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Rocks, Rocks and
More Rocks!
Igneous – “FIRE ROCKS”
(for now)
Extrusive Igneous Rocks
 Notes:
Extrusive Igneous Rocks
form when lava erupts from a volcano
or “bubbles up” from tectonic plates
moving apart - onto Earth’s surface.
Ex: Basalt – the most common
extrusive rock on Earth; forms most
of Earth’s ocean floor.
Ex: Obsidian – black volcanic glass

Earth's Most Abundant
Bedrock: Basalt
Basalt forms more of
Earth's surface than
any other rock type.
Most areas in ocean
basins are basalt.
May be on land from
lava flows, too.
Igneous Rocks
Igneous Rock - Obsidian
Obsidian was
used to make
knives, arrow
heads, spear
points,
scrapers and
many other
weapons and
tools
How igneous rocks form…
Extrusive
When a
volcano
erupts or
when magma
bubbles to
the surface
Intrusive
when
magma
cools
slowly
inside
earth
Intrusive Igneous Rocks
 Notes:
forms when magma hardens
beneath Earth’s surface (INSIDE).
Ex: Granite – the most abundant
intrusive rock on Earth’s continents.
Forms the core of many mountain
ranges.
Ex: Porphyry- looks like jello w/fruit
inside it
Intrusive Igneous
Rocks
Granite
is used to make many
objects

Granite is also well-known
from its many worldfamous natural
exposures.
These include: Stone
Mountain, GA; Mount
Rushmore, SD
Intrusive Igneous Rock
Porphyry
Igneous Rocks
 Notes:
Texture – fine grained
Rapidly cooling lava forms a finegrained rock with small crystals.
Ex: Basalt - crystals too small to be
seen without a microscope.
Ex: Obsidian –smooth, shiny texture
like thick glass & cooled without
forming crystals “Volcanic Glass”
Igneous Rocks
 Obsidian
Obsidian on the side of a
volcano
Igneous Rocks
 Notes:
Texture: coarse-grained
Slowly cooling magma forms coarsegrained rock with large crystals
Ex: granite and porphyry
Igneous Rocks –
Mineral Composition
 Notes:
When magma is high in silica lightcolored rocks form - like granite.
When lava is low in silica darkcolored rocks form - like basalt
High Silica vs. Low Silica
Uses of Igneous Rock
Granite – Statues, Bridges and old
public buildings, Paving streets with
cobblestone, floors, kitchen
countertops (thin polished sheets of
Granite)
Uses of Igneous Rock
 Basalt
- Gravel for construction
 Obsidian
- Sharp tools for cutting
and scraping (Native Am!), Starting
vegetable gardens (fertilizer) – it’s
called Perlite, which is formed from
heating Obsidian
Pumice is cool! It can float!
Pumice
polishing
– used for cleaning and
Uses of Igneous Rock
 Statues,
Public Buildings - Granite
Uses of Igneous Rocks
 Cobblestone
streets, kitchen
countertops – Granite
Questions…
 What
processes form Igneous Rocks?
 How are the 2 types of Igneous Rock
different?
 How can we use Igneous Rocks? How
have they been used in the past?
 Can you describe the most common
Intrusive Ign. Rock? Where is it
commonly found?
 Can you describe the most abundant
Extrusive Ign. Rock? What area on
Earth is mostly made of this type?