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5-1 Notes Volcanoes and Plate Tectonics
volcano – weak spot in the Earth’s crust where magma (molten material) reaches the surface.
-Volcanic belts form along the boundaries of Earth’s plates
-occur when 2 plates diverge (pull apart) causing fractures, rift valleys that allow
magma to reach Earth’s surface
-occur when 2 plates converge (push together) causing one plate to sink beneath the
other plate
hot spot – area where magma from deep within the mantle melts through the crust above it
-volcano forms above the hot spot when magma erupts through the crust and
reaches the surface
-can be along plate boundaries but doesn’t have to be along plate boundaries
Ring of Fire – one major belt of earthquakes and volcanoes that rim the Pacific Ocean
5-2 Notes Volcanic Eruptions
magma chamber – pocket beneath a volcano where magma collects
pipe – a long tube through which magma moves from the magma chamber to Earth’s surface
vent – the opening through which molten rock and gas leave the volcano
lava flow – the area covered by lava as it pours out of a volcano’s vent
crater – bowl-shaped area that may form at the top of a volcano around the central vent
When a volcano erupts, the force of the expanding gases pushes magma from the magma
chamber through the pipe until it flows or explodes out the vent.
Silica is made of oxygen and the element silicon and is found in magma along with water and
other materials.
5-2 Notes Volcanic Eruptions (Continued)
Volcanic particles vary in size:
ash – small pieces of rock particles as small as specks of dust
cinders – pebble sized particles
bombs – large particles from golf ball size to the size of a car
2 types of volcanic eruptions:
1. quiet – magma is hot with low silica so it is thin and runny flowing easily covering large area
-pahoehoe – (ropelike coils) forms from fast-moving hot, runny, thin lava
-aa – (rough jagged surface) forms from lava that is cooler, thicker, and slow moving
2. explosive – magma high in silica, thick, and sticky plugs up the volcano’s pipe allowing
the magma to explode when the pressure is too high
-pyroclastic flow – mixture of hot gases, ash, cinders, and bombs that slide
down the sides of a volcano when it erupts violently
stages of volcanic activity:
1. active volcano – a “live” volcano that is erupting or has shown signs that it may erupt
in the near future
2. dormant volcano – a “sleeping” volcano is expected to awaken in the future
3. extinct volcano – a “dead” volcano is unlikely to ever erupt again maybe because
they drift away from a hot spot
Changes in activity in and around volcanoes give warnings of possible eruptions. Ex A tiltmeter
detects surface changes in elevation, and ex rising temperature in underground water.
5-3 Notes Volcanic Landforms
Soil near volcanoes is great for farming.
Volcanic eruptions create landforms made of lava, ash, and other material creating 3 types of
volcanoes, plateaus and/or calderas (huge holes left by the collapse of volcanoes).
1. cinder cone volcanoes – steep, cone-shaped hill or small mountain made of ash,
cinders, and bombs piled up around a volcano opening
-has a high silica content causing it to be thick and sticky
- very explosive
2. composite volcanoes – tall, cone-shaped mountain in which layers of lava alternate
with layers of ash and other volcanic materials
-silica content varies, quiet runny lava alternating with
explosive eruptions of ash, cinder, and bombs
3. shield volcanoes- wide gently sloping mountain made of layers of lava with low silica
content which creates quiet, runny lava flows
lava plateaus – made from thin, runny lava flowing out of several long cracks traveling far
before cooling and hardening, after millions of years of repeated lava flows the
area can form a high level plateau
Features formed by magma:
1. volcanic necks – forms when magma harden in a volcanic pipe and the
surrounding rock wears away
2. dike – a slab of volcanic rock forms when magma forces itself across rock layers
3. sill – a slab of volcanic rock formed when magma squeezes between horizontal rock
layers
4. dome mountain –hardened magma are uplifted bending the layers of rock and
eventually rock above the mountain wears away leaving the dome mountain exposed
5. batholith – mass of rock formed when a large body of magma cools inside the crust
and forming the core of many mountains