Explain plate tectonic theory: the evidence for moving plates, mantle convection as the driving force, the features and motions at divergent, convergent and transform boundaries, and Virginia's geologic provinces (Virginia 2018 Earth Science SOL ES.7).

What this topic is asking

Virginia Earth Science SOL standard ES.7 asks you to explain plate tectonic theory: the evidence that plates move, the driving force (mantle convection), what happens at each boundary type, and how this built Virginia's geologic provinces. The EOC tests this with boundary diagrams (match the motion to the feature), with the historical evidence for continental drift, and with the global pattern of earthquakes and volcanoes. It is one of the most heavily tested standards.

The evidence and the driving force

Alfred Wegener proposed continental drift but lacked a mechanism, so it was rejected at first. The discovery of seafloor spreading decades later supplied the mechanism and turned the idea into the accepted theory of plate tectonics, a good example of how new evidence revises science.

Divergent boundaries

At a divergent boundary, two plates move apart. Magma rises into the gap, cools and forms new oceanic crust, a process called seafloor spreading that builds the mid-ocean ridges. On land, divergence pulls the crust apart to form a rift valley (the East African Rift). Divergent boundaries create crust, so the ocean floor is youngest at the ridge.

Convergent boundaries

At a convergent boundary, two plates collide, and the outcome depends on what collides:

• Oceanic meets continental: the denser oceanic plate subducts beneath the continent, forming a deep ocean trench, a chain of volcanoes on the continent, and mountain building (the Andes).
• Oceanic meets oceanic: one subducts, forming a trench and a volcanic island arc (Japan).
• Continental meets continental: neither subducts easily, so the crust crumples and is uplifted into high mountain ranges (the Himalayas).

Convergent boundaries destroy or thicken crust, the opposite of divergent ones.

Transform boundaries

At a transform boundary, two plates slide horizontally past each other. Crust is neither created nor destroyed, but the grinding motion stores and releases energy as frequent earthquakes. The San Andreas Fault in California is the classic example.

Hot spots

A hot spot is a plume of rising magma that stays roughly fixed while a plate moves over it, producing a line of volcanoes. As the plate carries each volcano away from the hot spot, a new one forms, building a chain of volcanic islands such as the Hawaiian Islands. Hot-spot volcanism happens away from plate boundaries, which is what makes it distinctive.

Virginia's geologic provinces

Virginia's landscape records ancient plate collisions that built the Appalachian Mountains. From east to west the provinces are the Coastal Plain (flat, young sediments near the Atlantic), the Piedmont (rolling hills of old metamorphic and igneous rock), the Blue Ridge (ancient, resistant rock forming the mountains), the Valley and Ridge (folded sedimentary rock, with limestone valleys and caves), and the Appalachian Plateau (uplifted, eroded sedimentary rock in the far southwest, where coal is found). These provinces show how convergence and later weathering and erosion shaped the state.

Try this

Q1. State what happens to the ocean floor at a divergent boundary and why it is youngest there. [2]

• Cue. Plates move apart and magma rises to form new oceanic crust (seafloor spreading), so the crust at the ridge has just formed and is youngest.

Q2. Explain why the Hawaiian Islands form a chain even though they are far from a plate boundary. [2]

• Cue. A fixed hot spot feeds a volcano; as the Pacific Plate moves over it, each volcano is carried away and a new one forms, building a chain.