Differentiate between relative dating and radiometric dating
Long, thin basalt flows with pahoehoe surfaces are common.
Intermediate composition magma such as andesite tends to form cinder cones of intermingled ash, tuff and lava, and may have viscosity similar to thick, cold molasses or even rubber when erupted.
Felsic magma such as rhyolite is usually erupted at low temperature and is up to 10,000 times as viscous as basalt.
Volcanoes with rhyolitic magma commonly erupt explosively, and rhyolitic lava flows typically are of limited extent and have steep margins, because the magma is so viscous.
Igneous rock are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body.
The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed.
Because the minerals are fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous rocks.
Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a microscope, so only an approximate classification can usually be made in the field.
The mineral grains in such rocks can generally be identified with the naked eye.
Extrusive igneous rocks are formed at the Earth's surface as a result of the partial melting of rocks within the mantle and crust.
The melt, with or without suspended crystals and gas bubbles, is called magma.
As noted above, igneous rocks may be either intrusive (plutonic) or extrusive (volcanic).
Intrusive igneous rocks are formed from magma that cools and solidifies within the earth.
Eruptions of volcanoes under the air are termed subaerial whereas those occurring underneath the ocean are termed submarine.