Lava and Pyroclastic Material

Image depicting a magma chamber, an underground pool of liquid rock found beneath the surface

Lava is simply defined as molten rock [magma] expelled by an opening/vent [volcano] during an eruption^{Weird Science Kids: Definition/Lava}. Lava can also be called "Former-Magma", or a term related to/of magma on the surface. What is "magma"? Magma is defined as Magma is a molten and semi-molten rock mixture found under the surface of the Earth^{National Geographic: Encyclopedia/Magma}. Magma ejected out of a volcano is known as lava, and Magma that is cooled down is known as an igneous rock (defined as: Igneous rock is formed when magma cools and solidifies^{Sciencekids.co.nz: Igenous Rocks}). Nonexplosive eruptions (these eruptions produce relatively calm flows of lava) mostly produce lava, but eruptions can alter between lava and what is known as pyroclastic material, which is material formed when a mixture of hot rock, ash and lava fragments is blasted into the air and hardens [while in the air].

Lava

Viscosity is how resistant a liquid is to flow. Water has a low viscosity (which means it flows fast) while a milkshake has a high viscosity (which means it flows slow). Similarly, we will review through four basic types of lava. Aa and Pillow Lava have lower viscosity while Blocky Lava and Pahoehoe have high viscosity. Thus, Aa and Pillow Lava flow quickly while Blocky Lava and Pahoehoe flow slowly.

Types of Lava

Aa (other spellings: ʻAʻā, aa, aʻa, ʻaʻa, and a-aa) is a form of lava that is named a name resembling a screech due to the painful experience of walking barefoot across this type of lava's surface (introduced by Clarence Edward Dutton, an American geologist and a former soldier). The lava typically erupts at temperatures of 1,000 to 1,100 °C quickly out of the volcano and forms a brittle, jagged, sharp, and spiny crust. Molten lava continues to flow underneath the hike-hazard crust.
Pahoehoe is a form of lava that means, when translated, "smooth, unbroken lava". This is because Pahoehoe flows quite slowly, somewhat like wax candles. This is because of lava of high viscosity, in which is covered by a wax-like surface (or crust). The glassy surface of Pahoehoe has rounded wrinkles.
Pillow Lava forms when lava erupts in water (underwater). Pillow lava, though it resembles similar to what we sleep on at night (a pillow), is very hard and lumpy.
Blocky Lava is cool and hard lava that does not have the capability to travel a long distance form its vent. Blockly lava typically oozes out from a volcano and forms heaps of shard-edged chunks.