Grade 6 Science Practice Test: Earth's Layers, Earthquakes and Volcanoes

The crust is only a few to several dozen kilometers thick, while the mantle is nearly 2,900 km thick. We live on the crust, so it seems large, but it is a thin shell.

The outer core is made of liquid iron and nickel. The inner core is even hotter but stays solid because of the enormous pressure.

The mantle is by far the thickest layer and holds most of Earth's volume. The core is dense and heavy, but it takes up less space.

Plates collide, pull apart or slide past each other at their edges, which causes earthquakes and lets magma rise. Maps of earthquakes trace the plate boundaries.

The focus is the underground point where the rock first breaks; the epicenter is the spot on the surface right above it. The two are easy to mix up.

A seismograph detects and records ground shaking. A barometer measures air pressure and an anemometer measures wind speed.

The Ring of Fire follows the plate boundaries around the edges of the Pacific Ocean, from South America to Alaska, Japan and New Zealand.

A hot spot stays in place while the Pacific Plate slowly moves over it, so a chain of volcanoes forms, older to the northwest. Subduction needs a plate boundary, which Hawaii does not have.

Heat from deep inside Earth makes mantle rock flow in slow loops, and the plates on top are carried along. Tides and winds are far too weak to move plates.

Ocean crust is denser, so it is pushed down into the mantle in a process called subduction, which often forms volcanoes and deep trenches. Plates pulling apart describes a different kind of boundary.