Forces Shaping Earth Notes Mrs. Seay 6 th Grade Science Geologists use earthquakes and surface rocks to indirectly observe Earths interior 1. Seismic waves change speed and direction depending on the density and material they travel through. 2. Forces bring rocks formed deep within Earth to the surface. 3. Evidence suggests that Earth is formed of different materials in layers. B. Earth has four layers. Inner coredense iron core; very hot and dense. Outer coremolten metal above the inner core Solid layer that flows slowly like putty is the mantle. Outermost layer is the crust. C. Earths structuretheories based on physical properties of density, temperature, and pressure that change with depth. Earths moving plates are sections of the lithosphere, the rigid upper part of the mantle and crust. Plate boundaries are edges where plates meet. Tension can pull plates apart, resulting in new lithosphere forming in the gaps. Colliding plates could cause mountains to form as rock crumbles and folds. Subduction occurs when a denser plate sinks underneath a less dense plate. Shearing causes faults and earthquakes as two plates slide past each other. Plate movement theoryconvection in the mantle circulates material and moves plates. Ridge-push at mid ocean ridges causes plates to slide down the slope. Slab-pull happens as plates move away from mid-ocean ridges and become denser. Mountains form in different ways. Fault-block mountains form from huge tilted blocks of rock separated from surrounding rock by large faults. Folded mountains form by compression forces folding rock layers. Upwarped mountainsforces push up Earths crust and allow the sedimentary rock to erode, leaving igneous or metamorphic rock. Over time, layers of lava can form a cone-shaped volcanic mountain. Hot spots formed by plumes of magma in the mantle can cause underwater volcanic mountains. Isostacy principle in dicates Earths crust and lithosphere float on the upper mantle. Earths crust is thicker under mountains. When mountains erode, the crust rises because the weight has been removed.