10.2 Intrusive Igneous
Activity
Plutons
Structures that result from the cooling & hardening of
magma at depth
Form deep down
Can only be studied once they are uplifted and erosion
have exposed them
Variety of shapes & sizes
Sills & Laccoliths
Dikes
Batholiths
Sills & Laccoliths
Plutons that form when magma is intruded close to the
surface
Sills & Laccoliths vary in shape & composition
Sills form when magma is injected along sedimentary
bedding surfaces, parallel to the bedding planes.
Horizontal Sills are the most common
To form, the overlying sedimentary rock is lifted to
equal height of the sill. Both have the same thickness.
Sills & Laccoliths
Laccoliths are formed when magma is intruded
between sedimentary layers close to the surface
Similar to Sills
Magma that makes Laccoliths are more viscous
Dikes
Form when magma is injected into fractures, cutting
across preexisting rock layers
Sheet-like structure
Thickness ranges from < 1 cm - > 1 km
Many dikes form when magma from a large magma
chamber invades fractures in the surrounding rocks
Batholiths
Largest intrusive igneous bodies
Very thick
An intrusive igneous body must have a surface
exposure > 100 km to be considered a batholith
Uplift and erosion have removed the surrounding rock
that then exposes the batholith
Origin of Magma
Geologists conclude that magma originates when
essentially solid rock partially melts by raising the
temperature
Heat
Hotter the further in you go
Pressure
 Pressure =  rock’s melting temp.
Water
Causes rocks to melt at  temps.
3 Ways Magma Can Form…
1. Heat
2.  Pressure
3. Water can  the melting temp. of mantle rock
10.3 Plate Tectonics &
Igneous Activity
Convergent Plate Boundaries
The main connection between plate tectonics &
volcanism is that plate motions provide the
mechanisms by which mantle rocks melt to generate
magma
Ocean – Ocean
Ocean – Continent
Divergent Plate Boundaries
Most magma is produced along the oceanic ridges
during seafloor spreading
Intraplate Igneous Activity
Intraplate volcanism occurs within a plate, NOT AT A
PLATE BOUNDARY
Most intraplate volcanism occurs where a mass of
hotter than normal mantle material called mantle
plume rises toward the surface.