Plate Tectonics: Interactions, Features and Geohazards

Earth’s Internal Structure

(USGS 1999, public domain)

How the Tectonic Plates Move

(modified from USGS, public domain)

Major Tectonic Plates

(USGS, public domain)

(Hughrance 2017, Wikimedia Creative Commons)

Divergent Boundaries

• Two plates are moving apart• Often under water (mid-ocean ridge) but can occur

on land

(H Grobe/ AWI 2007, Wikimedia Creative Commons)

Divergent Boundaries - Features

• New crust• Rift valley• Normal faults

Mid-ocean ridge (NASA 2004, public domain)

Divergent Boundaries - Geohazards

• Small, shallow earthquakes

• Centre is a special type of volcano – a fissure

Fissure eruption in HoluhraunIceland (Joschenbacher 2014, Wikimedia Creative Commons)

Age of the seafloor – NONE is older than 200 Ma!

(NOAA, public domain)

Convergent Boundaries: Continent-Ocean

• Continental and oceanic plates are moving toward each other

• The denser oceanic plate subducts (moves under) the less dense continental plate

(USGS, public domain)

• Trench• Reverse faults and folds• Volcanic mountain chain

Mt St Helens in the Cascade volcanic mountain range seen from the ISS (NASA 2017, public domain)

Convergent Boundaries: Continent-Ocean Features

• Frequent shallow earthquakes• Less frequent deep earthquakes• Composite/ stratovolcanoes can

be explosive and active for a long time

Eruption of Mt St Helens (USGS 1980, public domain)

Convergent Boundaries: Continent-Ocean Geohazards

• Two oceanic plates moving toward each other

• The denser plate (usually older) subducts under the other

(USGS, public domain)

Convergent Boundaries: Ocean-Ocean

• Trench (may be very deep)• Reverse faults and folds• Volcanic island arc

The Japanese archipelago is a volcanic island arc. (NASA Earth Observatory 2004, public domain)

Convergent Boundaries: Ocean-Ocean Features

• Frequent shallow earthquakes

• Less frequent deep earthquakes – may trigger tsunamis

• Composite/ stratovolcanoes can be very explosive

Sendai Airport after the 2011 earthquake and tsunami (Ministry of Land, Infrastructure, Transport and Tourism 2014, Wikimedia Creative Commons)

Convergent Boundaries: Ocean-Ocean Geohazards

• Two continental plates moving toward each other

• Both plates are too buoyant (light and floating on the mantle) to subduct

(USGS, public domain)

Convergent Boundaries: Continent-Continent

• High, steep mountain ranges

• Reverse faults and folds• Mountain ranges may have

seafloor sediments near the top – why?

Continental collision forming the Trans-North China Orogen (Zhao et al. 2019, Wikimedia Creative Commons)

Convergent Boundaries: Continent-Continent Features

• Infrequent shallow earthquakes

• Earthquakes may be damaging and can also cause landslides

• No (or rare) volcanic activity

Patan Durbar Square after the Nepal earthquake (Simoshace 2015, Wikimedia Creative Commons)

Convergent Boundaries: Continent-Continent Geohazards

• Two plates are moving past each other – alongside each other

• Plate edges are not smooth, so they can get ‘stuck’

San Andreas fault (USGS 2017, public domain)

Transform Boundaries

• Mountain chains and rocks may be displaced

• Faults

San Andreas fault (USGS 2011, public domain)

Transform Boundaries: Features

• Frequent shallow earthquakes

• Earthquakes may be devastating

Land displaced by the San Andreas fault after 1906 earthquake (public domain)

Transform Boundaries: Geohazards

Plate movements and volcanoes

(Astroshiandhike 2018, Wikimedia Creative Commons)