plate tectonic

• Aa (pronounced "ah-ah") is lava that has a rough, jagged, spiny, and generally clinkery surface.
• In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense interior.
• An active volcano is a volcano that is currently erupting, or has erupted during recorded history.
• An aftershock is an earthquake which follows a larger earthquake or main shock and originates in or near the rupture zone of the larger earthquake.
• Major earthquakes are generally followed by a larger number of aftershocks, decreasing in frequency with time.
• Amplitude is the maximum height of a wave crest or depth of a trough.
• Andesite is a medium-colored dark gray volcanic rock containing 53-63 percent silica with a moderate viscosity when in a molten state.
• Andesite is intermediate in color, composition, and eruptive character between basalt and dacite.
• Aseismic refers to something not associated with an earthquake, as in aseismic slip.
• Aseismic can also indicate an area with no record of earthquakes; an aseismic zone.
• Ash (volcanic) are fragments less than 2 millimeters (about 1/8 inch) in diameter of lava or rock blasted into the air by volcanic explosions.
• An ash cloud is the fine material that is generated by a pyroclastic flow and rises above it.
• An ash flow is a pyroclastic flow consisting predominantly of ash-sized (less than 4 millimeters in diameter) particles.
• Cinders are lava fragments about 1 centimeter (about 1/2 inch) in diameter.
• A cinder cone is a steep-sided volcano formed by the explosive eruption of cinders that form around a vent.
• A composite volcano is a steep-sided volcano built by lava flows and tephra deposits.
• The core is the innermost layers of the Earth.
• A plate is one of the huge sections which make up the Earth's crust, continuously moving.
• Except in special circumstances, earthquakes below magnitude 2.5 are not generally felt by humans.
• A mudflow is the flowing mixture of water and debris (intermediate between a volcanic avalanche and a water flood) that forms on the slopes of a volcano.
• A plate boundary is the place where two or more plates in the Earth's crust meet.
• A pluton pertains to igneous rock bodies that form at great depth.
• A magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake - or it takes about 900 magnitude 4.7 earthquakes to equal the energy released in a single 6.7 earthquake!
• Paleomagnetism is the study of the natural magnetic traces that reveal the intensity and direction of Earth's magnetic field in the geologic past.
• Metamorphic rocks are sometimes sedimentary and igneous rocks that are subject to pressures so intense or heat so high that they are completely changed, becoming metamorphic rocks, which form while buried within the Earth's crust.
• Pillow Lava is a structure formed when molten lava breaks through the thin walls of underwater tubes, squeezes out like toothpaste, and quickly solidifies as irregular, tongue-like protrusions.
• P waves, or primary, longitudinal, irrotational, push, pressure, dilatational, compressional, or push-pull waves, are the fastest body waves and arrive at stations before the S waves, or secondary waves, carrying energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave.
• Rock mechanics seems to preclude earthquakes smaller than about -1 or larger than about 9.5.
• The mantle is the layer of rock that lies between the crust and the outer core of the Earth, approximately 2900 kilometers thick and the largest of the Earth's major layers.
• Pumice is a light-colored, frothy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava.
• The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks.
• Pahoehoe is lava that in solidified form is characterized by a smooth, billowy, or ropy surface.
• A pyroclastic flow is a hot, fast-moving and high-density mixture of fine and coarse particles and gas formed during explosive eruptions or from the collapse of a lava dome.
• The Modified Mercalli scale is a scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, designated by Roman numerals, and is an arbitrary ranking based on observed effects.
• Plate Tectonics is the theory that the Earth's crust and upper mantle (the lithosphere) is broken into a number of more or less rigid, but constantly moving, segments or plates.
• A magnitude -1.0 event releases about 900 times less energy than a magnitude 1.0 quake.
• The inner core is solid and has a radius of about 1300 kilometers (The radius of the Earth is about 6371 kilometers).
• Scoria forms when blobs of gas-charged lava are thrown into the air during an eruption and cool in flight, falling as dark volcanic rock containing cavities created by trapped gas bubbles.
• A seismic sea wave is a tsunami generated by an undersea earthquake.
• Rhyolite is a volcanic rock (or lava) that characteristically is light in color, contains 69 percent silica or more, and is rich in potassium and sodium.