Shield volcanoes are composed dominantly of three different kinds of basaltic lava flows:
Pahoehoe- In Polynesian this means lava you can walk on with bare feet. Pahoehoe forms the "rivers" or "streams" of flowing lava so often see in pictures of Hawaiian Volcanoes. This kind of basalt lava is the least viscous or sticky of the three, it has the least resistance to flow. Surface features of Pahoehoe include long, curving, rope like features (called ropey Pahoehoe), or shells and slab-like pieces (called shelly pahoehoe). At its flow front it may form lava toes or tubes.
Aa- In Polynesian this means lava you cannot walk on in bare feet- Aa has more resistance to flow than Pahoehoe, its is stickier and there form more sluggish or slow. Instead of flowing like a river it tends to pile up at its flow front and this leads to an upper surface that is rougher than #1 grit sandpaper, an angular, clinkery surface with tall, jagged lava spikes sticking up out of it.
Both pahoehoe and aa work for the temperamental goddess Pele, so they can either flow calmly out of a fissure, or spurt and gush hundreds to thousands of feet into the air to form spectacular fountains of fire. These fountains feed glowing flows that move like coiled snakes down the side of the mountain.
Both aa and pahoehoe may contain magmatic gas (mostly water vapor, carbon dioxide, and sulfur), and this slowly passes out of the cooling lava leaving behind holes that look a lot like the ones in Swiss cheese. The holes can be angular or round, depending on whether the gas escaped during flow or after the lava stopped flowing. Called vesicles these gas holes vary from pinheads to rare watermelon size ones. Over the years as the lava cools and turns to stone, ground waters seeping through the rock precipitate out minerals that fill the gas holes. In this way are agates, thunder eggs and geodes made. Filled vesicles are called amygdules.
Lava that forms the underwater part of shield volcanoes usually looks and acts much differently than the above-water lava. When hot lava meets cold seawater, the lava undergoes great change:
Pillow Lava- Cold water quickly cools the lava, which causes a rapid increase in the lava's stickiness. This makes the lava slow down so much it takes on shapes that look like your fingers- but instead of ten fingers there are hundreds, all intertwined like pieces of spaghetti. This lava is called pillow lava because the ends of the fingers are oval and look like the pillow sitting on your bed. Pillow lava is the most common lava found on earth.
Hyaloclastite- Water can do so much more than just cool the hot lava. It can put so much stress on it that the lava shatters into zillions of small, block like pieces called hyaloclastite ( glassy fragmental rock).
Finally there are times when the lava pours out of a fissure so fast, and is so hot, the water does not get much of a chance to interact with it. Such lava may flow for a few hundred to a few thousand feet and behaves much like subaerial pahoehoe- this lava is called sheet lava or sheet flows.
Finally many shield volcanoes have small, cone-shaped features on their sides called cinder cones. These are made out of mafic pumice (called scoria) that has the shape and feel of cinder (similar to coal lumps after being burned and cooled). Cinder cones have steep sides and are less than 1000 feet high.