The internal structure of Earth is the solid portion of the Earth, excluding its atmosphere and hydrosphere. The structure consists of an outer silicate solid crust, a highly viscous asthenosphere and solid mantle, a liquid outer core whose flow generates the Earth's magnetic field, and a solid inner core. Scientific understanding of the internal structure of Earth is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanoes or volcanic activity, analysis of the seismic waves that pass through Earth, measurements of the gravitational and magnetic fields of Earth, and experiments with crystalline solids at pressures and temperatures characteristic of Earth's deep interior. Global properties Measurements of the force exerted by Earth's gravity can be used to calculate its mass. Astronomers can also calculate Earth's mass by observing the motion of orbiting satellites. Earth's average density can be determined through gravimetric experiments, which have historically involved pendulums. The mass of Earth is about . The average density of the Earth is . Layers The structure of Earth can be defined in two ways: by mechanical properties such as rheology, or chemically. Mechanically, it can be divided into lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core. Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core. The geologic component layers of Earth are at increasing depths below the surface: Crust and lithosphere The Earth's crust ranges from in depth and is the outermost layer. The thin parts are the oceanic crust, which underlie the ocean basins (5–10 km) and is mafic-rich (dense iron-magnesium silicate mineral or igneous rock). The thicker crust is the continental crust, which is less dense and is felsic-rich (igneous rocks rich in elements that form feldspar and quartz). The rocks of the crust fall into two major categories – sial (aluminium silicate) and sima (magnesium silicate). It is estimated that sima starts about 11 km below the Conrad discontinuity, though the discontinuity is not distinct and can be absent in some continental regions. The Earth's lithosphere consists of the crust and the uppermost mantle. The crust-mantle boundary occurs as two physically different phenomena. The Mohorovičić discontinuity is a distinct change of seismic wave velocity. This is caused by a change in the rock's density – Immediately above the Moho, the velocities of primary seismic waves (P