Section 18.3 Intrusive Activity
* Most of Earth’s volcanism happens below the surface.
Rock features formed from magma cooling and hardening on the surface would be known as:
extrusive igneous rock bodies.
Rock features formed from magma cooling and hardening underground would be known as:
intrusive igneous rock bodies
or
plutonic rock (plutons).
When this molten rock begins to cool off underground,
specific rock features will be formed.
Also, as the magma cools the minerals contained in the
magma will begin to crystallize. The deeper the magma, the
slower it will cool (and the larger the crystals).
The size of the crystals in the rock is known as its TEXTURE
Batholith – The largest intrusive igneous rock body that is irregularly shaped and cuts across
older rocks (forms 5 to 30 km beneath Earth’s surface).
Stock – The second largest intrusive igneous rock body that is irregularly shaped,
cuts across older rocks and generally forms 5 to 30 km beneath Earth’s surface.
Laccolith – An intrusive igneous rock body that forms close to the surface when magma
intrudes in between parallel rock layers and pushes the overlying rock layers
upward as a result of the intense pressure from the magma body.
Sill – An intrusive igneous rock body that forms when magma intrudes in between parallel
rock layers. This feature does not push up the overlying rock layers upward.
Veins – deposits of minerals within a rock that are foreign
to the host rock. A few millimeters to several meters across.
All kinds of minerals can occur in veins. Some of the better
known and sought after ones include Gold & Silver.
Volcanic Neck – It is a cylindrical-shaped landform standing
above the surface created by magma solidifying in the
vent of a volcano. Erosion of the sides of the volcano
exposes the neck.
Devil's Tower, WY – Is a great example of an exposed
volcanic neck.
* All of these Plutons can eventually be visible at the Earth’s surface due to the
uplifting of landmasses and also the erosion of the overlying rock layers.
* Batholiths are often found at the cores of many of Earth’s mountain ranges.
Texture (the size and presence of crystals) of Igneous Rocks
As magma cools the minerals can begin to crystallize:
Very slow cooling (deeper in the Earth)

Large crystals (can be seen with the unaided eye)

This would be known as a Coarse Grained Texture
Granite is a prime example. It varies in color
due to the differences in mineral content.
Uses of Granite (click here): Countertops, Tile
Flooring, Building Stone, Paving Stone, Street
Curbing, Tombstones, Decorative Facing Stone,
Monuments & Statues.
Rock of Ages – One of the largest granite providers in the world.
Faster cooling (closer to the surface the Earth)

Smaller crystals (need a microscope to see them)

This would be known as a Fine-Grained Texture
Slow partial cooling first (deep in the Earth) then rapid cooling the rest of the way

Large crystals embedded in a small crystal matrix

This would be known as a Porphyritic Texture
Very fast cooling (on the surface of the Earth)

No crystals

This would be known as a Glassy Texture (it looks like glass)
Obsidian (volcanic glass) is a prime example.
It varies in
color due to its mineral content. It has a glass-like appearance.
Uses of Obsidian: Used in the past for cutting tools
and also arrowheads.
Very fast cooling (on the surface of the Earth)

No crystals, loaded with gas bubbles that were trapped

This would be known as a Vesicular Texture
Pumice is a prime example.
It varies in color due to its mineral content.
Unique property of Pumice: Very light in weight
It will float on water.
Uses of Pumice: (more on these at the end of the notes)
* Abrasive hand soap
* Dental uses
* Rubbing compound
* Beauty aid
* Stonewashing of clothing * Gas grills
* Lightweight concrete
* Landscaping
Rock fragments that have been welded together by the heat of the
volcanic eruption or cemented together by minerals (on the surface of the Earth)

No crystals, made up of tephra blown out of the volcano

This would be known as a Pyroclastic Texture (also called fragmental)

