid asthenosphere
• The crust and the upper layer of the mantle make up the lithosphere, it is brittle and rigid
• There are several large plates and numerous smaller ones

Mantle

• The mantle is the largest layer of the Earth, composed of silicon, oxygen, magnesium, iron, aluminum, and calcium
• The middle mantle is made of hot, dense rock that flows like asphalt
• Convection currents in the middle mantle (asthenosphere) cause the Earth's crustal plates to move
• Convection currents are caused by very hot material in the deepest part of the mantle rising, cooling and sinking (repeating cycle)

Core

• The core is like a ball of very hot metals composed of two layers:
  • Outer core: so hot that the metals are in liquid state. The outer core is primarily composed of nickel and iron.
  • Inner core: temperatures and pressures are so great that metals are squeezed together, not able to move, but forced to vibrate in place as solids

Continental Drift Theory

• German scientist Alfred Wegener proposed the continental drift theory in the 1900s
• He hypothesized that the Earth was once a single supercontinent (Pangaea)
• Wegener believed Pangaea floated on the oceanic crust like an iceberg
• Most scientists at the time rejected the theory due to a lack of evidence

Evidence of Continental Drift

• Continents fit together like a puzzle
• Fossils match across oceans
• Rock types and mountain ranges match across oceans
• Climate evidence found in glacial deposits
• Ocean floor spreading

Sea-Floor Spreading

• The process adds new material to the ocean floor while pushing older rocks away from a ridge
• New ocean floor forms along cracks in the ocean crust
• Molten material erupts from the mantle, spreading outward, pushing older rocks to the sides of the crack
• This process happens continuously, causing the ocean floor to move like a conveyor belt, carrying continents
• Harry Hess (1960s)

Evidence of Sea-Floor Spreading

• Rocks shaped like pillows (rock pillows) show that molten material has erupted numerous times from cracks along the mid-ocean ridge and cooled quickly
• The mid-ocean ridge system is the most extensive chain of mountains on earth. More than 90% are in the deep ocean, extending over 65,000 km
• It occurs along plate boundaries where new ocean floor is created as plates spread apart (divergent plate boundary)
• Plates spread apart at rates ranging from 1 cm to 20 cm per year
• As plates move apart, rock melts, rising from tens of kilometers deep

Evidence from Magnetic Stripes

• Rocks that make up the ocean floor lie in a pattern of magnetized stripes. These stripes hold a record of magnetic field reversals in Earth.

Evidence from Drilling Samples

• Core samples from the ocean floor show that older rocks are found farther from the ridge; the youngest rocks are in the center of the ridge

Plate Tectonic Theory

• The idea that the Earth's landmasses have broken apart, rejoined, and moved to other parts of the globe.
• Pieces of Earth's top layer are called tectonic plates. They move very slowly.
• Along the mid-ocean ridge, the seafloor is pulling apart, and the two parts are moving in opposite directions, carrying continents and oceans.
• Earth's surface layers are currently divided into nine large and several smaller plates

Earth's Major Plates

• The Earth's plates are not anchored in place. They slide over a hot, bendable layer of the mantle.

Plate Boundaries

• There are three main types of plate boundaries:
  • Divergent (pulling away from each other)
  • Convergent (crashing head-on)
  • Transform (sliding past each other)

Mechanisms of Divergent Boundaries

• Boundary between two plates moving apart or rifting
• Rifting causes seafloor spreading
• Features of divergent boundaries include mid-ocean ridges, rift valleys, and fissure volcanoes

Pulling Apart

• When plates pull away from one another, they form a diverging plate boundary or spreading zone
• Iceland has a divergent plate boundary running through its middle, with Thingvellir as an example

Mechanisms of Convergent Boundaries

• Boundaries between two colliding plates
• There are three types: ocean-to-continent collision, ocean-to-ocean collision, and continent-to-continent collision

Ocean to Continent Collision

• Ocean plate colliding with a less dense continental plate
• Subduction zone: where the denser oceanic plate slides under the less dense continental plate. It heats and dehydrates as it subsides
• Volcanoes occur at subduction zones
• Deep ocean trenches are also found here

Continental/Oceanic Crush

• Subduction. Process of ocean floor sinking beneath a deep ocean trench and into the mantle
• Deep Ocean Trench. It occurs at subduction zones where deep underwater canyons are formed due to bending of oceanic crust downward

Ocean to Ocean Collision

• When an ocean plate collides with another ocean plate, the denser plate slides under the less dense plate (subduction zone)
• The subducting plate bends downward forming a deep depression in the ocean floor (trench)
• The world's deepest parts of the ocean are found along trenches

Both Oceanic Plates Crash (Converge)

• When both plates are oceanic, one slides under the other.
• Often, an island group forms at this boundary

Continent to Continent Collision

• A continental plate colliding with another
• Plates push against each other, causing the formation of mountain ranges (such as the Himalayas and the European Alps)

Both Continental Plates Crash (Converge)

• Earth's highest mountain range, the Himalayas, was formed millions of years ago when the Indo-Australian plate collided with the Eurasian plate
• This collision is still occurring at a rate of about 5 cm per year

Mechanisms of Transform Boundaries

• Boundary between two plates that are sliding past each other
• Earthquakes occur along faults

Transform Boundary

• The San Andreas Fault is an example of a transform boundary where the North American and Pacific plates slide past each other at a rate of 5-6 cm per year

Consequences of Plate Tectonics

• Pacific Ring of Fire: Shows the margins of the Pacific tectonic plate and surrounding area, where active volcanism is focused along plate boundaries
• Hotspot Volcanoes: Hot mantle plumes rising to the surface in the middle of tectonic plates (e.g., Hawaiian Islands)

Global Distribution of Earthquakes

• Earthquakes occur along faults in the boundaries of plates. These patterns resemble the Pacific Ring of Fire volcanism region.

Tsunami Formation

• Huge mass of water with tremendous momentum
• Shallow-focus earthquake
• Large volume of oceanic crust displaced
• The movement of the crust also displaces a large volume of water
• Tsunamis reach greater heights in narrow spaces
• Open coastline = 8m wave, narrow harbor = 30m

The Rock Cycle

• A continuous cyclical change of rocks from one type to another through: chemical, organic, cementation, pressure, compaction, heating, cooling, transformation, melting, weathering, decomposition
• The process of rocks changing from one type to another
• Igneous, Sedimentary, and Metamorphic rocks are involved

Types of Rock

• Igneous
• Sedimentary
• Metamorphic

Igneous Rock

• Formed by magma and lava cooling
• Two types
  • Rock from lava cools very fast, light/airy, or crystal (glassy)
  • Rock from magma forms underground, mostly hard, multiple colors

Sedimentary Rock

• Made when sediments (sand, gravel, and dirt) are pressed together over time
• Forms in layers near the Earth's surface at low temperatures and pressures
• Can be formed by precipitation, water, wind or ice, and biological processes

Metamorphic Rock

• Made from other rocks through intense heat and pressure in the Earth's crust
• Heat and pressure change the mineral structure and physical properties of the parent rock without changing the chemical makeup

Rocks Classification

• Volcanic rocks: extrusive, form from lava
• Plutonic rocks: intrusive, form from magma
• Rock Parts
  • Crystals
  • Grains