Section 5.1 Igneous Rocks pages 112-117

Lava – magma that flows out onto Earth’s surface.

Magma – molten rock below Earth’s surface.

Igneous rock – formed when lava or magma cools and minerals
crystallize







obsidian
Rocks melt at temperatures of 800°C - 1200°C (1472oF – 2192oF). These temps are present in the
upper mantle and lower crust.
The Earth’s thermal energy comes from Earth’s molten formation and from decay of radioactive
elements.

Magma composition – slushy form of molten rock, dissolved gases and mineral crystals.

Magma is classified as:
1. Basaltic
2. Andesitic
3. Rhyolitic
This is based on how much silica the rock contains.
Section 5.2 Classification of Igneous Rock pages 118-123
Mineral Composition of Igneous Rock
Igneous rock is either intrusive or extrusive
Intrusive rocks are when magma cools and crystallizes below Earth’s surface. Crystals of
intrusive rocks are generally large enough to see w/o magnification.
Extrusive rocks are when lava cools on the Earth’s surface. The crystals that form in extrusive
rocks are difficult to see w/o magnification.
Grain size and texture serve as clues in identifying igneous rock.
Basaltic rocks, such as gabbro, are dark – colored and have lower silica content.
The mineral content is mostly plagioclase and pyroxene.
Granitic rocks, such as granite, are light colored, and have high silica content.
The mineral content is mainly quartz, potassium feldspar and plagioclase
feldspar.
Intermediate rocks have a mineral composition between basaltic and
granitic. They contain plagioclase feldspar and hornblende. Diorite is an
example.
Texture is the size, shape and distribution of the crystals or grains in the rock. It is either coarse
grained or fine grained.
coarse
fine
Crystal size and cooling rates


Lava that flows on the Earth’s surface cools quickly so there is not enough time for large crystals
to form. The result is an extrusive rock that has crystals too small to see w/o magnification.
Rhyolite is an example. Sometimes the cooling is so fast, there are no crystals. Obsidian is an
example.
When the magma cools slowly beneath the Earth’s surface, large crystals form. Granite, diorite
and gabbro are examples of intrusive igneous rock.
Porphyritic rocks

Porphyritic texture is where the rock is characterized by large well-formed crystals surrounded by
smaller fine-grained crystals of the same mineral or different mineral.
This happens when magma cools slowly then suddenly begins to cool
rapidly. For example, if magma cools slowly the crystals have time to
grow large, then all of a sudden the magma gets pushed upward and
erupts out of the Earth. Now the lava will cool rapidly leaving small
crystals to form.
Vesicular rock

Vesicular rock is formed when the gasses in the magma cannot escape. If the lava is to thick holes
called vesicles are left behind. The result is a rock that looks spongy. Pumice and vesicular basalt
are examples.