This would appear grainy, like a sedimentary rock,
and there may be shards of glass embedded.
“Tuff” is a prime example. It is volcanic ash that has been
cemented together by minerals contained in the volcanic ash.
Other pyroclastic textured rocks can be formed when small
pieces of tephra are “welded” together by the great heat of a
volcanic eruption.
LAVA TUBE - are natural tunnels where lava flows underneath the ground, sometimes for
many kilometers. After the eruption is over, you can be left with a long empty
tunnel that seems almost man made. They are created when a channelized lava
flow gets crusted over (forming a hard outer shell). When the eruption stops, the
hot fluid core drains away leaving a long snake-like tunnel. Occasionally, the roof
of the lava tube will collapse creating a feature known as a skylight.
The feature shown here is
known as a skylight. It is
formed when the roof of a
lava tube collapses.
YouTube Video on Lava Tube formation.
Heavy-duty hand cleaner offers quick-acting lotion
formula with pumice scrubbing particles for cleaning a broad
range of industrial soils, medium dirt and grease. Contains
baby oil and an orange citrus scent.
The Lava Bar and Lava Liquid soaps contain finely ground
pumice which is a by-product of volcanic activity. This
provides the mildly abrasive quality in the soap.
Pumice Powder
The time honored method for rubbing
the final finish coat. Use pumice powder
number 2 grade for a satin sheen, for a
semi-gloss sheen follow up with pumice
4 grade. Use our felt pads to rub and
either water or rubbing oil as a lubricant.
Used as an abrasive and in abrasive
compounds for rubbing down wood
finishes. When used for rubbing down
finishes in its dry state, add water or
rubbing oil as lubricant for final finish
rubbing. (4F Grade Fine pictured).
How Stone Washing Works
Large pumice stones mixed in with jean fabric materials get washed together
2-3 times in an industrial sized washing machine. This process allows the
stones to rub and pound the material so it gets the stone washed look. This
wears out the cloth which leaves white spots and worn out areas of cloth
onto the jean fabric. Using this process instead of washing clothing over and
over to get the worn out look saves time and appeals to consumers because
it is already broken in to wear. Stone washing also makes the jean material
more supple so that it fits comfortably instead of being stiff and needing lots
of wear and washing to get the same effect. Once the fabric is washed with
the pumice stones, it will go through several rinsing cycles. This gets rid of the
stones that break apart and get caught into the material. Pumice is used in
this process because it is a lightweight, abrasive stone that gives the jean
material it is washed with that "stone washed look." Industrial-sized washing
machines are equipped to deal with the large pumice stones placed inside of
it where normal household washing machines can be damaged very easily.
Pumice-Crete?
Pumice is being used to make lightweight concrete for
durable, efficient and sustainable building projects.
Pumice is actually a lightweight volcanic rock. It can be
mixed with Portland cement and water to make a
concrete-like product called Pumice-Crete. It has many
characteristics of traditional concrete, but offers a few
distinct advantages:
An economical alternative to conventional building methods
and materials because it combines structural strength and
insulation in one product.
It provides a variety of design applications
It is durable, fireproof, and noise-resistant material
It can be used in a variety of climates.
Dental Uses
“nada” Pumice Paste
The perfect prep paste for adhesive dentistry!
This is what they use to polish your teeth!
No more messing with mixing pumice and water
Premixed fluoride with a variety of flavors
Fine, Medium, Coarse, X-tra Coarse grit
Beauty Aid -- PUMICE STONE
It's usually used to smooth rough spots on your feet. It can
also effectively reduce corns and calluses.
Cleanse and polish your tootsies with this gel-based
potion that hs finely ground pumice. It feels great on your
feet after a long walk.
Gas Grills -- Pumice Stone
The stones are usually placed on a metal rack just
above the natural gas burners.
Pumice stone heats up quickly and disperses the heat
to the interior of the grill.
Irregularities in the surface of the stone allow the food
drippings to collect and heat up putting a unique
flavoring into the food.
Landscaping Boulders
Blocks of pumice can be used for landscaping, artistic carvings, flower pots, water fountains,
and display ornaments
Smaller pieces of pumice, of various colors,
are used as decorative landscaping material
around bushes and trees.