LETS GO ON AN
IGNEOUS ROCKS TOUR
Joyce Knox
http://seis.natsci.csulb.edu/basicgeo/IGNEOUS_TOUR.html
Igneous
rocks form when molten rock cools and solidifies (crystallizes). Molten rock
trapped underground is called magma. Molten rock erupted at Earth's
surface is referred to as lava. The pictures below illustrate the
significant textures and compositions that enable geologists and geology
students to classify igneous rocks. For convenience of presentation the igneous
rocks are divided into intrusive rocks (or plutonic rocks) which form
underground, and extrusive rocks (or volcanic rocks) which form at
Earth's surface. Intrusive rocks are featured in cards 2 through 6. Extrusive
rocks are featured in cards 7 through 10.
Texture of Igneous Rocks The texture of an igneous rock is determined by
its rate of cooling. Intrusive rocks, formed due to slow cooling of magma
underground, are composed of large crystals because crystals had a long time to
grow before the crystallization process was finished. This forms a
coarse-grained or phaneritic texture. Observe picture 1 to see how this texture
appears. (Click on picture 1 for a better view.) Also note the interlocking
nature of the mineral crystals comprising this rock.
Extrusive rocks, formed due to rapid cooling of lava at Earth's surface,
are composed of tiny crystals (fine-grained) because crystallization was
completed within a few seconds, hours, or perhaps days. Click on pictures 2 and
4 to see how a fine-grained texture appears. Finally, click on picture 3 to view
the special igneous rock texture referred to as porphyritic. The large white
crystals formed as magma began to slowly crystallize underground. The rest of
the rock, the gray matrix, is composed of tiny crystals which formed as the
still-molten magma was erupted at Earth's surface where it cooled very rapidly.
Composition of Igneous Rocks Igneous rocks are divided into
three compositional groups:
1) felsic - composed mostly of the minerals quartz and potassium feldspar
which are generally white to pink in color. The rocks in pictures 1 and 2 are
felsic in composition.
2) intermediate - composed mostly of the minerals amphibole and
sodium-rich plagioclase feldspar, with some quartz and pyroxene possible.
Intermediate igneous rocks are usually light to dark gray in color. The rock in
picture 3 is intermediate in composition.
3) mafic - composed mostly of the minerals pyroxene, calcium-rich
plagioclase feldspar and olivine. Mafic rocks are usually black to dark green in
color. The rock in picture 4 is mafic in composition.
 QUESTIONS
Ø
Which pictures
show the rock granite?
Ø
Which picture
shows a mafic dike?
Ø
Which pictures
show xenoliths?
Ø
Which picture
shows pyroclastic texture?
Ø
Which pictures
show the rock rhyolite?
Ø
Wich picture
shows the rock diorite?
Ø
Which picture
shows pahoehoe texture?
Ø
Which pictures
show gabbro?
Ø
Which pictures
show felsic dikes?
Ø
Which pictures
show intrusive igneous rocks?
Ø
Which pictures
show basalt?
Ø
Which pictures
show felsic rocks?
Ø
Which pictures
show intermediate rocks?
Ø
Which pictures
show mafic rocks?
Ø
Which pictures
show extrusive rocks?
Ø
Which picture
shows andesite?
Ø
Which pictures
shows part of a batholith?
|
Support |
Links
