Global tectonics: Earth's interior, earthquakes and global geomorphology

- Continental is thicker (approx 35km) vs Oceanic (7km)

- Continental is older (approx 2 billion years) vs Oceanic (60 million)

- Continental is less dense than oceanic

- Fold mountains

- Ocean trenches

- Rift valleys

- Ocean ridges

- Island arcs

- Divergent

- Convergent

- Transform

- When one plate moves under another (usually oceanic under continental), creating a subduction zone, high pressure squeezes water out of the rock, lowering the melting temperature of the rock

- Melted rock (magma) rises up as it is less dense than surrounding rock and collects in magma chambers

- 'Eventually, some of the magma pushes through vents and fissures to the Earth's surface' (USGS,2020)

- Becomes lava when it erupts

- Convergent plate boundary

- Layers of accumulated rock crumple and fold

- Himalayas = newer fold mountains (50 million years ago). These tend to be higher. - Appalachians = older fold mountains (480 million years). These tend to be smaller as they have been subjected to erosion and weathering

- Convergent boundary (oceanic-oceanic or oceanic-continental)

- More dense plate subducts, forming an undersea trench

- Deepest is Marianas Trench (36,000ft below sea level)

- Divergent plate boundary

- Called ocean ridges in the ocean

- Continental rift valleys e.g. East African Rift

- Further deepened by erosion

- When the tensional forces are strong enough to cause the plate to split apart, a center block drops between the two blocks at its flanks, creating a rift valley

- Divergent boundary

- Plates move away from each other due to mantle convection and slab pull, creating new crust as magma rises when the mantle upwells

- When eruptions happen along the boundary, lava builds up to create ridges

- As the plates move, new crust is formed

- Created along a subduction zone, paralleling the trench

- Subducting plate expels water, lowering the melting point and creating magma which collects in chamber

- Eventually the magma ascends, forming a chain of volcanoes as the plate moves

- Fold mountains

- Island/volcanic arcs

- Ocean trenches

- Ocean ridges

- Rift valleys

- Fault lines (build up pressure due to friction creating an earthquake) e.g. San Andreas Fault

- Oceanic fracture zones (form large valleys, or trenches) e.g. Molokai fracture zone

- The Dead Sea Rift

- Shield

- Composite cone

- Lava dome

- Cinder and Scoria Cones

- Other (Rhyolitic caldera complexes and mid-ocean ridge)

- Broad, gently sloping flanks

- Dome shape

- Built almost entirely of layers of solidified basaltic lava flows

- Most feature a central summit vent, and often flank vents, that eject low-viscosity basaltic lava that flows long distances in all directions

- E.g. Mauna Loa and Kilauea (Hawaii)

• - Often form in the craters or on the flanks of composite volcanoes but they can form independently
• - When high-viscosity rhyolitic magma flows from composite volcanoes, it solidifies quickly around a vent
• - Pressure from magma within the volcano expands the dome and eventually it erupts
• - E.g. Mount St Helens

- Steep upper flanks and a symmetrical appearance

- Alternating layers of material such as ash and cinders, blocks and lava deposited by past eruptions

- Also called stratovolcanoes

- Higher risk (explosive)

- Flying rocks and lava bombs, mudslides

- E.g. Mount Fuji

- Simplest and smallest volcano type (usually less than 1000ft)

- Circular cone of hardened lava, ash and tephra around a single vent

- Formed when volcanic material fragments and falls to the ground after being ejected into the air from the vent 

- E.g. Lava Butte, Oregon

- Ancient volcanoes that erupted so explosively they collapsed into the magma chamber beneath them, forming a giant crater, or caldera (e.g. Yellowstone)

- Mid-ocean ridges are undersea areas along tectonic plate boundaries where the plates are diverging.

- Explosive (gas-driven explosions that propels magma and tephra)

- Effusive (outpouring of lava without significant explosive eruption)

- Phreatic (heat from magma interacting with water)

- Strombolian and Hawaiian eruptions. (least violent)

- Subplinian

- Magma/lava

- Gas (water vapour, CO2, SO2, hydrogen sulphide). Released through eruptions and fumaroles

- Pyroclastic material (tephra)

- Ash (small grain/glass)

- Bombs

- Lahars

• - Hotspot - an area in the mantle from which heat rises as a thermal plume from deep in the Earth
• - Magma is formed by melting rock (due to pressure and temperature) and magma rises and erupts to form volcanoes
• - As the plate moves over the hotspot, the volcanoes move away and are replaced by new ones
• - Leads to chains of volcanoes like Hawaiian Islands